Perception of coherent motion, biological motion and form-from-motion under dim-light conditions

Impaired visual recognition of biological motion in schizophrenia

BACKGROUND

Motion perception deficits have been suggested to be an important feature of schizophrenia but the behavioral consequences of such deficits are unknown. Biological motion refers to the movements generated by living beings. The human visual system rapidly and effortlessly detects and extracts socially relevant information from biological motion. A deficit in biological motion perception may have significant consequences for detecting and interpreting social information.

METHODS

Schizophrenia patients and matched healthy controls were tested on two visual tasks: recognition of human activity portrayed in point-light animations (biological motion task) and a perceptual control task involving detection of a grouped figure against the background noise (global-form task). Both tasks required detection of a global form against background noise but only the biological motion task required the extraction of motion-related information.

RESULTS

Schizophrenia patients performed as well as the controls in the global-form task, but were significantly impaired on the biological motion task. In addition, deficits in biological motion perception correlated with impaired social functioning as measured by the Zigler social competence scale [Zigler, E., Levine, J. (1981). Premorbid competence in schizophrenia: what is being measured? Journal of Consulting and Clinical Psychology, 49, 96-105.].

CONCLUSION

The deficit in biological motion processing, which may be related to the previously documented deficit in global motion processing, could contribute to abnormal social functioning in schizophrenia.

Temporal properties in masking biological motion

The perception of biological motion using point light animation techniques was investigated in several experiments. Animations simulating walking were presented with additional masking dots. The temporal properties of the walking motion or the temporal relationship between the walking and masking motions were systematically manipulated. Results showed that (1) perception of biological motion was sensitive to even small temporal perturbation within the walker, (2) the effectiveness of a mask depended upon the temporal phase difference between the mask and point light walker, (3) relatively small temporal differences between the mask and point light walker decreased the effectiveness of the mask, and (4) these effects were not due simply to observers detecting the phase offsets in the display. Temporal properties of the motion are important in perceiving the human form in action, just as in other types of figure-ground segregation. This information may be processed by both motion and form pathways for processing biological motion.

Perceptual limits of common fate

We studied the perception of a coherently moving group of collinearly arranged dots ("target dots") that traveled orthogonally to their linear orientation within a background of noise dots moving in random yet straight directions at constant speed ("random-direction noise"). Using a 2-interval forced-choice task we obtained coherence thresholds equal to a signal-to-noise ratio of 1-2%. These thresholds are lower than the 4-10% reported in the literature suggesting that the collinear arrangement of the target dots, in addition to movement, provided form information. Weber's Law was found to hold 4-7 target dots. Overall, sensitivity was constant for a broad range of dot speeds up to at least 6.5 deg/s. Lifetime required for optimal perception was 430 ms, far shorter than the threshold duration of 1 s reported for randomly distributed (i.e., nonaligned) target dots [Vis. Res. 41 (2001) 1891]. Angular deviations from parallel between adjacent motion trajectories were tolerated up to 27 deg for divergence and up to 19 deg for convergence. Diverging motion was detected earlier (after 600-800 ms) than converging motion (>1 s). Forced-choice discrimination yielded a higher proportion of correct responses than the actual (i.e., conscious) perception of the coherently moving group of dots. Our results are consistent with findings from neurophysiological recordings and neuroimaging of motion-sensitive neurons in areas V1 and MT showing broad tuning curves for speed and direction of a moving visual stimulus.

Texture segregation by motion under low luminance levels

We examined the effect of average luminance level on texture segregation by motion. We determined the minimum presentation duration required for subjects to detect a target defined by motion direction against a moving background. The average luminance level and retinal position of the target were systematically varied. We found that the minimum presentation duration needed for texture segregation depends significantly on the average luminance level and on retinal position. The minimum presentation duration increased as the mean luminance decreased. At a very low (presumably scotopic) luminance level, the motion-defined target was never detected rapidly. Under scotopic conditions, the minimum presentation duration was shorter in the periphery than in a near foveal region when the task was simple detection of the target. When the task included identifying the shape of the target patch, however, the target presented near the fovea was identified faster at all luminance levels. These results suggest that the performance of texture segregation is constrained by the spatiotemporal characteristics of the early visual system.

The integration of auditory and visual motion signals at threshold

To interpret our environment, we integrate information from all our senses. For moving objects, auditory and visual motion signals are correlated and provide information about the speed and the direction of the moving object. We investigated at what level the auditory and the visual modalities interact and whether the human brain integrates only motion signals that are ecologically valid. We found that the sensitivity for identifying motion was improved when motion signals were provided in both modalities. This improvement in sensitivity can be explained by probability summation. That is, auditory and visual stimuli are combined at a decision level, after the stimuli have been processed independently in the auditory and the visual pathways. Furthermore, this integration is direction blind and is not restricted to ecologically valid motion signals.

