Inefficient visual search for second-order motion

Influence of mental workload on motion perception: A direct comparison of luminance-based and contrast-based stimuli

In order to study the impact of increased mental workload on motion detection, twenty-four observers performed a motion discrimination task in which they had to detect odd moving patches. Two types of moving patches were used, namely luminance-based and contrast-based patches. For both types of patches, the motion discrimination task was performed with and without an additional N-Back task aimed at increasing the mental workload. The dual task decreased discrimination performance for both types of patches, but the difference was significantly larger for contrast-based patches, i.e., for second-order motion stimuli, both as an absolute and relative increment. This suggests that motion discrimination requires larger cognitive resources for contrast-based than for luminance-based stimuli, thereby hinting at the higher complexity of the cognitive mechanisms underlying second-order motion detection.

Searching for illusory motion

In a series of four experiments, standard visual search was used to explore whether the onset of illusory motion pre-attentively guides vision in the same way that the onset of real-motion is known to do. Participants searched for target stimuli based on Akiyoshi Kitaoka's classic illusions, configured so that they either did or did not give the subjective impression of illusory motion. Distractor items always contained the same elements as target items, but did not convey a sense of illusory motion. When target items contained illusory motion, they popped-out, with flat search slopes that were independent of set size. Search for control items without illusory motion - but with identical structural differences to distractors - was slow and serial in nature (> 200 ms/item). Using a nulling task, we estimated the speed of illusory rotation in our displays to be approximately 2 °/s. Direct comparison of illusory and real-motion targets moving with matched velocity showed that illusory motion targets were detected more quickly. Blurred target items that conveyed a weak subjective impression of illusory motion gave rise to serial but faster (< 100 ms/item) search than control items. Our behavioral findings of parallel detection across the visual field, together with previous imaging and neurophysiological studies, suggests that relatively early cortical areas play a causal role in the perception of illusory motion. Furthermore, we hope to re-emphasize the way in which visual search can be used as a flexible, objective measure of illusion strength.

Visual Search for Type of Motion is Based on Simple Motion Primitives

Can we search for items based on their type of motion? We consider here visual search based on three types of motion: (i) ballistic motion, in which objects move in a straight line until they encounter a display boundary; (ii) random-walk motion, in which objects change direction randomly; (iii) composite motion, in which objects move with random fluctuations around a generally ballistic trajectory. The asymmetric pattern of search efficiency can be explained by assuming that visual attention is guided by processes sensitive to the presence of linear motion and change in motion. The results do not reveal a more sophisticated ability to segregate items based on the nature of their motion.

Suppressive mechanisms in visual motion processing: From perception to intelligence

Perception operates on an immense amount of incoming information that greatly exceeds the brain's processing capacity. Because of this fundamental limitation, the ability to suppress irrelevant information is a key determinant of perceptual efficiency. Here, I will review a series of studies investigating suppressive mechanisms in visual motion processing, namely perceptual suppression of large, background-like motions. These spatial suppression mechanisms are adaptive, operating only when sensory inputs are sufficiently robust to guarantee visibility. Converging correlational and causal evidence links these behavioral results with inhibitory center-surround mechanisms, namely those in cortical area MT. Spatial suppression is abnormally weak in several special populations, including the elderly and individuals with schizophrenia-a deficit that is evidenced by better-than-normal direction discriminations of large moving stimuli. Theoretical work shows that this abnormal weakening of spatial suppression should result in motion segregation deficits, but direct behavioral support of this hypothesis is lacking. Finally, I will argue that the ability to suppress information is a fundamental neural process that applies not only to perception but also to cognition in general. Supporting this argument, I will discuss recent research that shows individual differences in spatial suppression of motion signals strongly predict individual variations in IQ scores.

