Functional organization of speed tuned neurons in visual area MT

Using camera-guided electrode microdrive navigation for precise 3D targeting of macaque brain sites

Spatial accuracy in electrophysiological investigations is paramount, as precise localization and reliable access to specific brain regions help the advancement of our understanding of the brain's complex neural activity. Here, we introduce a novel, multi camera-based, frameless neuronavigation technique for precise, 3-dimensional electrode positioning in awake monkeys. The investigation of neural functions in awake primates often requires stable access to the brain with thin and delicate recording electrodes. This is usually realized by implanting a chronic recording chamber onto the skull of the animal that allows direct access to the dura. Most recording and positioning techniques utilize this implanted recording chamber as a holder of the microdrive or to hold a grid. This in turn reduces the degrees of freedom in positioning. To solve this problem, we require innovative, flexible, but precise tools for neuronal recordings. We instead mount the electrode microdrive above the animal on an arch, equipped with a series of translational and rotational micromanipulators, allowing movements in all axes. Here, the positioning is controlled by infrared cameras tracking the location of the microdrive and the monkey, allowing precise and flexible trajectories. To verify the accuracy of this technique, we created iron deposits in the tissue that could be detected by MRI. Our results demonstrate a remarkable precision with the confirmed physical location of these deposits averaging less than 0.5 mm from their planned position. Pilot electrophysiological recordings additionally demonstrate the accuracy and flexibility of this method. Our innovative approach could significantly enhance the accuracy and flexibility of neural recordings, potentially catalyzing further advancements in neuroscientific research.

Table tennis players use superior saccadic eye movements to track moving visual targets

Introduction

Table tennis players perform visually guided visuomotor responses countlessly. The exposure of the visual system to frequent and long-term motion stimulation has been known to improve perceptual motion detection and discrimination abilities as a learning effect specific to that stimulus, so may also improve visuo-oculomotor performance. We hypothesized and verified that table tennis players have good spatial accuracy of saccades to moving targets.

Methods

University table tennis players (TT group) and control participants with no striking-sports experience (Control group) wore a virtual reality headset and performed two ball-tracking tasks to track moving and stationary targets in virtual reality. The ball moved from a predetermined position on the opponent's court toward the participant's court. A total of 54 conditions were examined for the moving targets in combinations of three ball trajectories (familiar parabolic, unfamiliar descent, and unfamiliar horizontal), three courses (left, right, and center), and six speeds.

Results and discussion

All participants primarily used catch-up saccades to track the moving ball. The TT group had lower mean and inter-trial variability in saccade endpoint error compared to the Control group, showing higher spatial accuracy and precision, respectively. It suggests their improvement of the ability to analyze the direction and speed of the ball's movement and predict its trajectory and future destination. The superiority of the spatial accuracy in the TT group was seen in both the right and the left courses for all trajectories but that of precision was for familiar parabolic only. The trajectory dependence of improved saccade precision in the TT group implies the possibility that the motion vision system is trained by the visual stimuli frequently encountered in table tennis. There was no difference between the two groups in the onset time or spatial accuracy of saccades for stationary targets appearing at various positions on the ping-pong table.

Conclusion

Table tennis players can obtain high performance (spatial accuracy and precision) of saccades to track moving targets as a result of motion vision ability improved through a vast amount of visual and visuo-ocular experience in their play.

Neural encoding of multiple motion speeds in visual cortical area MT

Segmenting objects from each other and their background is critical for vision. The speed at which objects move provides a salient cue for segmentation. However, how the visual system represents and differentiates multiple speeds is largely unknown. Here we investigated the neural encoding of multiple speeds of overlapping stimuli in the primate visual cortex. We first characterized the perceptual capacity of human and monkey subjects to segment spatially overlapping stimuli moving at different speeds. We then determined how neurons in the motion-sensitive, middle-temporal (MT) cortex of macaque monkeys encode multiple speeds. We made a novel finding that the responses of MT neurons to two speeds of overlapping stimuli showed a robust bias toward the faster speed component when both speeds were slow (≤ 20°/s). The faster-speed bias occurred even when a neuron had a slow preferred speed and responded more strongly to the slower component than the faster component when presented alone. The faster-speed bias emerged very early in neuronal response and was robust over time and to manipulations of motion direction and attention. As the stimulus speed increased, the faster-speed bias changed to response averaging. Our finding can be explained by a modified divisive normalization model, in which the weights for the speed components are proportional to the responses of a population of neurons elicited by the individual speeds. Our results suggest that the neuron population, referred to as the weighting pool, includes neurons that have a broad range of speed preferences. As a result, the response weights for the speed components are determined by the stimulus speeds and invariant to the speed preferences of individual neurons. Our findings help to define the neural encoding rule of multiple stimuli and provide new insight into the underlying neural mechanisms. The faster-speed bias would benefit behavioral tasks such as figure-ground segregation if figural objects tend to move faster than the background in the natural environment.

Mechanisms of speed encoding in the human middle temporal cortex measured by 7T fMRI

Perception of dynamic scenes in our environment results from the evaluation of visual features such as the fundamental spatial and temporal frequency components of a moving object. The ratio between these two components represents the object's speed of motion. The human middle temporal cortex hMT+ has a crucial biological role in the direct encoding of object speed. However, the link between hMT+ speed encoding and the spatiotemporal frequency components of a moving object is still under explored. Here, we recorded high resolution 7T blood oxygen level-dependent BOLD responses to different visual motion stimuli as a function of their fundamental spatial and temporal frequency components. We fitted each hMT+ BOLD response with a 2D Gaussian model allowing for two different speed encoding mechanisms: (1) distinct and independent selectivity for the spatial and temporal frequencies of the visual motion stimuli; (2) pure tuning for the speed of motion. We show that both mechanisms occur but in different neuronal groups within hMT+, with the largest subregion of the complex showing separable tuning for the spatial and temporal frequency of the visual stimuli. Both mechanisms were highly reproducible within participants, reconciling single cell recordings from MT in animals that have showed both encoding mechanisms. Our findings confirm that a more complex process is involved in the perception of speed than initially thought and suggest that hMT+ plays a primary role in the evaluation of the spatial features of the moving visual input.

The structural basis of age-related decline in global motion perception at fast and slow speeds

A decrease in global motion perception (GMP) has been reported in older adults, and this age-related decline in GMP varies with the speed of global motion. However, no studies have investigated whether the asynchronous age-related decline in GMP is related to degenerative changes in brain structure. In this study, the random dot kinematogram paradigm and structural magnetic resonance imaging were used to investigate the asynchronous aging of GMP at fast and slow speeds (called fast GMP and slow GMP, respectively) and their relationships with brain structure. Ninety-four older adults (65.74 ± 4.50 yrs) and 90 younger adults (22.83 ± 4.84 yrs) participated in the experiment. The results showed that older adults had higher motion coherence thresholds (MCT) than younger adults at both fast and slow speeds. Brain-behavior correlation analyses of younger adults revealed that none of the correlations between morphological measures and MCTs survived correction for multiple comparisons. For older adults, slow MCT was correlated with cortical thickness in the bilateral V4v, V5/MT+, left V7, V8, LO, and surface area in the right V7. Fast MCT was significantly correlated with gray matter volume in the right V7 and thickness in the left V5/MT+. These results support the view that global motion extraction occurs within two speed-tuned systems that are at least partially independent in terms of their neural substrates, which deteriorate with age at different speeds. Aging of GMP is also associated with morphological changes in the visual cortex. Age-related cerebral atrophy in the dorsal stream may impair both fast and slow GMP, whereas aging of the ventral stream specifically impairs slow GMP.

