Integration Of Local Motion Signals Research Articles

Integration of motion information in illusory motion perceived in stationary patterns

Illusory motion is a phenomenon in which stationary images with repeating luminance gradient patterns appear to be moving. In this study, we conducted experiments focusing on illusory motion to verify the hypothetical rule that velocity information, extracted from local luminance patterns, is integrated by summation in visual information processing. This rule is based on the hypothesis of velocity integration, and could estimate perceived velocity of stimulus. The summation rule was evaluated by a psychophysical experiment. Our results showed that the summation rule unbiasedly predicted perceived velocity, suggesting that an algorithm for integrating velocity information in illusory motion is based on the summation rule. These results would contribute to understanding of the spatial integration of local motion signals in visual information processing.

Internal noise measures in coarse and fine motion direction discrimination tasks and the correlation with autism traits.

Motion perception is essential for visual guidance of behavior and is known to be limited by both internal additive noise (i.e., a constant level of random fluctuations in neural activity independent of the stimulus) and motion pooling (global integration of local motion signals across space). People with autism spectrum disorder (ASD) display abnormalities in motion processing, which have been linked to both elevated noise and abnormal pooling. However, to date, the impact of a third limit—induced internal noise (internal noise that scales up with increases in external stimulus noise)—has not been investigated in motion perception of any group. Here, we describe an extension on the double-pass paradigm to quantify additive noise and induced noise in a motion paradigm. We also introduce a new way to experimentally estimate motion pooling. We measured the impact of induced noise on direction discrimination, which we ascribe to fluctuations in decision-related variables. Our results are suggestive of higher internal noise in individuals with high ASD traits only on coarse but not fine motion direction discrimination tasks. However, we report no significant correlations between autism traits and additive noise, induced noise, or motion pooling in either task. We conclude that, under some conditions, the internal noise may be higher in individuals with pronounced ASD traits and that the assessment of induced internal noise is a useful way of exploring decision-related limits on motion perception, irrespective of ASD traits.

Visual Motion Coherence Responses in Human Visual Cortex.

Random dot kinematograms (RDKs) have recently been used to train subjects with cortical scotomas to perform direction of motion discrimination, partially restoring visual motion perception. To study the recovery of visual perception, it is important to understand how visual areas in normal subjects and subjects with cortical scotomas respond to RDK stimuli. Studies in normal subjects have shown that blood oxygen level-dependent (BOLD) responses in human area hV5/MT+ increase monotonically with coherence, in general agreement with electrophysiology studies in primates. However, RDK responses in prior studies were obtained while the subject was performing fixation, not a motion discrimination condition. Furthermore, BOLD responses were gauged against a baseline condition of uniform illumination or static dots, potentially decreasing the specificity of responses for the spatial integration of local motion signals (motion coherence). Here, we revisit this question starting from a baseline RDK condition of no coherence, thereby isolating the component of BOLD response due specifically to the spatial integration of local motion signals. In agreement with prior studies, we found that responses in the area hV5/MT+ of healthy subjects were monotonically increasing when subjects fixated without performing a motion discrimination task. In contrast, when subjects were performing an RDK direction of motion discrimination task, responses in the area hV5/MT+ remained flat, changing minimally, if at all, as a function of motion coherence. A similar pattern of responses was seen in the area hV5/MT+ of subjects with dense cortical scotomas performing direction of motion discrimination for RDKs presented inside the scotoma. Passive RDK presentation within the scotoma elicited no significant hV5/MT+ responses. These observations shed further light on how visual cortex responses behave as a function of motion coherence, helping to prepare the ground for future studies using these methods to study visual system recovery after injury.

Supplemental Material for The False Aperture Problem: Global Motion Perception Without Integration of Local Motion Signals

Supplemental Material for The False Aperture Problem: Global Motion Perception Without Integration of Local Motion Signals

Facilitative integration of local motion signals in the peripheral visual field observed in monkey ocular following responses

To explore how local motion signals in the peripheral visual field are integrated, we recorded ocular following responses (OFRs) to moving vertical gratings extending across a wide horizontal width (1) of various vertical heights, (2) of a thin strip (4° height) at various vertical eccentricities, and (3) of two thin strips separated with various vertical distances (double-grating) in four monkeys. We found that the OFRs became larger and responsive more to lower spatial frequency than to higher as the vertical height of the motion stimuli increased up to 8° or 16°. This result is consistent with the incorporation of visual motion signals originating in the peripheral visual field; the OFRs to thin strips of drifting gratings were tuned for lower spatial frequency as the eccentricity increased. We also found that the OFRs to the double-gratings were quite similar to the sum of the responses to the single gratings in the initial period, but were significantly larger than the linear-sum in the final period. This result suggests that a non-linear mechanism, by which weak local motion signals are integrated into an effective ensemble, underlies the OFRs.