Motion dazzle and the effects of target patterning on capture success

BACKGROUND

Stripes and other high contrast patterns found on animals have been hypothesised to cause "motion dazzle", a type of defensive coloration that operates when in motion, causing predators to misjudge the speed and direction of object movement. Several recent studies have found some support for this idea, but little is currently understood about the mechanisms underlying this effect. Using humans as model 'predators' in a touch screen experiment we investigated further the effectiveness of striped targets in preventing capture, and considered how stripes compare to other types of patterning in order to understand what aspects of target patterning are important in making a target difficult to capture.

RESULTS

We find that striped targets are among the most difficult to capture, but that other patterning types are also highly effective at preventing capture in this task. Several target types, including background sampled targets and targets with a 'spot' on were significantly easier to capture than striped targets. We also show differences in capture attempt rates between different target types, but we find no differences in learning rates between target types.

CONCLUSIONS

We conclude that striped targets are effective in preventing capture, but are not uniquely difficult to catch, with luminance matched grey targets also showing a similar capture rate. We show that key factors in making capture easier are a lack of average background luminance matching and having trackable 'features' on the target body. We also find that striped patterns are attempted relatively quickly, despite being difficult to catch. We discuss these findings in relation to the motion dazzle hypothesis and how capture rates may be affected more generally by pattern type.

Multiple object tracking in autism spectrum disorders

Difficulties in visual attention are often implicated in autism spectrum disorders (ASD) but it remains unclear which aspects of attention are affected. Here, we used a multiple object tracking (MOT) task to quantitatively characterize dynamic attentional function in children with ASD aged 5-12. While the ASD group performed significantly worse overall, the group difference did not increase with increased object speed. This finding suggests that decreased MOT performance is not due to deficits in dynamic attention but instead to a diminished capacity to select and maintain attention on multiple targets. Further, MOT performance improved from 5 to 10 years in both typical and ASD groups with similar developmental trajectories. These results argue against a specific deficit in dynamic attention in ASD.

Increasing stimulus size impairs first- but not second-order motion perception

As stimulus size increases, the direction of high-contrast moving stimuli becomes increasingly difficult to perceive. This counterintuitive effect, termed spatial suppression, is believed to reflect antagonistic center-surround interactions--mechanisms that play key roles in tasks requiring sensitivity to relative motion. It is unknown, however, whether second-order motion also exhibits spatial suppression. To test this hypothesis, we measured direction discrimination thresholds for first- and second-order stimuli of varying sizes. The results revealed increasing thresholds with increasing size for first-order stimuli but demonstrated no spatial suppression of second-order motion. This selective impairment of first-order motion predicts increasing predominance of second-order cues as stimulus size increases. We confirmed this prediction by utilizing compound stimuli that contain first- and second-order information moving in opposite directions. Specifically, we found that for large stimuli, motion perception becomes increasingly determined by the direction of second-order cues. Overall, our findings show a lack of spatial suppression for second-order stimuli, suggesting that the second-order system may have distinct functional roles, roles that do not require high sensitivity to relative motion.

Direction-selective patterns of activity in human visual cortex suggest common neural substrates for different types of motion

A sense of motion can be elicited by the movement of both luminance- and texture-defined patterns, what is commonly referred to as first- and second-order, respectively. Although there are differences in the perception of these two classes of motion stimuli, including differences in temporal and spatial sensitivity, it is debated whether common or separate direction-selective mechanisms are responsible for processing these two types of motion. Here, we measured direction-selective responses to luminance- and texture-defined motion in the human visual cortex by using functional MRI (fMRI) in conjunction with multivariate pattern analysis (MVPA). We found evidence of direction selectivity for both types of motion in all early visual areas (V1, V2, V3, V3A, V4, and MT+), implying that none of these early visual areas is specialized for processing a specific type of motion. More importantly, linear classifiers trained with cortical activity patterns to one type of motion (e.g., first-order motion) could reliably classify the direction of motion defined by the other type (e.g., second-order motion). Our results suggest that the direction-selective mechanisms that respond to these two types of motion share similar spatial distributions in the early visual cortex, consistent with the possibility that common mechanisms are responsible for processing both types of motion.