Pop-out search instigates beta-gated feature selectivity enhancement across V4 layers

Visual search is a workhorse for investigating how attention interacts with processing of sensory information. Attentional selection has been linked to altered cortical sensory responses and feature preferences (i.e., tuning). However, attentional modulation of feature selectivity during search is largely unexplored. Here we map the spatiotemporal profile of feature selectivity during singleton search. Monkeys performed a search where a pop-out feature determined the target of attention. We recorded laminar neural responses from visual area V4. We first identified "feature columns" which showed preference for individual colors. In the unattended condition, feature columns were significantly more selective in superficial relative to middle and deep layers. Attending a stimulus increased selectivity in all layers but not equally. Feature selectivity increased most in the deep layers, leading to higher selectivity in extragranular layers as compared to the middle layer. This attention-induced enhancement was rhythmically gated in phase with the beta-band local field potential. Beta power dominated both extragranular laminar compartments, but current source density analysis pointed to an origin in superficial layers, specifically. While beta-band power was present regardless of attentional state, feature selectivity was only gated by beta in the attended condition. Neither the beta oscillation nor its gating of feature selectivity varied with microsaccade production. Importantly, beta modulation of neural activity predicted response times, suggesting a direct link between attentional gating and behavioral output. Together, these findings suggest beta-range synaptic activation in V4's superficial layers rhythmically gates attentional enhancement of feature tuning in a way that affects the speed of attentional selection.

Visual and Vestibular Selectivity for Self-Motion in Macaque Posterior Parietal Area 7a

We examined the responses of neurons in posterior parietal area 7a to passive rotational and translational self-motion stimuli, while systematically varying the speed of visually simulated (optic flow cues) or actual (vestibular cues) self-motion. Contrary to a general belief that responses in area 7a are predominantly visual, we found evidence for a vestibular dominance in self-motion processing. Only a small fraction of neurons showed multisensory convergence of visual/vestibular and linear/angular self-motion cues. These findings suggest possibly independent neuronal population codes for visual versus vestibular and linear versus angular self-motion. Neural responses scaled with self-motion magnitude (i.e., speed) but temporal dynamics were diverse across the population. Analyses of laminar recordings showed a strong distance-dependent decrease for correlations in stimulus-induced (signal correlation) and stimulus-independent (noise correlation) components of spike-count variability, supporting the notion that neurons are spatially clustered with respect to their sensory representation of motion. Single-unit and multiunit response patterns were also correlated, but no other systematic dependencies on cortical layers or columns were observed. These findings describe a likely independent multimodal neural code for linear and angular self-motion in a posterior parietal area of the macaque brain that is connected to the hippocampal formation.

Optic flow parsing in the macaque monkey

During self-motion, an independently moving object generates retinal motion that is the vector sum of its world-relative motion and the optic flow caused by the observer's self-motion. A hypothesized mechanism for the computation of an object's world-relative motion is flow parsing, in which the optic flow field due to self-motion is globally subtracted from the retinal flow field. This subtraction generates a bias in perceived object direction (in retinal coordinates) away from the optic flow vector at the object's location. Despite psychophysical evidence for flow parsing in humans, the neural mechanisms underlying the process are unknown. To build the framework for investigation of the neural basis of flow parsing, we trained macaque monkeys to discriminate the direction of a moving object in the presence of optic flow simulating self-motion. Like humans, monkeys showed biases in object direction perception consistent with subtraction of background optic flow attributable to self-motion. The size of perceptual biases generally depended on the magnitude of the expected optic flow vector at the location of the object, which was contingent on object position and self-motion velocity. There was a modest effect of an object's depth on flow-parsing biases, which reached significance in only one of two subjects. Adding vestibular self-motion signals to optic flow facilitated flow parsing, increasing biases in direction perception. Our findings indicate that monkeys exhibit perceptual hallmarks of flow parsing, setting the stage for the examination of the neural mechanisms underlying this phenomenon.

Spatial metric in blindness: behavioural and cortical processing

Visual modality dominates spatial perception and, in lack of vision, space representation might be altered. Here we review our work showing that blind individuals have a strong deficit when performing spatial bisection tasks (Gori et al., 2014). We also describe the neural correlates associated with this deficit, as blind individuals do not show the same ERP response mimicking the visual C1 reported in sighted people during spatial bisection (Campus et al., 2019). Interestingly, the deficit is not always evident in late blind individuals, and it is dependent on blindness duration. We report that the deficit disappears when one presents coherent temporal and spatial cues to blind people. This suggests that they may use time information to infer spatial maps (Gori et al., 2018). Finally, we propose a model to explain why blind individuals are impaired in this task, speculating that a lack of vision drives the construction of a multi-sensory cortical network that codes space based on temporal, rather than spatial, coordinates.

A quantitative framework for motion visibility in human cortex

Despite the central use of motion visibility to reveal the neural basis of perception, perceptual decision making, and sensory inference there exists no comprehensive quantitative framework establishing how motion visibility parameters modulate human cortical response. Random-dot motion stimuli can be made less visible by reducing image contrast or motion coherence, or by shortening the stimulus duration. Because each of these manipulations modulates the strength of sensory neural responses they have all been extensively used to reveal cognitive and other nonsensory phenomena such as the influence of priors, attention, and choice-history biases. However, each of these manipulations is thought to influence response in different ways across different cortical regions and a comprehensive study is required to interpret this literature. Here, human participants observed random-dot stimuli varying across a large range of contrast, coherence, and stimulus durations as we measured blood-oxygen-level dependent responses. We developed a framework for modeling these responses that quantifies their functional form and sensitivity across areas. Our framework demonstrates the sensitivity of all visual areas to each parameter, with early visual areas V1-V4 showing more parametric sensitivity to changes in contrast and V3A and the human middle temporal area to coherence. Our results suggest that while motion contrast, coherence, and duration share cortical representation, they are encoded with distinct functional forms and sensitivity. Thus, our quantitative framework serves as a reference for interpretation of the vast perceptual literature manipulating these parameters and shows that different manipulations of visibility will have different effects across human visual cortex and need to be interpreted accordingly. NEW & NOTEWORTHY Manipulations of motion visibility have served as a key tool for understanding the neural basis for visual perception. Here we measured human cortical response to changes in visibility across a comprehensive range of motion visibility parameters and modeled these with a quantitative framework. Our quantitative framework can be used as a reference for linking human cortical response to perception and underscores that different manipulations of motion visibility can have greatly different effects on cortical representation.

Neural Correlates of Speed-Tuned Motion Perception in Healthy Adults

It has been suggested that slow and medium-to-fast speeds of motion may be processed by at least partially separate mechanisms. The purpose of this study was to establish the cortical areas activated during motion-defined form and global motion tasks as a function of speed, using functional magnetic resonance imaging. Participants performed discrimination tasks with random dot stimuli at high coherence, at coherence near their own thresholds, and for random motion. Stimuli were moving at 0.1 or 5 deg/s. In the motion-defined form task, lateral occipital complex, V5/MT+ and intraparietal sulcus showed greater activation by high or near-threshold coherence than by random motion stimuli; V5/MT+ and intraparietal sulcus demonstrated greater activation for 5 than 0.1 deg/s dot motion. In the global motion task, only high coherence stimuli elicited significant activation over random motion; this activation was primarily in nonclassical motion areas. V5/MT+ was active for all motion conditions and showed similar activation for coherent and random motion. No regions demonstrated speed-tuning effects for global motion. These results suggest that similar cortical systems are activated by slow- and medium-speed stimuli during these tasks in healthy adults.

