Motion-Sensitive Neurones in V5/MT Modulate Perceived Spatial Position

Facilitation and inhibition effects of anodal and cathodal tDCS over areas MT+ on the flash-lag effect

The perceived position of a moving object in vision entails an accumulation of neural signals over space and time. Due to neural signal transmission delays, the visual system can not acquire immediate information about the moving object's position. Although physiological and psychophysical studies on the flash-lag effect (FLE), a moving object is perceived ahead of a flash even they are aligned at the same location, have shown that the visual system develops the mechanisms of predicting the object's location to compensate for the neural delays, the neural mechanisms of motion-induced location prediction are not still understood well. Here, we investigated the role of neural activity changes in areas MT+ (specialized for motion processing) and the potential contralateral processing preference of MT+ in modulating the FLE. Using transcranial direct current stimulations (tDCS) over the left and right MT+ between pre-and post-tests of the FLE in different motion directions, we measured the effects of tDCS on the FLE. The results found that anodal and cathodal tDCS enhanced and reduced the FLE with the moving object heading to but not deviating from the side of the brain stimulated, respectively, compared to sham tDCS. These findings suggest a causal role of area MT+ in motion-induced location prediction, which may involve the integration of position information.

Motion-form interaction: Motion and form aftereffects induced by distorted static natural scenes

Spatially varying distortions (SVDs) are common artifacts of spectacles like progressive additional lenses (PALs). To habituate to distortions of PALs, the visual system has to adapt to distortion-induced image alterations, termed skew adaptation. But how this visual adjustment is achieved is largely unknown. This study examines the properties of visual adaptation to distortions of PALs in natural scenes. The visual adaptation in response to altered form and motion features of the natural stimuli were probed in two different psychophysical experiments. Observers were exposed to distortions in natural images, and form and motion aftereffects were tested subsequently in a constant stimuli procedure where subjects were asked to judge the skew, or the motion direction of an according test stimulus. Exposure to skewed natural stimuli induced a shift in perceived undistorted form as well as motion direction, when viewing distorted dynamic natural scenes, and also after exposure to static distorted natural images. Therefore, skew adaptation occurred in form and motion for dynamic visual scenes as well as static images. Thus, specifically in the condition of static skewed images and the test feature of motion direction, cortical interactions between motion-form processing presumably contributed to the adaptation process. In a nutshell, interfeature cortical interactions constituted the adaptation process to distortion of PALs. Thus, comprehensive investigation of adaptation to distortions of PALs would benefit from taking into account content richness of the stimuli to be used, like natural images.

Idiosyncratic perception: a link between acuity, perceived position and apparent size

Perceiving the positions of objects is a prerequisite for most other visual and visuomotor functions, but human perception of object position varies from one individual to the next. The source of these individual differences in perceived position and their perceptual consequences are unknown. Here, we tested whether idiosyncratic biases in the underlying representation of visual space propagate across different levels of visual processing. In Experiment 1, using a position matching task, we found stable, observer-specific compressions and expansions within local regions throughout the visual field. We then measured Vernier acuity (Experiment 2) and perceived size of objects (Experiment 3) across the visual field and found that individualized spatial distortions were closely associated with variations in both visual acuity and apparent object size. Our results reveal idiosyncratic biases in perceived position and size, originating from a heterogeneous spatial resolution that carries across the visual hierarchy.

The edge of awareness: Mask spatial density, but not color, determines optimal temporal frequency for continuous flash suppression

The study of how visual processing functions in the absence of visual awareness has become a major research interest in the vision-science community. One of the main sources of evidence that stimuli that do not reach conscious awareness-and are thus "invisible"-are still processed to some degree by the visual system comes from studies using continuous flash suppression (CFS). Why and how CFS works may provide more general insight into how stimuli access awareness. As spatial and temporal properties of stimuli are major determinants of visual perception, we hypothesized that these properties of the CFS masks would be of significant importance to the achieved suppression depth. In previous studies however, the spatial and temporal properties of the masks themselves have received little study, and masking parameters vary widely across studies, making a metacomparison difficult. To investigate the factors that determine the effectiveness of CFS, we varied both the temporal frequency and the spatial density of Mondrian-style masks. We consistently found the longest suppression duration for a mask temporal frequency of around 6 Hz. In trials using masks with reduced spatial density, suppression was weaker and frequency tuning was less precise. In contrast, removing color reduced mask effectiveness but did not change the pattern of suppression strength as a function of frequency. Overall, this pattern of results stresses the importance of CFS mask parameters and is consistent with the idea that CFS works by disrupting the spatiotemporal mechanisms that underlie conscious access to visual input.