Visual motion mechanisms under low retinal illuminance revealed by motion reversal

The aim of this study is to determine what kinds of motion mechanisms operate at low luminance levels. We used a motion reversal phenomenon in which the perceived direction of motion is reversed when a blank inter-stimulus interval (ISI) frame is inserted between two image frames of similar mean luminance. At low luminance levels, we found that motion reversal was perceived when the moving pattern was presented in the retinal periphery, but no motion reversal was observed when the stimulus was presented in the central retina. When a large stimulus that covers both central and peripheral visual fields was presented, motion reversal did not occur. We conclude that as retinal illuminance decreases, the relative contribution of a feature-tracking mechanism in the central retina becomes larger, while motion perception in the peripheral retina continues to depend on a biphasic, first-order motion mechanism. When both central and peripheral visual fields are stimulated simultaneously, the motion mechanism that dominates in the central retina determines the perceived direction of motion at low luminance levels.

Contrast detection in infants with fragile X syndrome

Studies have reported that a selective deficit in visual motion processing is present in certain developmental disorders, including Williams syndrome and autism. More recent evidence suggests a visual motion impairment is also present in adults with fragile X syndrome (FXS), the most common form of inherited mental retardation. The goal of the current study was to examine low-level cortical visual processing in infants diagnosed with FXS in order to explore the developmental origin of this putative deficit. We measured contrast detection of first-order (luminance-defined) and second-order (contrast-defined) gratings at two levels of temporal frequency, 0 Hz (static) and 4 Hz (moving). Results indicate that infants with FXS display significantly higher detection thresholds only for the second-order, moving stimuli compared to mental age-matched typically developing controls.

Human cortical response to various apparent motions: A magnetoencephalographic study

The human visual system is considered to have at least two different mechanisms for perceiving motions: one for luminance-based (first-order) motions and another for non-luminance-based (second-order) motions. In this study, we examined the perception of first- and second-order motions using four different types of stimulus cues (luminance, contrast, texture, and flicker) while using whole head magnetoencephalography (MEG) to measure human brain responses to those apparent motions. MEG responses to all stimuli were recorded from the occipito-temporal area (possibly human MT/V5+), and response properties (peak latency and amplitude) varied with stimulus cues. Further, we observed various effects of luminance-addition to the non-luminance cues on the response properties that could not be explained by the magnetic field distribution and/or the visibility of the stationary object. The results indicate that differences in response properties elicited by various stimulus cues represent differences in the neural processes underlying apparent motions with various cues. We suggest that the distinct "preprocessing" of each stimulus cue occurs before the common process for apparent motion, and the response property changes associated with different cues are related to differences in preprocessing that may occur in a distributed cortical network that include the striate and extrastriate visual cortex.

Maturation of the sensitivity for luminance and contrast modulated patterns during development of normal and pathological human children

Any object may contain at least two spatio-temporal components referred to as first- and second-order, respectively, defined by spatial-temporal luminance modulation or by contrast, texture or depth modulation. This study investigates form sensitivity of infants, normals, premature or strabismic. A two-alternative forced-choice preferential looking procedure was used in monocular and binocular condition. Maturation profile for both stimuli was similar in the control group. Strabismic infants showed a vertical offset in maturation, which affected the second-order more severely. The pre-term group showed a lag of second-order sensitivity. Our results underline differences between first- and second-order processing.

Second-order motion without awareness: passive adaptation to second-order motion produces a motion aftereffect

Although second-order motion may be detected by early and automatic mechanisms, some models suggest that perceiving second-order motion requires higher-order processes, such as feature or attentive tracking. These types of attentionally mediated mechanisms could explain the motion aftereffect (MAE) perceived in dynamic displays after adapting to second-order motion. Here we tested whether there is a second-order MAE in the absence of attention or awareness. If awareness of motion, mediated by high-level or top-down mechanisms, is necessary for the second-order MAE, then there should be no measurable MAE if the ability to detect directionality is impaired during adaptation. To eliminate the subject's ability to detect directionality of the adapting stimulus, a second-order drifting Gabor was embedded in a dense array of additional crowding Gabors. We found that a significant MAE was perceived even after adaptation to second-order motion in crowded displays that prevented awareness. The results demonstrate that second-order motion can be passively coded in the absence of awareness and without top-down attentional control.