Neural mechanisms of motion perceptual learning in noise

Practice improves our perceptual ability. However, the neural mechanisms underlying this experience-dependent plasticity in adult brain remain unclear. Here, we studied the long-term neural correlates of motion perceptual learning. Subjects' behavioral performance and BOLD signals were tracked before, immediately after, and 2 weeks after practicing a motion direction discrimination task in noise over six daily sessions. Parallel to the specificity and persistency of the behavioral learning effect, we found that training sharpened the cortical tuning in MT, and enhanced the connectivity strength from MT to the intraparietal sulcus (IPS, a motion decision-making area). In addition, the decoding accuracy for the trained motion direction was improved in IPS 2 weeks after training. The dual changes in the sensory and the high-level cortical areas suggest that learning refines the neural representation of the trained stimulus and facilitates the information transmission in the decision process. Our findings are consistent with the functional specialization in the visual cortex, and provide empirical evidence to the reweighting theory of perceptual learning at a large spatial scale. Hum Brain Mapp 38:6029-6042, 2017. © 2017 Wiley Periodicals, Inc.

End-Stopping Predicts Curvature Tuning along the Ventral Stream

Neurons in primate inferotemporal cortex (IT) are clustered into patches of shared image preferences. Functional imaging has shown that these patches are activated by natural categories (e.g., faces, body parts, and places), artificial categories (numerals, words) and geometric features (curvature and real-world size). These domains develop in the same cortical locations across monkeys and humans, which raises the possibility of common innate mechanisms. Although these commonalities could be high-level template-based categories, it is alternatively possible that the domain locations are constrained by low-level properties such as end-stopping, eccentricity, and the shape of the preferred images. To explore this, we looked for correlations among curvature preference, receptive field (RF) end-stopping, and RF eccentricity in the ventral stream. We recorded from sites in V1, V4, and posterior IT (PIT) from six monkeys using microelectrode arrays. Across all visual areas, we found a tendency for end-stopped sites to prefer curved over straight contours. Further, we found a progression in population curvature preferences along the visual hierarchy, where, on average, V1 sites preferred straight Gabors, V4 sites preferred curved stimuli, and many PIT sites showed a preference for curvature that was concave relative to fixation. Our results provide evidence that high-level functional domains may be mapped according to early rudimentary properties of the visual system.

SIGNIFICANCE STATEMENT

The macaque occipitotemporal cortex contains clusters of neurons with preferences for categories such as faces, body parts, and places. One common question is how these clusters (or "domains") acquire their cortical position along the ventral stream. We and other investigators previously established an fMRI-level correlation among these category domains, retinotopy, and curvature preferences: for example, in inferotemporal cortex, face- and curvature-preferring domains show a central visual field bias whereas place- and rectilinear-preferring domains show a more peripheral visual field bias. Here, we have found an electrophysiological-level explanation for the correlation among domain preference, curvature, and retinotopy based on neuronal preference for short over long contours, also called end-stopping.

A massively asynchronous, parallel brain

Whether the visual brain uses a parallel or a serial, hierarchical, strategy to process visual signals, the end result appears to be that different attributes of the visual scene are perceived asynchronously--with colour leading form (orientation) by 40 ms and direction of motion by about 80 ms. Whatever the neural root of this asynchrony, it creates a problem that has not been properly addressed, namely how visual attributes that are perceived asynchronously over brief time windows after stimulus onset are bound together in the longer term to give us a unified experience of the visual world, in which all attributes are apparently seen in perfect registration. In this review, I suggest that there is no central neural clock in the (visual) brain that synchronizes the activity of different processing systems. More likely, activity in each of the parallel processing-perceptual systems of the visual brain is reset independently, making of the brain a massively asynchronous organ, just like the new generation of more efficient computers promise to be. Given the asynchronous operations of the brain, it is likely that the results of activities in the different processing-perceptual systems are not bound by physiological interactions between cells in the specialized visual areas, but post-perceptually, outside the visual brain.

A high-density EEG study of differences between three high speeds of simulated forward motion from optic flow in adult participants

A high-density EEG study was conducted to investigate evoked and oscillatory brain activity in response to high speeds of simulated forward motion. Participants were shown an optic flow pattern consisting of a virtual road with moving poles at either side of it, simulating structured forward motion at different driving speeds (25, 50, and 75 km/h) with a static control condition between each motion condition. Significant differences in N2 latencies and peak amplitudes between the three speeds of visual motion were found in parietal channels of interest P3 and P4. As motion speed increased, peak latency increased while peak amplitude decreased which might indicate that higher driving speeds are perceived as more demanding resulting in longer latencies, and as fewer neurons in the motion sensitive areas of the adult brain appear to be attuned to such high visual speeds this could explain the observed inverse relationship between speed and amplitude. In addition, significant differences between alpha de-synchronizations for forward motion and alpha synchronizations in the static condition were found in the parietal midline (PM) source. It was suggested that the alpha de-synchronizations reflect an activated state related to the visual processing of simulated forward motion, whereas the alpha synchronizations in response to the static condition reflect a deactivated resting period.

Shared sensory estimates for human motion perception and pursuit eye movements

Are sensory estimates formed centrally in the brain and then shared between perceptual and motor pathways or is centrally represented sensory activity decoded independently to drive awareness and action? Questions about the brain's information flow pose a challenge because systems-level estimates of environmental signals are only accessible indirectly as behavior. Assessing whether sensory estimates are shared between perceptual and motor circuits requires comparing perceptual reports with motor behavior arising from the same sensory activity. Extrastriate visual cortex both mediates the perception of visual motion and provides the visual inputs for behaviors such as smooth pursuit eye movements. Pursuit has been a valuable testing ground for theories of sensory information processing because the neural circuits and physiological response properties of motion-responsive cortical areas are well studied, sensory estimates of visual motion signals are formed quickly, and the initiation of pursuit is closely coupled to sensory estimates of target motion. Here, we analyzed variability in visually driven smooth pursuit and perceptual reports of target direction and speed in human subjects while we manipulated the signal-to-noise level of motion estimates. Comparable levels of variability throughout viewing time and across conditions provide evidence for shared noise sources in the perception and action pathways arising from a common sensory estimate. We found that conditions that create poor, low-gain pursuit create a discrepancy between the precision of perception and that of pursuit. Differences in pursuit gain arising from differences in optic flow strength in the stimulus reconcile much of the controversy on this topic.

Acute alcohol exposure impairs neural representation of visual motion speed in the visual cortex area posteromedial lateral suprasylvian cortex of cats

BACKGROUND

Psychophysical and behavioral studies have demonstrated that perception of motion can be impaired by acute alcohol exposure. The neural activities of posteromedial lateral suprasylvian cortex (PMLS) of cats are directly linked to the perception of visual motion speed. To date, there have been no studies on the effects of acute alcohol exposure in vivo upon the representation of speed in PMLS neurons.

METHODS

Alcohol was administered intravenously as a 20% (v/v) saline solution via a syringe at a dose levels of 0.5, 1, or 2 g/kg to generate a series of blood alcohol concentrations. Using extracellular single-unit recording technique, we recorded the speed-tuning properties of PMLS neurons that responded to random-dot patterns before and after alcohol administration, and simultaneously monitored the concentration of ethanol by detecting the breath alcohol concentration using a breath analyzer.

RESULTS

After acute alcohol treatment, PMLS cells preferred lower speeds. A broadened speed-tuning bandwidth of PMLS cells was also observed after acute alcohol administration. Additionally, response modulation and discriminative capacity for speed of visual motion in the PMLS cells were significantly impaired after acute alcohol exposure. Concurrently, PMLS cells after acute alcohol exposure showed decreased spontaneous activity, peak responses, and signal-to-noise ratios.