Visual crowding is unaffected by adaptation-induced spatial compression

It has recently been shown that adapting to a densely textured stimulus alters the perception of visual space, such that the distance between two points subsequently presented in the adapted region appears reduced (Hisakata, Nishida, & Johnston, 2016). We asked whether this form of adaptation-induced spatial compression alters visual crowding. To address this question, we first adapted observers to a dynamic dot texture presented within an annular region surrounding the test location. Following adaptation, observers perceived a test array comprised of multiple oriented dot dipoles as spatially compressed, resulting in an overall reduction in perceived size. We then tested to what extent this spatial compression influences crowding by measuring orientation discrimination of a single dipole flanked by randomly oriented dipoles across a range of separations. Following adaptation, we found that the magnitude of crowding was predicted by the physical rather than perceptual separation between center and flanking dipoles. These findings contrast with previous studies in which crowding has been shown to increase when motion-induced position shifts act to reduce apparent separation (Dakin, Greenwood, Carlson, & Bex, 2011; Maus, Fischer, & Whitney, 2011).

Veridical stimulus localization is linked to human area V5/MT+ activity

How the brain represents visual space is an unsolved mystery. Spatial localization becomes particularly challenging when visual information processing is briefly disrupted, as in the case of saccadic eye movements, blinks, or visual masks. As we have recently reported, a compression of visual space, illustrated by displacements of shortly flashed stimuli, can be observed in the temporal vicinity of masking stimuli during ocular fixation (Zimmermann et al., 2013). We here aimed at investigating the neural mechanisms underlying these displacements using functional magnetic resonance imaging. On the behavioral level, we detected significant stimulus displacement when visual masks were simultaneously presented. At the neural level, we observed decreased human motion complex V5/MT+ activation associated with these displacements: When comparing trials with a perceived stimulus shift in space to trials of veridical perception of stimulus localization, human V5/MT+ was significantly less activated although no differences in perceived motion can account for this. Data suggest an important role of human V5/MT+ in the process of spatial localization of briefly presented objects and thus extend current concepts of the functions of human V5/MT+.

Motion-Induced Position Shifts Activate Early Visual Cortex

The ability to correctly determine the position of objects in space is a fundamental task of the visual system. The perceived position of briefly presented static objects can be influenced by nearby moving contours, as demonstrated by various illusions collectively known as motion-induced position shifts. Here we use a stimulus that produces a particularly strong effect of motion on perceived position. We test whether several regions-of-interest (ROIs), at different stages of visual processing, encode the perceived rather than retinotopically veridical position. Specifically, we collect functional MRI data while participants experience motion-induced position shifts and use a multivariate pattern analysis approach to compare the activation patterns evoked by illusory position shifts with those evoked by matched physical shifts. We find that the illusory perceived position is represented at the earliest stages of the visual processing stream, including primary visual cortex. Surprisingly, we found no evidence of percept-based encoding of position in visual areas beyond area V3. This result suggests that while it is likely that higher-level visual areas are involved in position encoding, early visual cortex also plays an important role.

Psychophysical and rTMS Evidence for the Presence of Motion Opponency in Human V5

BACKGROUND

Motion sensitive cells within macaque V5, but not V1, exhibit motion opponency whereby their firing is suppressed by motion in their anti-preferred direction. fMRI studies indicate the presence of motion opponent mechanisms in human V5.

OBJECTIVE/HYPOTHESIS

We tested two hypotheses. 1) Performance of a motion discrimination task would be poorer when stimuli were constructed from pairs of dots that moved in counter-phase vs. in-phase, because counter-phase dots would activate motion opponent mechanisms in V5. 2) Offline 1 Hz rTMS of V5 would impair discrimination performance for in-phase stimuli but not counter-phase stimuli, and the opposite effect would be found for rTMS of V1.

METHODS

Stimuli were constructed from 100 dot pairs. Paired dots moved along a fixed motion axis either in counter-phase (motion opponent stimulus) or in-phase (non-opponent motion stimulus). Motion axis orientation discrimination thresholds were measured for each stimulus. Blocks of 300 trials were then presented at 85% correct threshold and discrimination accuracy was measured before and after 1 Hz offline rTMS of either V1 or V5. Subjects were 8 healthy adults.

RESULTS

Discrimination thresholds were significantly larger (worse) for counter-phase than in-phase stimuli (p = 0.02). V5 rTMS mildly impaired discrimination accuracy for the in-phase dot stimuli (p = 0.02) but not the counter-phase dot stimuli. The opposite effect occurred for V1 rTMS (p = 0.05).

CONCLUSIONS

Opponent motion mechanisms are present within human V5 and activation of these mechanisms impairs motion discrimination. In addition, perception of the motion axis within opponent motion stimuli involves processing within V1.