How do we track invisible objects?

We previously demonstrated that observers in multiple object tracking experiments can successfully track targets when all the objects simultaneously vanish for periods lasting several hundred milliseconds (Alvarez, Horowitz, Arsenio, Dimase, and Wolfe, 2005). How do observers do this? Since observers can track objects that move behind occluders (e.g., Scholl and Pylyshyn, 1999), they may treat a temporal gap as a case of complete occlusion. If so, performance should improve if occlusion cues (deletion and accretion) are provided and items disappear and reappear one by one (asynchronously), rather than simultaneously. However, we found better performance with simultaneous than with asynchronous disappearance (Experiment 1), whereas occlusion cues were detrimental (Experiment 2). We propose that observers tolerate a gap in tracking by storing the current task state when objects vanish and resuming tracking on the basis of that memory when the objects reappear (a task-switching account).

Magnetoencephalography: In search of neural processes for visual motion information

Magnetoencephalography (MEG) has become a standard approach to the investigation of human brain functions. This review starts with a brief review of the human visual system and studies on visual motion detection mechanisms is followed by the presentation of MEG studies that have contributed to the field. Emphasis is placed on the fact that because the neural activities measured in functional magnetic resonance imaging (fMRI) differ substantially from those measured in MEG--fMRI data cannot be used directly to estimate MEG signal sources. The basic ideas behind the methods of signal processing and analyses generally used in MEG studies are described and theoretical considerations of the neural mechanisms determining MEG response latency and amplitude changes are discussed. Here, scalar fields theory is proposed to explain MEG responses to incoherent motions, and the ways in which detection of complex motions such as transparency, rotation and expansion can be explained by this theory are also presented. Relationships between human behavioral reaction time and MEG response latency suggest a new concept underlying the reasons why humans are late in detecting slow motion.

FMRI adaptation reveals separate mechanisms for first-order and second-order motion

A key unresolved debate in human vision concerns whether we have two different low-level mechanisms for encoding image motion. Separate neural mechanisms have been suggested for first-order (luminance modulation) and second-order (e.g., contrast modulation) motion in the retinal image but a single mechanism could handle both. Human functional magnetic resonance imaging (fMRI) has not so far convincingly revealed separate anatomical substrates. To examine whether two separate but co-localized mechanisms might exist, we used the technique of fast fMRI adaptation. We found direction-selective adaptation independently for each type of motion in the motion area V5/MT+ of the human brain. However, there was a total absence of cross-adaptation between first-order and second-order motion stimuli. This was true in both of the two subcomponents of MT+ (MT and MST) and similar results were found in V3A. This pattern of adaptation was consistent with psychophysical measurements of detection thresholds in similar stimulus sequences. The results provide strong evidence for separate neural populations that are responsible for detecting first- and second-order motion.

Attention-driven discrete sampling of motion perception

In movies or on TV, a wheel can seem to rotate backwards, due to the temporal subsampling inherent in the recording process (the wagon wheel illusion). Surprisingly, this effect has also been reported under continuous light, suggesting that our visual system, too, might sample motion in discrete "snapshots." Recently, these results and their interpretation have been challenged. Here, we investigate the continuous wagon wheel illusion as a form of bistable percept. We observe a strong temporal frequency dependence: the illusion is maximal at alternation rates around 10 Hz but shows no spatial frequency dependence. We introduce an objective method, based on unbalanced counterphase gratings, for measuring this phenomenon and demonstrate that the effect critically depends on attention: the continuous wagon wheel illusion was almost abolished in the absence of focused attention. A motion-energy model, coupled with attention-dependent temporal subsampling of the perceptual stream at rates between 10 and 20 Hz, can quantitatively account for the observed data.