CONCLUSIONS

There is a significant functional degradation in the neural representation of visual motion speed in PMLS of cats after acute alcohol exposure. These neural changes may contribute to the alcohol-related deficits in visual motion perception observed in behavioral studies.

Area V5-a microcosm of the visual brain

Area V5 of the visual brain, first identified anatomically in 1969 as a separate visual area, is critical for the perception of visual motion. As one of the most intensively studied parts of the visual brain, it has yielded many insights into how the visual brain operates. Among these are: the diversity of signals that determine the functional capacities of a visual area; the relationship between single cell activity in a specialized visual area and perception of, and preference for, attributes of a visual stimulus; the multiple asynchronous inputs into, and outputs from, an area as well as the multiple operations that it undertakes asynchronously; the relationship between activity at given, specialized, areas of the visual brain and conscious awareness; and the mechanisms used to “bind” signals from one area with those from another, with a different specialization, to give us our unitary perception of the visual world. Hence V5 is, in a sense, a microcosm of the visual world and its study gives important insights into how the whole visual brain is organized—anatomically, functionally and perceptually.

A common framework for the analysis of complex motion? Standstill and capture illusions

A series of illusions was created by presenting stimuli, which consisted of two overlapping surfaces each defined by textures of independent visual features (i.e., modulation of luminance, color, depth, etc.). When presented concurrently with a stationary 2-D luminance texture, observers often fail to perceive the motion of an overlapping stereoscopically defined depth-texture. This illusory motion standstill arises due to a failure to represent two independent surfaces (one for luminance and one for depth textures) and motion transparency (the ability to perceive motion of both surfaces simultaneously). Instead the stimulus is represented as a single non-transparent surface taking on the stationary nature of the luminance-defined texture. By contrast, if it is the 2D-luminance defined texture that is in motion, observers often perceive the stationary depth texture as also moving. In this latter case, the failure to represent the motion transparency of the two textures gives rise to illusionary motion capture. Our past work demonstrated that the illusions of motion standstill and motion capture can occur for depth-textures that are rotating, or expanding / contracting, or else spiraling. Here I extend these findings to include stereo-shearing. More importantly, it is the motion (or lack thereof) of the luminance texture that determines how the motion of the depth will be perceived. This observation is strongly in favor of a single pathway for complex motion that operates on luminance-defines texture motion signals only. In addition, these complex motion illusions arise with chromatically-defined textures with smooth transitions between their colors. This suggests that in respect to color motion perception the complex motions' pathway is only able to accurately process signals from isoluminant colored textures with sharp transitions between colors, and/or moving at high speeds, which is conceivable if it relies on inputs from a hypothetical dual opponent color pathway.

Averaging, not internal noise, limits the development of coherent motion processing

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Motion processing abilities develop gradually through childhood.

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This lengthy development could be due to local noise and/or poor averaging.

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5–11-year-olds and adults performed equivalent noise and motion coherence tasks.

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Through childhood, internal noise reduces and averaging increases.

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Yet, only improved averaging explains developments in motion coherence sensitivity.

The development of motion processing is a critical part of visual development, allowing children to interact with moving objects and navigate within a dynamic environment. However, global motion processing, which requires pooling motion information across space, develops late, reaching adult-like levels only by mid-to-late childhood. The reasons underlying this protracted development are not yet fully understood. In this study, we sought to determine whether the development of motion coherence sensitivity is limited by internal noise (i.e., imprecision in estimating the directions of individual elements) and/or global pooling across local estimates. To this end, we presented equivalent noise direction discrimination tasks and motion coherence tasks at both slow (1.5°/s) and fast (6°/s) speeds to children aged 5, 7, 9 and 11 years, and adults. We show that, as children get older, their levels of internal noise reduce, and they are able to average across more local motion estimates. Regression analyses indicated, however, that age-related improvements in coherent motion perception are driven solely by improvements in averaging and not by reductions in internal noise. Our results suggest that the development of coherent motion sensitivity is primarily limited by developmental changes within brain regions involved in integrating motion signals (e.g., MT/V5).

Hands in motion: an upper-limb-selective area in the occipitotemporal cortex shows sensitivity to viewed hand kinematics

Regions in the occipitotemporal cortex (OTC) show clear selectivity to static images of human body parts, and upper limbs in particular, with respect to other object categories. Such selectivity was previously attributed to shape aspects, which presumably vary across categories. Alternatively, it has been proposed that functional selectivity for upper limbs is driven by processing of their distinctive motion features. In the present study we show that selectivity to static upper-limb images and motion processing go hand in hand. Using resting-state and task-based functional MRI, we demonstrate that OTC voxels showing greater preference to static images of arms and hands also show stronger functional connectivity with motion coding regions within the human middle temporal complex (hMT+), but not with shape-selective midtier areas, such as hV4 or LO-1, suggesting a tight link between upper-limb selectivity and motion processing. To test this directly, we created a set of natural arm-movement videos where kinematic patterns were parametrically manipulated, while keeping shape information constant. Using multivariate pattern analysis, we show that the degree of (dis)similarity in arm-velocity profiles across the video set predicts, to a significant extent, the degree of (dis)similarity in multivoxel activation patterns in both upper-limb-selective OTC regions and the hMT+. Together, these results suggest that the functional specificity of upper-limb-selective regions may be partially determined by their involvement in the processing of upper-limb dynamics. We propose that the selectivity to static upper-limb images in the OTC may be a result of experience-dependent association between shape elements, which characterize upper limbs, and upper-limb-specific motion patterns.

Decision-related activity in sensory neurons may depend on the columnar architecture of cerebral cortex

Many studies have reported correlations between the activity of sensory neurons and animals' judgments in discrimination tasks. Here, we suggest that such neuron-behavior correlations may require a cortical map for the task relevant features. This would explain why studies using discrimination tasks based on disparity in area V1 have not found these correlations: V1 contains no map for disparity. This scheme predicts that activity of V1 neurons correlates with decisions in an orientation-discrimination task. To test this prediction, we trained two macaque monkeys in a coarse orientation discrimination task using band-pass-filtered dynamic noise. The two orientations were always 90° apart and task difficulty was controlled by varying the orientation bandwidth of the filter. While the trained animals performed this task, we recorded from orientation-selective V1 neurons (n = 82, n = 31 for Monkey 1, n = 51 for Monkey 2). For both monkeys, we observed significant correlation (quantified as "choice probabilities") of the V1 activity with the monkeys' perceptual judgments (mean choice probability 0.54, p = 10(-5)). In one of these animals, we had previously measured choice probabilities in a disparity discrimination task in V1, which had been at chance (0.49, not significantly different from 0.5). The choice probabilities in this monkey for the orientation discrimination task were significantly larger than those for the disparity discrimination task (p = 0.032). These results are predicted by our suggestion that choice probabilities are only observed for cortical sensory neurons that are organized in maps for the task-relevant feature.