Visual Transients Reveal the Veridical Position of a Moving Object

The position of a moving object is often mislocalised in the direction of movement. At the input stage of visual processing, the position of a moving object should still be represented veridically, whereas it should become closer to the mislocalised position at a later processing stage responsible for positional judgment. Here, we show that visual transients expose the veridical position of a moving object represented in early visual areas. For example, when a ring is flashed on a moving bar, the part of the bar within the ring is perceived at the veridical position, whereas the part outside the ring is perceived to be ahead of the ring as in the flash-lag effect. Our observations suggest that a filling-in process is triggered at the edges of the flash. This indicates that, in early cortical areas, moving objects are still represented at their veridical positions, and the perceived location is determined by the higher visual areas.

Unifying account of visual motion and position perception

Despite growing evidence for perceptual interactions between motion and position, no unifying framework exists to account for these two key features of our visual experience. We show that percepts of both object position and motion derive from a common object-tracking system--a system that optimally integrates sensory signals with a realistic model of motion dynamics, effectively inferring their generative causes. The object-tracking model provides an excellent fit to both position and motion judgments in simple stimuli. With no changes in model parameters, the same model also accounts for subjects' novel illusory percepts in more complex moving stimuli. The resulting framework is characterized by a strong bidirectional coupling between position and motion estimates and provides a rational, unifying account of a number of motion and position phenomena that are currently thought to arise from independent mechanisms. This includes motion-induced shifts in perceived position, perceptual slow-speed biases, slowing of motions shown in visual periphery, and the well-known curveball illusion. These results reveal that motion perception cannot be isolated from position signals. Even in the simplest displays with no changes in object position, our perception is driven by the output of an object-tracking system that rationally infers different generative causes of motion signals. Taken together, we show that object tracking plays a fundamental role in perception of visual motion and position.

Action can amplify motion-induced illusory displacement

Local motion is known to produce strong illusory displacement in the perceived position of globally static objects. For example, if a dot-cloud or grating drifts to the left within a stationary aperture, the perceived position of the whole aperture will also be shifted to the left. Previously, we used a simple tracking task to demonstrate that active control over the global position of an object did not eliminate this form of illusion. Here, we used a new iPad task to directly compare the magnitude of illusory displacement under active and passive conditions. In the active condition, participants guided a drifting Gabor patch along a virtual slalom course by using the tilt control of an iPad. The task was to position the patch so that it entered each gate at the direct center, and we used the left/right deviations from that point as our dependent measure. In the passive condition, participants watched playback of standardized trajectories along the same course. We systematically varied deviation from midpoint at gate entry, and participants made 2AFC left/right judgments. We fitted cumulative normal functions to individual distributions and extracted the point of subjective equality (PSE) as our dependent measure. To our surprise, the magnitude of displacement was consistently larger under active than under passive conditions. Importantly, control conditions ruled out the possibility that such amplification results from lack of motor control or differences in global trajectories as performance estimates were equivalent in the two conditions in the absence of local motion. Our results suggest that the illusion penetrates multiple levels of the perception-action cycle, indicating that one important direction for the future of perceptual illusions may be to more fully explore their influence during active vision.

Motion-dependent representation of space in area MT+

How is visual space represented in cortical area MT+? At a relatively coarse scale, the organization of MT+ is debated; retinotopic, spatiotopic, or mixed representations have all been proposed. However, none of these representations entirely explain the perceptual localization of objects at a fine spatial scale--a scale relevant for tasks like navigating or manipulating objects. For example, perceived positions of objects are strongly modulated by visual motion; stationary flashes appear shifted in the direction of nearby motion. Does spatial coding in MT+ reflect these shifts in perceived position? We performed an fMRI experiment employing this "flash-drag" effect and found that flashes presented near motion produced patterns of activity similar to physically shifted flashes in the absence of motion. This reveals a motion-dependent change in the neural representation of object position in human MT+, a process that could help compensate for perceptual and motor delays in localizing objects in dynamic scenes.

Motion-form interactions beyond the motion integration level: evidence for interactions between orientation and optic flow signals

Motion and form encoding are closely coupled in the visual system. A number of physiological studies have shown that neurons in the striate and extrastriate cortex (e.g., V1 and MT) are selective for motion direction parallel to their preferred orientation, but some neurons also respond to motion orthogonal to their preferred spatial orientation. Recent psychophysical research (Mather, Pavan, Bellacosa, & Casco, 2012) has demonstrated that the strength of adaptation to two fields of transparently moving dots is modulated by simultaneously presented orientation signals, suggesting that the interaction occurs at the level of motion integrating receptive fields in the extrastriate cortex. In the present psychophysical study, we investigated whether motion-form interactions take place at a higher level of neural processing where optic flow components are extracted. In Experiment 1, we measured the duration of the motion aftereffect (MAE) generated by contracting or expanding dot fields in the presence of either radial (parallel) or concentric (orthogonal) counterphase pedestal gratings. To tap the stage at which optic flow is extracted, we measured the duration of the phantom MAE (Weisstein, Maguire, & Berbaum, 1977) in which we adapted and tested different parts of the visual field, with orientation signals presented either in the adapting (Experiment 2) or nonadapting (Experiments 3 and 4) sectors. Overall, the results showed that motion adaptation is suppressed most by orientation signals orthogonal to optic flow direction, suggesting that motion-form interactions also take place at the global motion level where optic flow is extracted.