Poor encoding of position by contrast-defined motion

Second-order (contrast-defined) motion stimuli lead to poor performance on a number of tasks, including discriminating form from motion and visual search. To investigate this deficiency, we tested the ability of human observers to monitor multiple regions for motion, to code the relative positions of shapes defined by motion, and to simultaneously encode motion direction and location. Performance with shapes from contrast-defined motion was compared with that obtained from luminance-defined (first-order) stimuli. When the position of coherent motion was uncertain, direction-discrimination thresholds were elevated similarly for both luminance-defined and contrast-defined motion, compared to when the stimulus location was known. The motion of both luminance- and contrast-defined structure can be monitored in multiple visual field locations. Only under conditions that greatly advantaged contrast-defined motion, were observers able to discriminate the positional offset of shapes defined by either type of motion. When shapes from contrast-defined and luminance-defined motion were presented under comparable conditions, the positional accuracy of contrast-defined motion was found to be poorer than its luminance-defined counterpart. These results may explain some, but possibly not all, of the deficits found previously with second-order motion.

Visual Mechanisms of Motion Analysis and Motion Perception

Psychophysical experiments on feature tracking suggest that most of our sensitivity to chromatic motion and to second-order motion depends on feature tracking. There is no reason to suppose that the visual system contains motion sensors dedicated to the analysis of second-order motion. Current psychophysical and physiological data indicate that local motion sensors are selective for orientation and spatial frequency but they do not eliminate any of the three main models-the Reichardt detector, the motion-energy filter, and gradient-based sensors. Both psychophysical and physiological data suggest that both broadly oriented and narrowly oriented motion sensors are important in the early analysis of motion in two dimensions.

Texture contrast attracts overt visual attention in natural scenes

In natural vision, the central nervous system actively selects information for detailed processing through mechanisms of visual attention. It is widely held that simple stimulus features such as color, orientation and intensity contribute to the determination of visual salience and thus can act to guide the selection process in a bottom-up fashion. Contrary to this view, Einhäuser, W. & König, P. [(2003) Eur. J. Neurosci., 17, 1089-1097] conclude from their study of human eye movements that luminance contrast does not contribute to the calculation of stimulus salience and that top-down, rather than bottom-up, factors therefore determine attentional allocation in natural scenes. In this article, we dispute their conclusion and argue that the Einhäuser and König study has a number of methodological problems, the most prominent of which is the unintentional introduction of changes in texture contrast. We hypothesize that texture contrast, like luminance contrast, can contribute to the guidance of attention in a bottom-up fashion, and that an appeal to top-down factors is not necessary. To test this hypothesis, we implement a purely bottom-up model of visual selective attention where salience is derived from both luminance and texture contrast. We find that the model can quantitatively account for Einhäuser and König's results and that texture contrast strongly influences attentional guidance in this particular paradigm. The significance of this result for attentional guidance in other paradigms is discussed.

Physiological evidence of interaction of first- and second-order motion processes in the human visual system: a magnetoencephalographic study

Humans have several mechanisms for the visual perception of motion, including one that is luminance-based (first-order) and another that is luminance-independent (second-order). Recent psychophysical studies have suggested that significant interaction occurs between these two neural processes. We investigated whether such interactions are represented as neural activity measured by magnetoencephalography (MEG). The second-order motion of a drifting sinusoidal grating, which is defined by the speed of the dot motion, did not generate a response. Apparent motion (AM) of the square area, defined by the speed of randomly moving dots, evoked a magnetic response whose latency and amplitude changed with the distance that the area moved (a second-order characteristic), though the response properties were significantly different from those for the first-order AM. AM, defined by both first- and second-order attributes, evoked an MEG response and the latencies and the amplitudes were distributed between those for the first- and second-order motions. The cortical source of the response was estimated to be around MT+. The results show a distinct difference in the neural processing of the second-order motion that cannot be explained by the difference in visibility, and they indicate that the interaction of the neural processes underlying first- and second-order motion detection occurs before the MEG response. Our study provides the first physiological evidence of a neural interaction between the two types of early motion detection.