Gamma synchrony predicts neuron-neuron correlations and correlations with motor behavior in extrastriate visual area MT

Correlated variability of neuronal responses is an important factor in estimating sensory parameters from a population response. Large correlations among neurons reduce the effective size of a neural population and increase the variation of the estimates. They also allow the activity of one neuron to be informative about impending perceptual decisions or motor actions on single trials. In extrastriate visual area MT of the rhesus macaque, for example, some but not all neurons show nonzero "choice probabilities" for perceptual decisions or non-zero "MT-pursuit" correlations between the trial-by-trial variations in neural activity and smooth pursuit eye movements. To understand the functional implications of zero versus nonzero correlations between neural responses and impending perceptions or actions, we took advantage of prior observations that specific frequencies of local field potentials reflect the correlated activity of neurons. We found that the strength of the spike-field coherence of a neuron in the gamma-band frequency range is related to the size of its MT-pursuit correlations for eye direction, as well as to the size of the neuron-neuron correlations. Spike-field coherence predicts MT-pursuit correlations better for direction than for speed, perhaps because the topographic organization of direction preference in MT is more amenable to creating meaningful local field potentials. We suggest that the relationship between spiking and local-field potentials is stronger for neurons that have larger correlations with their neighbors; larger neuron-neuron correlations create stronger MT-pursuit correlations. Neurons that lack strong correlations with their neighbors also have weaker correlations with pursuit behavior, but still could drive pursuit strongly.

The development of global motion discrimination in school aged children

Global motion perception matures during childhood and involves the detection of local directional signals that are integrated across space. We examine the maturation of local directional selectivity and global motion integration with an equivalent noise paradigm applied to direction discrimination. One hundred and three observers (6-17 years) identified the global direction of motion in a 2AFC task. The 8° central stimuli consisted of 100 dots of 10% Michelson contrast moving 2.8°/s or 9.8°/s. Local directional selectivity and global sampling efficiency were estimated from direction discrimination thresholds as a function of external directional noise, speed, and age. Direction discrimination thresholds improved gradually until the age of 14 years (linear regression, p < 0.05) for both speeds. This improvement was associated with a gradual increase in sampling efficiency (linear regression, p < 0.05), with no significant change in internal noise. Direction sensitivity was lower for dots moving at 2.8°/s than at 9.8°/s for all ages (paired t test, p < 0.05) and is mainly due to lower sampling efficiency. Global motion perception improves gradually during development and matures by age 14. There was no change in internal noise after the age of 6, suggesting that local direction selectivity is mature by that age. The improvement in global motion perception is underpinned by a steady increase in the efficiency with which direction signals are pooled, suggesting that global motion pooling processes mature for longer and later than local motion processing.

Responses to random dot motion reveal prevalence of pattern-motion selectivity in area MT

How the visual system reconstructs global patterns of motion from components is an important issue in vision. Conventional studies using plaids have shown that approximately one-third of neurons in cortical area MT respond to one-dimensional (1D) components of a moving pattern (component cells), whereas another third responds to the global two-dimensional (2D) motion of a pattern (pattern cells). Conversely, studies using spots of light or random dots that contain multiple orientations have seldom reported directional tuning that is consistent with 1D motion preference. To bridge the gap between these studies, we recorded from isolated neurons in macaque area MT and measured tuning for velocity (direction and speed) using random dot stimuli. We used the "intersection of constraints" principle to classify our population into pattern-direction-selective (PDS) neurons and component-direction-selective (CDS) neurons. We found a larger proportion of PDS cells (68%) and a smaller proportion of CDS cells (8%) compared with prior studies using plaids. We further compared velocity tuning, measured using random dot stimuli, with direction tuning, measured using plaids. Although there was a correlation between the degree of preference for 2D over 1D motion of the two measurements, tuning seemed to prefer 2D motion using random dot stimuli. Modeling analyses suggest that integration across orientations contributes to the 2D motion preference of both dots and plaids, but opponent inhibition mainly contributes to the 2D motion preference of plaids. We conclude that MT neurons become more capable of identifying a particular 2D velocity when stimuli contain multiple orientations.

Joint representation of depth from motion parallax and binocular disparity cues in macaque area MT

Perception of depth is based on a variety of cues, with binocular disparity and motion parallax generally providing more precise depth information than pictorial cues. Much is known about how neurons in visual cortex represent depth from binocular disparity or motion parallax, but little is known about the joint neural representation of these depth cues. We recently described neurons in the middle temporal (MT) area that signal depth sign (near vs far) from motion parallax; here, we examine whether and how these neurons also signal depth from binocular disparity. We find that most MT neurons in rhesus monkeys (Macaca Mulatta) are selective for depth sign based on both disparity and motion parallax cues. However, the depth-sign preferences (near or far) are not always aligned: 56% of MT neurons have matched depth-sign preferences ("congruent" cells) whereas the remaining 44% of neurons prefer near depth from motion parallax and far depth from disparity, or vice versa ("opposite" cells). For congruent cells, depth-sign selectivity increases when disparity cues are added to motion parallax, but this enhancement does not occur for opposite cells. This suggests that congruent cells might contribute to perceptual integration of depth cues. We also found that neurons are clustered in MT according to their depth tuning based on motion parallax, similar to the known clustering of MT neurons for binocular disparity. Together, these findings suggest that area MT is involved in constructing a representation of 3D scene structure that takes advantage of multiple depth cues available to mobile observers.

Processing slow and fast motion in children with autism spectrum conditions

Consistent with the dorsal stream hypothesis, difficulties processing dynamic information have previously been reported in individuals with autism spectrum conditions (ASC). However, no research has systematically compared motion processing abilities for slow and fast speeds. Here, we measured speed discrimination thresholds and motion coherence thresholds in slow (1.5 deg/sec) and fast (6 deg/sec) speed conditions in children with an ASC aged 7 to 14 years, and age- and ability-matched typically developing children. Unexpectedly, children with ASC were as sensitive as typically developing children to differences in speed at both slow and fast reference speeds. Yet, elevated motion coherence thresholds were found in children with ASC, but in the slow stimulus speed condition only. Rather than having pervasive difficulties in motion processing, as predicted by the dorsal stream hypothesis, these results suggest that children with ASC have a selective difficulty in extracting coherent motion information specifically at slow speeds. Understanding the effects of stimulus parameters such as stimulus speed will be important for resolving discrepancies between previous studies examining motion coherence thresholds in ASC and also for refining theoretical models of altered autistic perception.

Common rules guide comparisons of speed and direction of motion in the dorsolateral prefrontal cortex

When a monkey needs to decide whether motion direction of one stimulus is the same or different as that of another held in working memory, neurons in dorsolateral prefrontal cortex (DLPFC) faithfully represent the motion directions being evaluated and contribute to their comparison. Here, we examined whether DLPFC neurons are more generally involved in other types of sensory comparisons. Such involvement would support the existence of generalized sensory comparison mechanisms within DLPFC, shedding light on top-down influences this region is likely to provide to the upstream sensory neurons during comparison tasks. We recorded activity of individual neurons in the DLPFC while monkeys performed a memory-guided decision task in which the important dimension was the speed of two sequentially presented moving random-dot stimuli. We found that many neurons, both narrow-spiking putative local interneurons and broad-spiking putative pyramidal output cells, were speed-selective, with tuning reminiscent of that observed in the motion processing middle temporal (MT) cortical area. Throughout the delay, broad-spiking neurons were more active, showing anticipatory rate modulation and transient periods of speed selectivity. During the comparison stimulus, responses of both cell types were modulated by the speed of the first stimulus, and their activity was highly predictive of the animals' behavioral report. These results are similar to those found for comparisons of motion direction, suggesting the existence of generalized neural mechanisms in the DLPFC subserving the comparison of sensory signals.