The perceived position of moving objects: transcranial magnetic stimulation of area MT+ reduces the flash-lag effect

How does the visual system assign the perceived position of a moving object? This question is surprisingly complex, since sluggish responses of photoreceptors and transmission delays along the visual pathway mean that visual cortex does not have immediate information about a moving object's position. In the flash-lag effect (FLE), a moving object is perceived ahead of an aligned flash. Psychophysical work on this illusion has inspired models for visual localization of moving objects. However, little is known about the underlying neural mechanisms. Here, we investigated the role of neural activity in areas MT+ and V1/V2 in localizing moving objects. Using short trains of repetitive Transcranial Magnetic Stimulation (TMS) or single pulses at different time points, we measured the influence of TMS on the perceived location of a moving object. We found that TMS delivered to MT+ significantly reduced the FLE; single pulse timings revealed a broad temporal tuning with maximum effect for TMS pulses, 200 ms after the flash. Stimulation of V1/V2 did not significantly influence perceived position. Our results demonstrate that area MT+ contributes to the perceptual localization of moving objects and is involved in the integration of position information over a long time window.

Crowding is tuned for perceived (not physical) location

In the peripheral visual field, nearby objects can make one another difficult to recognize (crowding) in a manner that critically depends on their separation. We manipulated the apparent separation of objects using the illusory shifts in perceived location that arise from local motion to determine if crowding depends on physical or perceived location. Flickering Gabor targets displayed between either flickering or drifting flankers were used to (a) quantify the perceived target-flanker separation and (b) measure discrimination of the target orientation or spatial frequency as a function of physical target-flanker separation. Relative to performance with flickering targets, we find that flankers drifting away from the target improve discrimination, while those drifting toward the target degrade it. When plotted as a function of perceived separation across conditions, the data collapse onto a single function indicating that it is perceived and not physical location that determines the magnitude of visual crowding. There was no measurable spatial distortion of the target that could explain the effects. This suggests that crowding operates predominantly in extrastriate visual cortex and not in early visual areas where the response of neurons is retinotopically aligned with the physical position of a stimulus.

Perceived positions determine crowding

Crowding is a fundamental bottleneck in object recognition. In crowding, an object in the periphery becomes unrecognizable when surrounded by clutter or distractor objects. Crowding depends on the positions of target and distractors, both their eccentricity and their relative spacing. In all previous studies, position has been expressed in terms of retinal position. However, in a number of situations retinal and perceived positions can be dissociated. Does retinal or perceived position determine the magnitude of crowding? Here observers performed an orientation judgment on a target Gabor patch surrounded by distractors that drifted toward or away from the target, causing an illusory motion-induced position shift. Distractors in identical physical positions led to worse performance when they drifted towards the target (appearing closer) versus away from the target (appearing further). This difference in crowding corresponded to the difference in perceived positions. Further, the perceptual mislocalization was necessary for the change in crowding, and both the mislocalization and crowding scaled with drift speed. The results show that crowding occurs after perceived positions have been assigned by the visual system. Crowding does not operate in a purely retinal coordinate system; perceived positions need to be taken into account.

Functional inhibition of the human middle temporal cortex affects non-visual motion perception: a repetitive transcranial magnetic stimulation study during tactile speed discrimination

The visual motion-responsive middle temporal complex (hMT+) is activated during tactile and aural motion discrimination in both sighted and congenitally blind individuals, suggesting a supramodal organization of this area. Specifically, non-visual motion processing has been found to activate the more anterior portion of the hMT+. In the present study, repetitive transcranial magnetic stimulation (rTMS) was used to determine whether this more anterior portion of hMT+ truly plays a functional role in tactile motion processing. Sixteen blindfolded, young, healthy volunteers were asked to detect changes in the rotation velocity of a random Braille-like dot pattern by using the index or middle finger of their right hand. rTMS was applied for 600 ms (10 Hz, 110% motor threshold), 200 ms after the stimulus onset with a figure-of-eight coil over either the anterior portion of hMT+ or a midline parieto-occipital site (as a control). Accuracy and reaction times were significantly impaired only when TMS was applied on hMT+, but not on the control area. These results indicate that the recruitment of hMT+ is necessary for tactile motion processing, and thus corroborate the hypothesis of a 'supramodal' functional organization for this sensory motion processing area.