Two forms of directional bias revealed by multistable motion stimuli

Two superimposed sinusoidal gratings drifting in different directions can combine to appear as a single rigidly moving pattern whose motion is unambiguous. Adding a third grating can create a multistable motion stimulus. We superimposed three gratings with evenly-spaced directions of motion, 120 deg apart. Any two of these can combine and be perceived as a plaid, while the third grating is seen in isolation, moving in the opposite direction. Only one plaid is perceived at a time, and the symmetry of the stimulus suggests that each of the three possible percepts should be seen equally often. In reality, some percepts are strongly favored, so we measured and characterized this unexpected perceptual bias. Five subjects fixated the center of a display on which we presented ‘tri-plaid’ stimuli for 500 ms; this was too brief for multistability to become evident within a single trial. Stimuli were placed at one of four randomly chosen locations symmetrically disposed around the center of gaze; after each presentation the observers reported the perceived direction of plaid motion. The triplaids were oriented so that no motion or orientation was closer than 15 deg to a cardinal. Across all locations, our observers reported near-vertical motions more often than near-horizontal, and either cardinal direction more often than oblique. They also had an independent bias to report motion toward fixation from any location. These two forms of bias may have different origins. Bias for motion in cardinal directions could be related to similar biases in the orientation domain (e.g. Andrews & Schluppeck, 2000), but our effects seem to be tied to motion rather than orientation. The centripetal bias might reflect the action of mechanisms tuned to complex global motion patterns associated with optic flow, which could influence the integration of local motion signals.

Non-linear integration of local motion signals underlying ocular following responses in monkeys

Non-linear integration of local motion signals underlying ocular following responses in monkeys

Effects of the orientation of moving objects on the perception of streaming/bouncing motion displays

In this study, we examined the contribution of the orientation of moving objects to perception of a streaming/bouncing motion display. In three experiments, participants reported which of the two types of motion, streaming or bouncing, they perceived. The following independent variables were used: orientation differences between Gabor micropatterns (Gabors) and their path of motion (all the experiments) and the presence/absence of a transient tone (Experiment 1), transient visual flash (Experiment 2), or concurrent secondary task (Experiment 3) at the coincidence of Gabors. The results showed that the events at coincidence generally biased responses toward the perception of bouncing. On the other hand, alignment of Gabors with their motion axes significantly reduced the frequency of bounce perception. The results also indicated that an object whose orientation was parallel to its motion path strengthened the spatiotemporal integration of local motion signals along a straight motion path, resulting in the perception of streaming. We suggest that the effect of collinearity between Gabors and their motion path is relatively free from the effect of attention distraction.

Illusory surfaces affect the integration of local motion signals

A 2IFC paradigm was used to measure speed discrimination thresholds for pairs of Gabor patches. When one of these patches was phenomenally placed over an illusory surface (IS), we observed higher thresholds relative to control conditions without ISs. Additional controls demonstrated that this effect was due to the placement of the patch on a different phenomenal depth plane rather than to the mere presence of an IS. We conclude that (i) ISs can affect the long-range integration of local motion signals, and (ii) long-range motion integration obeys a coplanarity principle.

Intact perception of biological motion in the face of profound spatial deficits: Williams syndrome.

Williams syndrome (WS) is a rare genetic disorder that results in profound spatial cognitive deficits. We examined whether individuals with WS have intact perception of biological motion, which requires global spatial integration of local motion signals into a unitary percept of a human form. Children with WS, normal mental-age-matched children, and normal adults viewed point-light-walker (PLW) displays portraying a human figure walking to the left or right. Children with WS were as good as or better than control children in their ability to judge the walker's direction, even when it was masked with dynamic noise that mimicked the local motion of the PLW lights. These results show that mechanisms underlying the perception of at least some kinds of biological motion are unimpaired in children with WS. They provide the first evidence of selective sparing of a specialized spatial system in individuals with a known genetic impairment.