Neural correlates of induced motion perception in the human brain

A physically stationary stimulus surrounded by a moving stimulus appears to move in the opposite direction. There are similarities between the characteristics of this phenomenon of induced motion and surround suppression of directionally selective neurons in the brain. Here, functional magnetic resonance imaging was used to investigate the link between the subjective perception of induced motion and cortical activity. The visual stimuli consisted of a central drifting sinusoid surrounded by a moving random-dot pattern. The change in cortical activity in response to changes in speed and direction of the central stimulus was measured. The human cortical area hMT+ showed the greatest activation when the central stimulus moved at a fast speed in the direction opposite to that of the surround. More importantly, the activity in this area was the lowest when the central stimulus moved in the same direction as the surround and at a speed such that the central stimulus appeared to be stationary. The results indicate that the activity in hMT+ is related to perceived speed modulated by induced motion rather than to physical speed or a kinetic boundary. Early visual areas (V1, V2, V3, and V3A) showed a similar pattern; however, the relationship to perceived speed was not as clear as that in hMT+. These results suggest that hMT+ may be a neural correlate of induced motion perception and play an important role in contrasting motion signals in relation to their surrounding context and adaptively modulating our motion perception depending on the spatial context.

Independent effects of adaptation and attention on perceived speed

Adaptation and attention are two mechanisms by which sensory systems manage limited bioenergetic resources: Whereas adaptation decreases sensitivity to stimuli just encountered, attention increases sensitivity to behaviorally relevant stimuli. In the visual system, these changes in sensitivity are accompanied by a change in the appearance of different stimulus dimensions, such as speed. Adaptation causes an underestimation of speed, whereas attention leads to an overestimation of speed. In the two experiments reported here, we investigated whether the effects of these mechanisms interact and how they affect the appearance of stimulus features. We tested the effects of adaptation and the subsequent allocation of attention on perceived speed. A quickly moving adaptor decreased the perceived speed of subsequent stimuli, whereas a slow adaptor did not alter perceived speed. Attention increased perceived speed regardless of the adaptation effect, which indicates that adaptation and attention affect perceived speed independently. Moreover, the finding that attention can alter perceived speed after adaptation indicates that adaptation is not merely a by-product of neuronal fatigue.

The development of speed discrimination abilities

The processing of speed is a critical part of a child's visual development, allowing children to track and interact with moving objects. Despite such importance, no study has investigated the developmental trajectory of speed discrimination abilities or precisely when these abilities become adult-like. Here, we measured speed discrimination thresholds in 5-, 7-, 9-, 11-year-olds and adults using random dot stimuli with two different reference speeds (slow: 1.5 deg/s; fast: 6 deg/s). Sensitivity for both reference speeds improved exponentially with age and, at all ages, participants were more sensitive to the faster reference speed. However, sensitivity to slow speeds followed a more protracted developmental trajectory than that for faster speeds. Furthermore, sensitivity to the faster reference speed reached adult-like levels by 11 years, whereas sensitivity to the slower reference speed was not yet adult-like by this age. Different developmental trajectories may reflect distinct systems for processing fast and slow speeds. The reasonably late development of speed processing abilities may be due to inherent limits in the integration of neuronal responses in motion-sensitive areas in early childhood.

Functional organization of envelope-responsive neurons in early visual cortex: organization of carrier tuning properties

It is well established that visual cortex neurons having similar selectivity for orientation, direction of motion, ocular dominance, and other properties of first-order (luminance-defined) stimuli are clustered into a columnar organization. However, the cortical architecture of neuronal responses to second-order (contrast/texture-defined) stimuli is poorly understood. A useful second-order stimulus is a contrast envelope, consisting of a finely detailed pattern (carrier) whose contrast varies on a coarse spatial scale (envelope). In this study, we analyzed the cortical organization of carrier tuning properties of neurons, which responded to contrast-modulated stimuli. We examined whether neurons tuned to similar carrier properties are clustered spatially and whether such spatial clusters are arranged in columns. To address these questions, we recorded single-unit activity, multiunit activity, and local field potentials simultaneously from area 18 of anesthetized cats, using single-channel microelectrodes and multielectrode arrays. Our data showed that neurons tuned to similar carrier spatial frequency are distributed in a highly clustered manner; neurons tuned to similar carrier orientation are also significantly clustered. Neurons along linear arrays perpendicular to the brain surface always exhibited similar optimal carrier spatial frequency, indicating a columnar organization. Multi-pronged tetrode recordings indicated that the diameter of these columns is ≥450 μm. Optimal carrier orientation was also significantly clustered but with finer-grain organization and greater scatter. These results indicate a fine anatomical structure of cortical organization of second-order information processing and suggest that there are probably more maps in cat area 18 than previously believed.

Neural correlates of visual motion prediction

Predicting the trajectories of moving objects in our surroundings is important for many life scenarios, such as driving, walking, reaching, hunting and combat. We determined human subjects’ performance and task-related brain activity in a motion trajectory prediction task. The task required spatial and motion working memory as well as the ability to extrapolate motion information in time to predict future object locations. We showed that the neural circuits associated with motion prediction included frontal, parietal and insular cortex, as well as the thalamus and the visual cortex. Interestingly, deactivation of many of these regions seemed to be more closely related to task performance. The differential activity during motion prediction vs. direct observation was also correlated with task performance. The neural networks involved in our visual motion prediction task are significantly different from those that underlie visual motion memory and imagery. Our results set the stage for the examination of the effects of deficiencies in these networks, such as those caused by aging and mental disorders, on visual motion prediction and its consequences on mobility related daily activities.

Frontal eye field neurons assess visual stability across saccades

The image on the retina may move because the eyes move, or because something in the visual scene moves. The brain is not fooled by this ambiguity. Even as we make saccades, we are able to detect whether visual objects remain stable or move. Here we test whether this ability to assess visual stability across saccades is present at the single-neuron level in the frontal eye field (FEF), an area that receives both visual input and information about imminent saccades. Our hypothesis was that neurons in the FEF report whether a visual stimulus remains stable or moves as a saccade is made. Monkeys made saccades in the presence of a visual stimulus outside of the receptive field. In some trials, the stimulus remained stable, but in other trials, it moved during the saccade. In every trial, the stimulus occupied the center of the receptive field after the saccade, thus evoking a reafferent visual response. We found that many FEF neurons signaled, in the strength and timing of their reafferent response, whether the stimulus had remained stable or moved. Reafferent responses were tuned for the amount of stimulus translation, and, in accordance with human psychophysics, tuning was better (more prevalent, stronger, and quicker) for stimuli that moved perpendicular, rather than parallel, to the saccade. Tuning was sometimes present as well for nonspatial transaccadic changes (in color, size, or both). Our results indicate that FEF neurons evaluate visual stability during saccades and may be general purpose detectors of transaccadic visual change.

Contributions of indirect pathways to visual response properties in macaque middle temporal area MT

The primate visual cortex exhibits a remarkable degree of interconnectivity. Each visual area receives an average of 10 to 15 inputs, many of them from cortical areas with overlapping, but not identical, functional properties. In this study, we assessed the functional significance of this anatomical parallelism to the middle temporal area (MT) of the macaque visual cortex. MT receives major feedforward inputs from areas V1, V2, and V3, but little is known about the properties of each of these pathways. We previously demonstrated that reversible inactivation of V2 and V3 causes a disproportionate degradation of tuning for binocular disparity of MT neurons, relative to direction tuning (Ponce et al., 2008). Here we show that MT neurons continued to encode speed and size information during V2/3 inactivation; however, many became significantly less responsive to fast speeds and others showed a modest decrease in surround suppression. These changes resemble previously reported effects of reducing stimulus contrast (Pack et al., 2005; Krekelberg et al., 2006), but we show here that they differ in their temporal dynamics. We find no evidence that the indirect pathways selectively target different functional regions within MT. Overall, our findings suggest that the indirect pathways to MT primarily convey modality-specific information on binocular disparity, but that they also contribute to the processing of stimuli moving at fast speeds.