Spatial distortions produced by purely dichoptic-based visual motion

It is known that local, monocular motion (short-range motion) can produce local distortions of visual space. We wanted to know if local monocular motion was both sufficient and necessary for producing motion-based spatial distortions. We used a previously reported dichoptic motion stimulus in which the directional motion signal is not present in either eye's input but is only present after binocular combination. We show that such a stimulus can also induce perceived changes in spatial position. This suggests that local, monocular motion while being sufficient is not necessary for the production of motion-based illusions. It suggests that one source of motion signals responsible for this illusion is from binocular motion mechanisms.

Illusory position shift induced by plaid motion

In the motion-induced position shift (MIPS), the position of a moving pattern tapered by a stationary envelope is perceived to shift in the direction of the motion. It was found that plaid motion also elicited a MIPS in the direction of global motion and this global MIPS could not be predicted by the average of the local MIPSs due to component motions. We also used a pseudo plaid pattern and again observed a global MIPS that could not be predicted by the local MIPSs due to the components of the pseudo plaid pattern. We suggest the possibility that the receptive-field positions of global motion detectors shift in the direction opposite to global motion, resulting in a positional displacement in activation via population coding.

Motion-induced position shifts occur after motion integration

Low-level motion processing in the primate visual system involves two stages. The first stage (in V1) contains specialised motion sensors which respond to local retinal motion, and the second stage (in MT) pools local signals to encode rigid surface motion. Recent psychophysical research shows that motion signals influence the perceived position of an object (motion-induced position shift, MIPS). In the present paper we investigate the role played by the two processing stages in generating MIPS. We compared MIPS induced by single grating components (Gabor patches) to MIPS induced by plaids created by combining pairs of components. If motion signals at the lowest level of motion analysis (V1) influence position assignment, MIPS from plaids should reflect the position shift induced by each component when presented separately. On the other hand, if signals generated in MT (or later) influence perceived position, then MIPS from plaids should be consistent with a motion integration computation on the components. Results showed that MIPS from plaids is larger than the MIPS obtained from individual components, and can be explained by the output of an integration process that combines intersection-of-constraints and vector-sum computations.

The movement of motion-defined contours can bias perceived position

Illusory position shifts induced by motion suggest that motion processing can interfere with perceived position. This may be because accurate position representation is lost during successive visual processing steps. We found that complex motion patterns, which can only be extracted at a global level by pooling and segmenting local motion signals and integrating over time, can influence perceived position. We used motion-defined Gabor patterns containing motion-defined boundaries, which themselves moved over time. This 'motion-defined motion' induced position biases of up to 0.5 degrees , much larger than has been found with luminance-defined motion. The size of the shift correlated with how detectable the motion-defined motion direction was, suggesting that the amount of bias increased with the magnitude of this complex directional signal. However, positional shifts did occur even when participants were not aware of the direction of the motion-defined motion. The size of the perceptual position shift was greatly reduced when the position judgement was made relative to the location of a static luminance-defined square, but not eliminated. These results suggest that motion-induced position shifts are a result of general mechanisms matching dynamic object properties with spatial location.

Contributions of the human temporoparietal junction and MT/V5+ to the timing of interception revealed by transcranial magnetic stimulation

To intercept a fast target at destination, hand movements must be centrally triggered ahead of target arrival to compensate for neuromechanical delays. The role of visual-motion cortical areas is unclear. They likely feed downstream parietofrontal networks with signals reflecting target motion, but do they also contribute internal timing signals to trigger the motor response? We disrupted the activity of human temporoparietal junction (TPJ) and middle temporal area (hMT/V5+) by means of transcranial magnetic stimulation (TMS) while subjects pressed a button to intercept targets accelerated or decelerated in the vertical or horizontal direction. Target speed was randomized, making arrival time unpredictable across trials. We used either repetitive TMS (rTMS) before task execution or double-pulse TMS (dpTMS) during target motion. We found that after rTMS and dpTMS at 100-200 ms from motion onset, but not after dpTMS at 300-400 ms, the button-press responses occurred earlier than in the control, with time shifts independent of target speed. This suggests that activity in TPJ and hMT/V5+ can feed downstream regions not only with visual-motion information, but also with internal timing signals used for interception at destination. Moreover, we found that TMS of hMT/V5+ affected interception of all tested motion types, whereas TMS of TPJ significantly affected only interception of motion coherent with natural gravity. TPJ might specifically gate visual-motion information according to an internal model of the effects of gravity.

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.