Dynamic properties of neurons in cortical area MT in alert and anaesthetized macaque monkeys.

In order to see the world with high spatial acuity, an animal must sample the visual image with many detectors that restrict their analyses to extremely small regions of space. The visual cortex must then integrate the information from these localized receptive fields to obtain a more global picture of the surrounding environment. We studied this process in single neurons within the middle temporal visual area (MT) of macaques using stimuli that produced conflicting local and global information about stimulus motion. Neuronal responses in alert animals initially reflected predominantly the ambiguous local motion features, but gradually converged to an unambiguous global representation. When the same animals were anaesthetized, the integration of local motion signals was markedly impaired even though neuronal responses remained vigorous and directional tuning characteristics were intact. Our results suggest that anaesthesia preferentially affects the visual processing responsible for integrating local signals into a global visual representation.

The directional tuning of the barber-pole illusion.

In order to study the integration of local motion signals in the human visual system, we measured directional tuning curves for the barber-pole illusion by varying two crucial aspects of the stimulus layout independently across a wide a range in the same experiment. These were the orientation of the grating presented behind the rectangular aperture and the aspect ratio of the aperture, which in combination determine the relative contributions of local motion signals perpendicular to the gratings and parallel to the aperture borders, respectively. The strength of the illusion, ie the tendency to perceive motion along the major axis of the aperture, obviously depends on the spatial layout of the aperture, but also on grating orientation. Subjects were asked which direction they perceived and how compelling their motion percept was, revealing different strategies of the visual system to deal with the barber-pole stimulus. Some individuals respond strongly to the unambiguous motion information at the boundaries, leading to multistable percepts and multimodal distributions of responses. Others tend to report intermediate directions, apparently being less influenced by the actual boundaries. The general pattern of deviations from the motion direction perpendicular to grating orientation--a decrease with aspect ratio approaching unity (ie square-shaped apertures) and with gratings approaching parallel orientation to the shorter aperture boundary--is discussed in the context of simple phenomenological models of motion integration. The best fit between model predictions and experimental data is found for an interaction between two stimulus parameters: (i) cycle ratio, which is the sine-wave gratings equivalent of the terminator ratio for line gratings, describing the effects from the aperture boundaries, and (ii) the grating orientation, responsible for perpendicular motion components, which describes the influence of motion signals from inside the aperture. This suggests that the most simple cycle (terminator) ratio explanation cannot fully account for the quantitative properties of the barber-pole illusion.

Pursuit affects precision of perceived heading for small viewing apertures

We investigated the interaction between extra-retinal rotation signals and retinal motion signals in heading perception during pursuit eye movement. For limited viewing aperture, the variability in perceived heading strongly depends on the pattern of motion directions. Heading towards a point outside the aperture generates nearly parallel aperture flow. This results in lower precision of perceived heading than heading that renders the radial pattern of flow visible. We ask if the precision is limited by the pattern of flow visible on the retina or that on the screen. During fixation, the two patterns are identical. They are decoupled during pursuit, since pursuit changes radial flow within the aperture on the screen into nearly parallel flow on the retina, and vice versa. The extra-retinal signal is known to reduce systematic errors in the direction of pursuit, thus compensating for the rotational flow during pursuit. We now ask if the extra-retinal signal also affects the precision of heading percepts. It might if at the spatial integration stage the rotational flow has been subtracted out already. A compensation beyond the integration stage, however, cannot undo the change in retinal motion directions so that an effect of pursuit on precision cannot be avoided. We measured the variable and systematic errors in perceived heading during fixation and pursuit for a frontal plane approach, while varying duration, dot lifetime and aperture size. We found precision is effected by pursuit as much as predicted from the pattern of retinal flow, while compensation is significantly greater than zero. This means that the interaction between the extra-retinal signal and visual motion signals takes place after spatial integration of local motion signals. Furthermore, compensation increased significantly with longer duration (0.5–3.0 s), but not with larger aperture size (10–50°). A larger aperture size did increase the eccentricity of perceived heading.