Misperceptions of speed are accounted for by the responses of neurons in macaque cortical area MT

In humans, the perceived speed of random dot patterns (RDP) moving within small apertures is faster than that of RDPs moving within larger apertures at the same physical speed. To investigate the neural basis of this illusion, we recorded the responses of direction- and speed-selective neurons in the middle temporal area (MT) of macaque monkeys to stimuli varying in size and speed. Our results show that the preferred speed of MT neurons is slower for smaller stimuli. This effect was larger for neurons preferring faster speeds, matching our psychophysical observation in human subjects that the magnitude of the misperception is larger at higher stimulus speeds. Our physiological data indicate that, across a population of speed-tuned neurons in MT, decreasing the size of a stimulus would shift the activity profile to neurons tuned for higher speeds. Modeling a labeled-line readout of this shifted profile, we show an increased apparent speed, in line with the psychophysical observations. This link strengthens the evidence for a causal role of area MT in speed perception. The systematic shift in tuning curves of single neurons with stimulus size might reflect a general mechanism for feature-mismatch illusions in visual perception.

Visual motion processing by neurons in area MT of macaque monkeys with experimental amblyopia

Early experience affects the development of the visual system. Ocular misalignment or unilateral blur often causes amblyopia, a disorder that has become a standard for understanding developmental plasticity. Neurophysiological studies of amblyopia have focused almost entirely on the first stage of cortical processing in striate cortex. Here we provide the first extensive study of how amblyopia affects extrastriate cortex in nonhuman primates. We studied macaque monkeys (Macaca nemestrina) for which we have detailed psychophysical data, directly comparing physiological findings to perceptual capabilities. Because these subjects showed deficits in motion discrimination, we focused on area MT/V5, which plays a central role in motion processing. Most neurons in normal MT respond equally to visual stimuli presented through either eye; most recorded in amblyopes strongly preferred stimulation of the nonamblyopic (fellow) eye. The pooled responses of neurons driven by the amblyopic eye showed reduced sensitivity to coherent motion and preferred higher speeds, in agreement with behavioral measurements. MT neurons were more limited in their capacity to integrate motion information over time than expected from behavioral performance; neurons driven by the amblyopic eye had even shorter integration times than those driven by the fellow eye. We conclude that some, but not all, of the motion sensitivity deficits associated with amblyopia can be explained by abnormal development of MT.

How Motion Signals Are Integrated Across Frequencies: Study on Motion Perception and Ocular Following Responses Using Multiple-Slit Stimuli

Visual motion signals, which are initially extracted in parallel at multiple spatial frequencies, are subsequently integrated into a unified motion percept. Cross-frequency integration plays a crucial role when directional information conflicts across frequencies due to such factors as occlusion. We investigated the human observers' open-loop oculomotor tracking responses (ocular following responses, or OFRs) and the perceived motion direction in an idealized situation of occlusion—multiple-slits viewing (MSV)—in which a moving pattern is visible only through an array of slits. We also tested a more challenging viewing condition, contrast-alternating MSV (CA-MSV), in which the contrast polarity of the moving pattern alternates when it passes the slits. We found that changes in the distribution of the spectral content of the slit stimuli, introduced by variations of both the interval between the slits and the frame rate of the image stream, modulated the OFR and the reported motion direction in a rather complex manner. We show that those complex modulations could be explained by the weighted sum of the motion signal (motion contrast) of each spatiotemporal frequency. The estimated distribution of frequency weights (tuning maps) indicate that the cross-frequency integration of supra-threshold motion signals gives strong weight to low spatial frequency components (<0.25 cpd) for both OFR and motion perception. However, the tuning map estimated with the MSV stimuli were significantly different from those estimated with the CA-MSV (and from those measured in a more direct manner using grating stimuli), suggesting that inter-frequency interactions (e.g., interaction producing speed-dependent tuning) was involved.

Neural activity in the middle temporal area and lateral intraparietal area during endogenously cued shifts of attention

We measured the behavioral time course of endogenously cued attentional shifts while recording from neurons in the middle temporal area (MT) and lateral intraparietal area (LIP) of two macaque monkeys. The monkeys were required to detect a subtle speed change of one of two continuously moving stimuli. The likely location of the speed change was cued throughout each trial but could switch at an unpredictable time. Attention was evident as an improvement in detection ability and reaction time at the cued location, and the focus of attention shifted over a 400 ms period in response to a switch of the cued stimulus. Attention modulated the ongoing neural response in both MT and LIP, and the sign of this modulation also rapidly shifted after a cue switch. Our data provide a framework for understanding the link between the neural and behavioral effects of attention. The responses of single neurons to the test stimulus in MT and LIP were correlated with stimulus detection and reaction time and, at the population level, a spike-rate threshold model was able to account for the effect of attention on detection rate and reaction time. In this view, the time course of the attentional shift can be understood as an interaction between the emerging attentional modulation and the neural response to the test stimulus in LIP. We also present evidence that the threshold model is not wholly explained by sensory (feedforward) information but may also be influenced by cognitive (feedback) processes at the time of stimulus detection.

Aging affects the neural representation of speed in Macaque area MT

Human perception of speed declines with age. Much of the decline is probably mediated by changes in the middle temporal (MT) area, an extrastriate area whose neural activity is linked to the perception of speed. In the present study, we used random-dot patterns to study the effects of aging on speed-tuning curves in cortical area MT of macaque visual cortex. Our results provide evidence for a significant degradation of speed selectivity in MT. Cells in old animals preferred lower speeds than did those in young animals. Response modulation and discriminative capacity for speed in old monkeys were also significantly weaker than those in young ones. Concurrently, MT cells in old monkeys showed increased baseline responses, peak responses and response variability, and these changes were accompanied by decreased signal-to-noise ratios. We also found that speed discrimination thresholds in old animals were higher than in young ones. The foregoing neural changes may mediate the declines in visual motion perception that occur during senescence.

Parallel processing strategies of the primate visual system

Incoming sensory information is sent to the brain along modality-specific channels corresponding to the five senses. Each of these channels further parses the incoming signals into parallel streams to provide a compact, efficient input to the brain. Ultimately, these parallel input signals must be elaborated on and integrated in the cortex to provide a unified and coherent percept. Recent studies in the primate visual cortex have greatly contributed to our understanding of how this goal is accomplished. Multiple strategies including retinal tiling, hierarchical and parallel processing and modularity, defined spatially and by cell type-specific connectivity, are used by the visual system to recover the intricate detail of our visual surroundings.

Visual motion direction is represented in population-level neural response as measured by magnetoencephalography

We investigated whether direction information is represented in the population-level neural response evoked by the visual motion stimulus, as measured by magnetoencephalography. Coherent motions with varied speed, varied direction, and different coherence level were presented using random dot kinematography. Peak latency of responses to motion onset was inversely related to speed in all directions, as previously reported, but no significant effect of direction on latency changes was identified. Mutual information entropy (IE) calculated using four-direction response data increased significantly (>2.14) after motion onset in 41.3% of response data and maximum IE was distributed at approximately 20 ms after peak response latency. When response waveforms showing significant differences (by multivariate discriminant analysis) in distribution of the three waveform parameters (peak amplitude, peak latency, and 75% waveform width) with stimulus directions were analyzed, 87 waveform stimulus directions (80.6%) were correctly estimated using these parameters. Correct estimation rate was unaffected by stimulus speed, but was affected by coherence level, even though both speed and coherence affected response amplitude similarly. Our results indicate that speed and direction of stimulus motion are represented in the distinct properties of a response waveform, suggesting that the human brain processes speed and direction separately, at least in part.