Distinct position assignment mechanisms revealed by cross-order motion

Motion perception influences perceived position. It has been shown that first-order (luminance defined) motion shifts perceived position across a wide range of spatial and temporal frequencies. On the other hand, second-order (contrast defined) motion shifts perceived position over a narrow range of temporal frequencies, regardless of spatial frequency [Bressler, D. W., & Whitney, D. (2006). Second-order motion shifts perceived position. Vision Research, 46(6-7), 1120-1128]. These results suggest the presence of distinct position assignment mechanisms for first- and second-order motion. We investigated whether the first- and second-order systems independently encode and assign the position of a moving stimulus. To measure motion induced position shift we presented two horizontally offset Gabors placed above and below a central fixation point, with sine wave carriers drifting in opposite directions. Subjects judged the position of the top Gabor relative to the bottom one. We used both first-order Gabors (sinusoidal luminance modulation of a dynamic noise carrier enveloped by a static Gaussian) and second-order Gabors (sinusoidal contrast modulation of a dynamic noise carrier enveloped by a static Gaussian). Results showed a strong position shift in the direction of the carrier motion when both Gabors were first-order, a weak position shift when both Gabors were second-order, and no appreciable position shift when one Gabor was first-order and the other was second-order (cross-order motion). The absence of a position shift using cross-order motion supports the hypothesis that the two motion systems independently encode and assign the position of a moving object. These results are consistent with those of experiments investigating global spatial interactions between static first-order and second-order Gabor patches, indicating a commonality in the underlying spatial integration processes.

Sensory and association cortex in time perception

The recent upsurge of interest in brain mechanisms of time perception is beginning to converge on some new starting points for investigating this long under studied aspect of our experience. In four experiments, we asked whether disruption of normal activity in human MT/V5 would interfere with temporal discrimination. Although clearly associated with both spatial and motion processing, MT/V5 has not yet been implicated in temporal processes. Following predictions from brain imaging studies that have shown the parietal cortex to be important in human time perception, we also asked whether disruption of either the left or right parietal cortex would interfere with time perception preferentially in the auditory or visual domain. The results show that the right posterior parietal cortex is important for timing of auditory and visual stimuli and that MT/V5 is necessary for timing only of visual events.

Flash-lag: Prediction or emergent property of directional selectivity mechanisms?

3D FORMOTION, a unified cortical model of motion integration and segmentation, explains how brain mechanisms of form and motion processing interact to generate coherent percepts of object motion from spatially distributed and ambiguous visual information. The same cortical circuits reproduce motion-induced distortion of position maps, including both flash-lag and flash-drag effects.

Neural activation state determines behavioral susceptibility to modified theta burst transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) allows one to investigate the effects of temporary interference of neural processing in neurologically intact subjects. In a previous study [J. Silvanto et al. (2007) Eur. J. Neurosci., 25, 1874-1881] we found that online TMS perceptually facilitates the attributes encoded by the least active neural populations. The objective of the present experiment was to extend this work to determine whether such state-dependent effects can be observed when offline high-frequency TMS is applied to suppress neural activity. The activity levels of direction-selective neural populations in the V1/V2 region were modulated by asking subjects to passively view either leftward or rightward motion during offline TMS. In a subsequent motion direction-discrimination task, their ability to discriminate motion direction was dependent on the type of motion they had passively viewed during offline TMS: detection of the congruent direction (i.e. direction viewed during offline TMS) was unaffected, whereas detection of the incongruent direction (i.e. opposite direction to the one viewed during offline TMS) was impaired. As the activity level of neurons tuned to the incongruent direction was presumably lower during the TMS than of those tuned to the congruent direction, this behavioral result demonstrates that the offline TMS preferentially suppressed attributes encoded by the least active neural populations. In contrast to direction discrimination, motion detection was not impaired in a direction-specific manner. This shows that the requirements of the psychophysical task, in conjunction with the relative activity states of neuronal populations when TMS is applied, can be used to selectively interfere with overlapping neuronal populations.

Motion and position coding

Motion contained within a static object can cause illusory position shifts toward the direction of internal motion. Here we present data suggesting this illusion is driven by modulations of apparent contrast. We observe position shifts at blurred stimulus regions without corresponding changes to internal structure, and find that low-contrast targets are more difficult to detect at the trailing, as opposed to leading, edges of movement. Motion induced position shifts are also shown to occur without conscious appreciation of motion direction. Our data suggests that motion can influence spatial coding via interactions that modulate apparent contrast, thereby changing the regions of the stimulus that are visible.