How is complex second-order motion processed?

Converging psychophysical and electrophysiological evidence suggests that first-order (luminance-defined) complex motion types i.e., radial and rotational motion, are processed by specialized extrastriate motion mechanisms. We ask whether radial and rotational second-order (texture-defined) motion patterns are processed in a similar manner. The motion sensitivity to translating, radiating and rotating motion patterns of both first-order (luminance-modulated noise) and second-order (contrast-modulated noise) were measured for patterns presented at four different exposure durations (106, 240, 500 and 750 ms). No significant difference in motion sensitivity was found across motion type for the first-order motion class across exposure duration (i.e., from 240 to 750 ms) whereas direction-identification thresholds for radiating and rotating second-order motion were significantly greater than that of the second-order translational stimuli. Furthermore, thresholds to all second-order motion stimuli increased at a significantly faster rate with decreasing exposure duration compared to those of first-order motion. Interestingly, simple and complex second-order thresholds increased at similar rates. Taken together, the results suggest that complex second-order motion is not analyzed in a sequential manner. Rather, it seems that the same 'hard-wired' mechanisms responsible for complex first-order motion processing also mediate complex second-order motion, but not before the pre-processing (i.e., rectification) of local second-order motion signals.

Attentional modulation of threshold sensitivity to first-order motion and second-order motion patterns

Previous studies [e.g. Vision Research 40 (2000) 173] have shown that when observers are required to selectively attend to one of two, spatially-adjacent patches containing either first-order (luminance-defined) or second-order (contrast-defined) motion, threshold sensitivity for identifying the direction of second-order motion, but not first-order motion, is enhanced for the attended stimuli. The processing of second-order motion, unlike first-order motion, may, therefore, require attention. However, other studies have found little evidence for differential effects of attention on the processing of first-order and second-order motion [Investigative Ophthalmology and Visual Science 42(4) (2001) 5061]. We investigated the effects of attention instructions on the ability of observers to identify the directions and spatial orientations of luminance-defined and contrast-defined motion stimuli. Pairs of motion stimuli were presented simultaneously and threshold performance was measured over a wide range of drift temporal frequencies and stimulus durations. We found: (1) direction discrimination thresholds for attended motion stimuli were lower than those for unattended stimuli for both types of motion. The magnitude of this effect was reduced when the observers were not given prior knowledge of which patch of motion (attended or unattended) they had to judge first. (2) Direction discrimination for first-order motion was similarly affected at all temporal frequencies and durations examined, but for second-order motion the effects of attention depended critically on the drift temporal frequency and stimulus duration used. (3) Orientation discrimination showed little or no influence of attention instructions. Thus, whether or not attention influences the processing of second-order motion depends crucially on the precise stimulus parameters tested. Furthermore under appropriate conditions the processing of first-order motion is also influenced by attention, albeit to a lesser extent than second-order motion.

Integration of first- and second-order orientation

The problem of how visual information such as orientation is combined across space bears on key visual abiities, such as texture perception. Orientation signals can be derived from both luminance and contrast, but it is not well understood how such information is pooled or how these different orientation signals interact in the integration process. We measured orientation discrimination thresholds for arrays of equivisible first-order and second-order Gabors. Thresholds were measured as the orientation variability in the arrays increased, and we estimated the number of samples (or efficiency) and internal noise of the mechanism being used. Observers were able to judge the mean orientation of arrays of either first- or second-order Gabors. For arrays of first-order and arrays of second-order Gabors, estimates of the number of samples used increased as the number of Gabors increased. When judging the orientation of arrays of either order, observers were able to ignore randomly oriented Gabors of the opposite order. If observers did not know which Gabor type carried the more useful orientation information, they tended to use the information from first-order Gabors (even when this was poorer information). Observers were unable to combine information from first- and second-order Gabors, though this would have improved their performance. The visual system appears to have separate integrators for combining local orientation across space for luminance- and contrast-defined features.