Close similarity between spatiotemporal frequency tunings of human cortical responses and involuntary manual following responses to visual motion

Human brain uses visual motion inputs not only for generating subjective sensation of motion but also for directly guiding involuntary actions. For instance, during arm reaching, a large-field visual motion is quickly and involuntarily transformed into a manual response in the direction of visual motion (manual following response, MFR). Previous attempts to correlate motion-evoked cortical activities, revealed by brain imaging techniques, with conscious motion perception have resulted only in partial success. In contrast, here we show a surprising degree of similarity between the MFR and the population neural activity measured by magnetoencephalography (MEG). We measured the MFR and MEG induced by the same motion onset of a large-field sinusoidal drifting grating with changing the spatiotemporal frequency of the grating. The initial transient phase of these two responses had very similar spatiotemporal tunings. Specifically, both the MEG and MFR amplitudes increased as the spatial frequency was decreased to, at most, 0.05 c/deg, or as the temporal frequency was increased to, at least, 10 Hz. We also found in peak latency a quantitative agreement (approximately 100-150 ms) and correlated changes against spatiotemporal frequency changes between MEG and MFR. In comparison with these two responses, conscious visual motion detection is known to be most sensitive (i.e., have the lowest detection threshold) at higher spatial frequencies and have longer and more variable response latencies. Our results suggest a close relationship between the properties of involuntary motor responses and motion-evoked cortical activity as reflected by the MEG.

Clustering of self-motion selectivity and visual response properties in macaque area MSTd

Neurons in the dorsal subdivision of the medial superior temporal area (MSTd) show directionally selective responses to both visual (optic flow) and vestibular stimuli that correspond to translational or rotational movements of the subject. Previous work has shown that MSTd neurons are clustered within the cortex according to their directional preferences for optic flow, suggesting that there may be a topographic mapping of self-motion vectors in MSTd. If MSTd provides a multisensory representation of self-motion information, then MSTd neurons may also be expected to show clustering according to their directional preferences for vestibular signals, but this has not been tested previously. We have examined clustering of vestibular signals by comparing the tuning of isolated single units (SUs) with the undifferentiated multiunit (MU) activity of several neighboring neurons recorded from the same microelectrode. We find that directional preferences for both translational and rotational vestibular stimuli, like those for optic flow, are clustered within area MSTd. MU activity often shows significant tuning for vestibular stimuli, although this MU selectivity is generally weaker for translation than for rotation. When directional tuning is observed in MU activity, the direction preference generally agrees closely with that of a simultaneously recorded SU. We also examined clustering of visual receptive field properties in MSTd by analyzing receptive field maps obtained using a reverse-correlation technique. We find that both the local directional preferences and overall spatial receptive field profiles are well clustered in MSTd. Overall, our findings have implications for how visual and vestibular signals regarding self-motion may be decoded from populations of MSTd neurons.

Distortions of perceived auditory and visual space following adaptation to motion

Adaptation to visual motion can induce marked distortions of the perceived spatial location of subsequently viewed stationary objects. These positional shifts are direction specific and exhibit tuning for the speed of the adapting stimulus. In this study, we sought to establish whether comparable motion-induced distortions of space can be induced in the auditory domain. Using individually measured head related transfer functions (HRTFs) we created auditory stimuli that moved either leftward or rightward in the horizontal plane. Participants adapted to unidirectional auditory motion presented at a range of speeds and then judged the spatial location of a brief stationary test stimulus. All participants displayed direction-dependent and speed-tuned shifts in perceived auditory position relative to a 'no adaptation' baseline measure. To permit direct comparison between effects in different sensory domains, measurements of visual motion-induced distortions of perceived position were also made using stimuli equated in positional sensitivity for each participant. Both the overall magnitude of the observed positional shifts, and the nature of their tuning with respect to adaptor speed were similar in each case. A third experiment was carried out where participants adapted to visual motion prior to making auditory position judgements. Similar to the previous experiments, shifts in the direction opposite to that of the adapting motion were observed. These results add to a growing body of evidence suggesting that the neural mechanisms that encode visual and auditory motion are more similar than previously thought.

Neuronal correlates of perceptual stability during eye movements

We are usually unaware of retinal image motion resulting from our own movement. For instance, during slow-tracking eye movements the world around us remains perceptually stable despite the retinal image slip induced by the eye movement. It is commonly held that this example of perceptual invariance is achieved by subtracting an internal reference signal, reflecting the eye movement, from the retinal motion signal. If the two cancel each other, visual objects, which do not move, will also be perceived as non-moving. If, however, the reference signal is too small or too large, a false eye movement-induced motion of the external world, the Filehne illusion, will be perceived. We have exploited our ability to manipulate the size of the reference signal in an attempt to identify neurons in the visual cortex of monkeys, influenced by the percept of self-induced visual motion or the reference signal rather than the retinal motion signal. We report here that such 'percept-related' neurons can already be found in the primary visual cortex area, although few in numbers. They become more frequent in areas middle temporal and medial superior temporal in the superior temporal sulcus, and comprise almost 50% of all neurons in area visual posterior sylvian (VPS) in the posterior part of the lateral sulcus. In summary, our findings suggest that our ability to perceive a visual world, which is stable despite self-motion, is based on a neuronal network, which culminates in the VPS located in the lateral sulcus below the classical dorsal stream of visual processing.

Comparing the feature selectivity of the gamma-band of the local field potential and the underlying spiking activity in primate visual cortex

The local field potential (LFP), comprised of low-frequency extra-cellular voltage fluctuations, has been used extensively to study the mechanisms of brain function. In particular, oscillations in the gamma-band (30-90 Hz) are ubiquitous in the cortex of many species during various cognitive processes. Surprisingly little is known about the underlying biophysical processes generating this signal. Here, we examine the relationship of the local field potential to the activity of localized populations of neurons by simultaneously recording spiking activity and LFP from the primary visual cortex (V1) of awake, behaving macaques. The spatial organization of orientation tuning and ocular dominance in this area provides an excellent opportunity to study this question, because orientation tuning is organized at a scale around one order of magnitude finer than the size of ocular dominance columns. While we find a surprisingly weak correlation between the preferred orientation of multi-unit activity and gamma-band LFP recorded on the same tetrode, there is a strong correlation between the ocular preferences of both signals. Given the spatial arrangement of orientation tuning and ocular dominance, this leads us to conclude that the gamma-band of the LFP seems to sample an area considerably larger than orientation columns. Rather, its spatial resolution lies at the scale of ocular dominance columns.

Linking neuronal and behavioral performance in a reaction-time visual detection task

Perceptual decisions are likely to be based on signals that are provided by populations of neurons in early sensory cortical areas. How these neural responses are combined across neurons and over time to mediate behavior is unknown. To study the link between neural responses and perceptual decisions, we recorded the activity of single units (SU) and multiple units (MU) in the primary visual cortex (V1) of monkeys while they performed a reaction-time visual detection task. We then determined how well the target could be detected from these neural signals. We found that, on average, the detection sensitivities supported by SU and MU in V1 are comparable with the detection sensitivity of the monkey even when considering neural responses during brief temporal intervals (median duration, 137 ms) that ended shortly before the monkey's reaction time. However, we observed systematic differences between the overall shape of the neurometric functions and the monkey's psychometric functions. We also examined the quantitative relationship between SU and MU activity and found that MU responses are consistent with the sum of the responses of multiple SU, most of which have low stimulus selectivity. Finally, we found weak but significant trial-to-trial covariations between V1 activity and behavioral choices, demonstrating for the first time that choice probability can be observed at the earliest stages of cortical sensory processing. Together, these results suggest that the activity of a large population of V1 neurons is combined suboptimally by subsequent processing stages to mediate behavioral performance in visual detection tasks.