Neural adaptation reveals state-dependent effects of transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) is now widely used as a 'virtual' lesion paradigm to investigate behavioural functions, but the mechanisms through which it influences neural processing are unclear. To understand the differential effects of TMS on spatially overlapping populations of neurons we manipulated the relative activity levels of visual neurons by adapting subjects to a range of visual stimuli. By applying TMS to the visual cortex representing the central visual field we have shown in two experiments that the behavioural and perceptual effects of TMS depend on the state of adaptation of the neural population stimulated by TMS. Specifically, we have demonstrated that within the stimulated area TMS perceptually facilitates the attributes encoded by the less active neural population. We have demonstrated the generality of this principle for both suprathreshold and subthreshold TMS as well as for colour and orientation-contingent colour using both subjective reports and psychophsyical measures. These findings can explain how TMS disrupts cognitive functions and therefore have implications for all studies which use TMS to disrupt behaviour.

Visually guided reaching depends on motion area MT+

Visual information is crucial for goal-directed reaching. A number of studies have recently shown that motion in particular is an important source of information for the visuomotor system. For example, when reaching a stationary object, movement of the background can influence the trajectory of the hand, even when the background motion is irrelevant to the object and task. This manual following response may be a compensatory response to changes in body position, but the underlying mechanism remains unclear. Here we tested whether visual motion area MT+ is necessary to generate the manual following response. We found that stimulation of MT+ with transcranial magnetic stimulation significantly reduced a strong manual following response. MT+ is therefore necessary for generating the manual following response, indicating that it plays a crucial role in guiding goal-directed reaching movements by taking into account background motion in scenes.

Apparent position in depth of stationary moving three-dimensional objects

Motion signals contained within a stationary object projected on the fronto-parallel plane shift the object's apparent spatial position in the direction of the motion [see De Valois, R. L., & De Valois, K. K. (1991). Vernier acuity with stationary moving Gabors. Vision Research, 31(9), 1619-1626]. We report an analogous apparent position shift of three-dimensional objects that contain local elements that move in depth. Our stimulus was a transparent three-dimensional cylinder defined by 150 limited-lifetime dots, oriented such that it was end on and its tangent plane was circular. Dots moved in depth by changes in their binocular disparities. In the first experiment, observers judged the positions of the near and far ends of the cylinder, by moving marker lines in depth, for different dot speeds. The results showed that when dots moved towards the observer, the perceived location of the two ends of the cylinder appeared closer in depth. When dots moved away from the observer, the opposite effect was produced. Additionally, the amount of apparent position shift produced was dependent on dot speed, with faster speeds producing larger positional offsets. However, we found in the second experiment that when the cylinder contained randomly moving dots, or when the cylinder contained equal amounts of dots moving towards and away from the observer, positional shifts were very much reduced, or abolished. Our findings suggest that motion signals can induce a misperception of position in depth that is similar manner to that produced by motion within an object in the two-dimensional image plane.

Left frontal eye field remembers “where” but not “what”

Short-term memory of basic stimulus features seems to rely upon low-level functional components of the visual pathways. By using a repetition priming paradigm, we previously showed that visual area V5/MT is important for holding motion direction information, but not spatial position information. Here we extend our previous findings and investigate the possible locus of spatial position priming. We compare the effect of repetitive transcranial magnetic stimulation (rTMS) over right angular gyrus and left and right frontal eye fields on priming for spatial position and motion direction. TMS over left frontal eye field selectively and significantly reduced priming for spatial position but there was no significant effect of TMS over right parietal or right frontal eye field. These results suggest that FEF neurons are implicated in short-term memory storage of spatial position, and extend and support the idea that memory for basic stimulus features is retained within the sensory areas that respond to primary stimulus attributes. They add to a growing body of evidence that the frontal eye fields are involved in many visual functions independent of eye movements.

Spatially asymmetric response to moving patterns in the visual cortex: re-examining the local sign hypothesis

One of the most fundamental functions of the visual system is to code the positions of objects. Most studies, especially those using fMRI, widely assume that the location of the peak retinotopic activity generated in the visual cortex by an object is the position assigned to that object-this is a simplified version of the local sign hypothesis. Here, we employed a novel technique to compare the pattern of responses to moving and stationary objects and found that the local sign hypothesis is false. By spatially correlating populations of voxel responses to different moving and stationary stimuli in different positions, we recovered the modulation transfer function for moving patterns. The results show that the pattern of responses to a moving object is best correlated with the response to a static object that is located behind the moving one. The pattern of responses across the visual cortex was able to distinguish object positions separated by about 0.25 deg visual angle, equivalent to approximately 0.25 mm cortical distance. We also found that the position assigned to a pattern is not simply dictated by the peak activity-the shape of the luminance envelope and the resulting shape of the population response, including the shape and skew in the response at the edges of the pattern, influences where the visual cortex assigns the object's position. Therefore, visually coded position is not conveyed by the peak but by the overall profile of activity.