Spatial structure, contrast polarity and motion integration

It has been shown that in the initial stages of motion processing, the ON and OFF pathways stay more or less separated. There is evidence that this distinction between motion signals from opposite contrast polarities remains at least partly intact in the integration stage of local motion information. At the same time, interactions between the two systems are also apparent. Here we constructed stimuli that contained a constant number of moving checks. The checks were either assigned only one contrast polarity, or contrast polarity was distributed across the checks either randomly or evenly. We investigated how the spatial configuration of the moving stimulus affected direction discrimination thresholds for the different polarity distributions. Our results provide new evidence for contrast-sign-specific integration of local motion signals within areas of limited size, and inhibitory interactions between these separate ON and OFF motion sensor pools.

Integration and segregation of local motion signals: the role of contrast polarity.

In the initial stages of visual processing in primates, more or less separated ON and OFF pathways have been shown to exist. There is ample evidence, that this separation includes the initial stages of motion processing. In the present study, experiments were conducted to investigate whether this ON versus OFF distinction persists into the integration stage of local motion information. We constructed stimuli that consisted of clusters of checks with equal contrast polarity, which could be varied in size, and compared them to stimuli with a random polarity distribution. We found that the ON versus OFF distinction remains partly intact, while interactions between the two systems are also apparent. These interactions prove to be highly correlated with the spatial structure of the stimulus. We propose a mechanism of contrast-sign specific integration of local motion signals, after which these separate ON and OFF pools engage in mutually inhibitory interactions.

Paradoxical rest.

Paradoxical absence of motion in a rotating pattern is reported. The effect requires that the motion of the paradoxically stationary figure be underspecified by local motion signals, and that the paradoxically stationary figure be well segregated. This is consistent with proposals that figural segregation affects the integration of local motion signals.

Is global motion really based on spatial integration of local motion signals?

Previous studies have shown that a random-dot kinematogram (RDK) comprising dots, each of which takes a random walk in direction or speed over time, can appear to flow in a single direction. This has been interpreted as evidence for the existence of a co-operative network linking neurons sensitive to different directions/speeds and different spatial locations. We have investigated the possibility that global motion perception in such patterns might simply reflect motion energy detection at a coarse spatial scale (such that many dots fall in the receptive field of one energy detector) without the need to encode local dot motions on a fine spatial scale and then integrate their motions over space. We created random-walk RDKs and then spatially high-pass filtered them to remove low spatial frequencies. Perception of global motion was unimpaired for both direction and speed random walks, showing that the phenomenon is not reliant on low spatial frequencies and must, therefore, involve integration of local motion signals across space, as originally postulated.

Motion perception following lesions of the superior temporal sulcus in the monkey.

We examined the effect of bilateral ibotenic acid lesions, aimed at areas MT/MST in three macaques, on their perception of motion. The medial boundary of the lesions in the three monkeys was near the dorsal end of the STS, but the lesions extended different lengths ventrally along the STS. The lesions extended the shortest distance ventrally monkeys 1 and 2, covering most of MST but possibly sparing a portion of lateral MT. That in monkey 3 damaged all of MT and MST bilaterally and extended through most of FST. All three lesions caused a temporary disruption, followed by at least partial recovery, of most motion thresholds. Permanent effects of the lesions on visual sensitivity were graded with lesion extent. Contrast sensitivity for detecting low-spatial-frequency (1 cycle/degree) drifting gratings over a wide range of drift rates, as well as for identifying their direction of motion, was slightly affected only in monkey 3. Only monkeys 2 and 3 showed a deficit in discriminating stimulus speed, and the size of the loss was two- to fourfold. Discrimination of opposite directions of dot pattern motion, which required integration of local motion signals, was mildly affected in monkeys 2 and 3, and not affected in monkey 1. However, addition of directional noise to this discrimination caused the performance of all monkeys to be permanently disrupted, especially that of monkeys 2 and 3. Finally, direction difference thresholds were elevated by a factor of 2-4 after the lesions in all three monkeys. Many of these deficits were more pronounced during the first 2 months of testing following the lesion. Thus, our results demonstrate that areas within dorsal STS make an important contribution to the performance of various motion perception tasks including the discrimination of small differences in direction and speed, and the perception of global motion in the presence of directional noise. The residual motion perception, even in the monkey with virtually complete removal of areas MT/MST, may suggest either that these tasks are normally mediated in part by cortical areas outside of areas MT and MST, or that the disrupted functions were partially assumed by other cortical areas after lesions.