The segregation and integration of colour in motion processing revealed by motion after-effects

Analysis of the colour and motion of objects is widely believed to take place within segregated processing pathways in the primate visual system. However, it is apparent that this segregation cannot remain absolute and that there must be some capacity for integration across these sub-modalities. In this study, we have assessed the extent to which colour constitutes a separable entity in human motion processing by measuring the chromatic selectivity of two kinds of after-effect resulting from motion adaptation. First, the traditional motion after-effect, where prolonged inspection of a unidirectional moving stimulus results in illusory motion in the opposite direction, was found to exhibit a high degree of chromatic selectivity. The second type of after-effect, in which motion adaptation induces misperceptions in the spatial position of stationary objects, was completely insensitive to chromatic composition. This dissociation between the chromatic selectivities of these after-effects shows that chromatic inputs remain segregated at early stages of motion analysis, while at higher levels of cortical processing there is integration across chromatic, as well as achromatic inputs, to produce a unified perceptual output.

Second-order motion shifts perceived position

Many studies have documented that first-order motion influences perceived position. Here, we show that second-order (contrast defined) motion influences the perceived positions of stationary objects as well. We used a Gabor pattern as our second-order stimulus, which consisted of a drifting sinusoidal contrast modulation of a dynamic random-dot background; this second-order carrier was enveloped by a static Gaussian contrast modulation. Two vertically aligned Gabors had carrier motion in opposite directions. Subjects judged the relative positions of the Gabors' static envelopes. The positions of the Gabors appeared shifted in the direction of the carrier motion, but the effect was narrowly tuned to low temporal frequencies across all tested spatial frequencies. In contrast, first-order (luminance defined) motion shifted perceived positions across a wide range of temporal frequencies, and this differential tuning could not be explained by differences in the visibility of the patterns. The results show that second-order motion detection mechanisms contribute to perceived position. Further, the differential spatial and temporal tuning of the illusion supports the idea that there are distinct position assignment mechanisms for first and second-order motion.

The motion-induced position shift depends on the visual awareness of motion

Visual motion signals distort the perceived positions of briefly presented stimuli; a briefly-flashed, stationary stimulus appears spatially displaced in the direction of a nearby motion. The present study examined the role of the visual awareness of motion in the motion-induced position shift by using exclusive dominance and suppression of binocular rivalry. Observers dichoptically viewed a flickering radial checkerboard and two sinusoidal gratings that drifted vertically in opposite directions. When observers viewed exclusively either the checkerboard or motion stimulus, two horizontal lines were flashed, one for each side of the rivalry stimulus. During the exclusive dominance of the grating motion, the lines appeared to shift in the directions of the nearby motions. The position shift was identical to that during non-rivalry, monocular viewing of the motion stimulus. However, when the grating motions were completely suppressed, no position shift was observed. These results demonstrate that the motion-induced position shift depends on the visual awareness of motion.

Role of primary visual cortex in the binocular integration of plaid motion perception

This study assessed the early mechanisms underlying perception of plaid motion. Thus, two superimposed gratings drifting in a rightward direction composed plaid stimuli whose global motion direction was perceived as the vector sum of the two components. The first experiment was aimed at comparing the perception of plaid motion when both components were presented to both eyes (dioptic) or separately to each eye (dichoptic). When components of the patterns had identical spatial frequencies, coherent motion was correctly perceived under dioptic and dichoptic viewing condition. However, the perceived direction deviated from the predicted direction when spatial frequency differences were introduced between components in both conditions. The results suggest that motion integration follows similar rules for dioptic and dichoptic plaids even though performance under dichoptic viewing did not reach dioptic levels. In the second experiment, the role of early cortical areas in the processing of both plaids was examined. As convergence of monocular inputs is needed for dichoptic perception, we tested the hypothesis that primary visual cortex (V1) is required for dichoptic plaid processing by delivering repetitive transcranial magnetic stimulation to this area. Ten minutes of magnetic stimulation disrupted subsequent dichoptic perception for approximately 15 min, whereas no significant changes were observed for dioptic plaid perception. Taken together, these findings suggest that V1 is not crucial for the processing of dioptic plaids but it is necessary for the binocular integration underlying dichoptic plaid motion perception.

Double Dissociation of V1 and V5/MT activity in Visual Awareness

The critical time windows of the contribution of V1 and V5/MT to visual awareness of moving visual stimuli were compared by administering transcranial magnetic stimulation (TMS) to V1 or V5/MT in various time intervals from stimulus offset during performance of a simple motion detection task. Our results show a double dissociation in which the critical period of V1 both predates and postdates that of V5/MT, and where stimulation of either V1 at V5/MT's critical period or V5/MT at V1's critical period does not impair performance. These findings demonstrate the importance of back-projections from V5/MT to V1 in awareness of real motion stimuli.