FMRI responses to video and point-light displays of moving humans and manipulable objects

We used fMRI to study the organization of brain responses to different types of complex visual motion. In a rapid event-related design, subjects viewed video clips of humans performing different whole-body motions, video clips of manmade manipulable objects (tools) moving with their characteristic natural motion, point-light displays of human whole-body motion, and point-light displays of manipulable objects. The lateral temporal cortex showed strong responses to both moving videos and moving point-light displays, supporting the hypothesis that the lateral temporal cortex is the cortical locus for processing complex visual motion. Within the lateral temporal cortex, we observed segregated responses to different types of motion. The superior temporal sulcus (STS) responded strongly to human videos and human point-light displays, while the middle temporal gyrus (MTG) and the inferior temporal sulcus responded strongly to tool videos and tool point-light displays. In the ventral temporal cortex, the lateral fusiform responded more to human videos than to any other stimulus category while the medial fusiform preferred tool videos. The relatively weak responses observed to point-light displays in the ventral temporal cortex suggests that form, color, and texture (present in video but not point-light displays) are the main contributors to ventral temporal activity. In contrast, in the lateral temporal cortex, the MTG responded as strongly to point-light displays as to videos, suggesting that motion is the key determinant of response in the MTG. Whereas the STS responded strongly to point-light displays, it showed an even larger response to video displays, suggesting that the STS integrates form, color, and motion information.

Perception of biological motion in parietal patients

Three unilateral parietal patients were tested on their perception of biological motion, a special case of form-from-motion. Two patients had the lesion in the right, and one in the left parietal area. All patients could easily perform a classical form-from-motion task [Neuron 32 (2001) 985], but they were severely impaired in a visual search task using biological motion sequences. In particular, the left parietal patient showed a more severe loss. He was unable to identify even a single item. Overall our patients seemed to perform differently from the classical motion-blind patients described in the literature [Visual Cognition 3 (1996) 363; Eur. J. Neurol. 9 (2002) 463; Visual Neurosci. 5 (1990) 353] whose lesions included the visual cortical area V5. Since our patients' low-level motion mechanisms are preserved, we suggest that the perception of biological motion relies on a high-level description of dynamic patterns [Cognition 80 (2001) 47], a mechanism that is impaired in parietal lobe patients. We discuss our results at the light of the recent theories suggesting that biological motion is performed by visual associative areas outside the classical motion pathways and that it is an active process dependent on attentional resources [Cognition 80 (2001) 47].

Display of polarization information by coherently moving dots

It is known that human eyes are effectively polarization-blind. Therefore, in order to display the polarization information in an image, one may require exhibiting such information using other visual cues that are compatible with the human visual system and can be easily detectable by a human observer. Here, we present a technique for displaying polarization information in an image using coherently moving dots that are superimposed on the image. Our examples show that this technique would allow the image segments with polarization signals to "pop out" easily, which will lead to better target feature detection and visibility enhancement.

Perception of shape-from-motion in macaque monkeys and humans

Motion is one of the most efficient cues for shape perception. We conducted behavioral experiments to examine how monkeys perceive shapes defined by motion cues and whether they perceive them as humans do. We trained monkeys to perform a shape discrimination task in which shapes were defined by the motion of random dots. Effects of dot density and dot speed on the shape perception of monkeys were examined. Human subjects were also tested using the same paradigm and the test results were compared with those of monkeys. In both monkeys and humans, correct performance rates declined when density or speed of random dots was reduced. Both of them tended to confuse the same combinations of shapes frequently. These results suggest that monkeys and humans perceive shapes defined by motion cues in a similar manner and probably have common neural mechanisms to perceive them.

Visual recognition of biological motion is impaired in children with autism

Autistic children and typically developing control children were tested on two visual tasks, one involving grouping of small line elements into a global figure and the other involving perception of human activity portrayed in point-light animations. Performance of the two groups was equivalent on the figure task, but autistic children were significantly impaired on the biological motion task. This latter deficit may be related to the impaired social skills characteristic of autism, and we speculate that this deficit may implicate abnormalities in brain areas mediating perception of human movement.

Biological motion shown backwards: the apparent-facing effect

We examined how showing a film backwards (reverse transformation) affects the visual perception of biological motion. Adults and 6-year-old children saw first a point-light quadruped moving normally as if on a treadmill, and then saw the same display in reverse transformation. For other groups the order of presentation was the opposite. Irrespective of the presentation mode (normal or reverse) and of the facing of the point-light figure (rightward or leftward), a pronounced apparent-facing effect was observed: the perceptual identification of a display was mainly determined by the apparent direction of locomotion. The findings suggest that in interpreting impoverished point-light biological-motion stimuli the visual system may neglect distortions caused by showing a film backwards. This property appears to be robust across perceptual development. Possible explanations of the apparent-facing effect are discussed.

Spatio-temporal tuning of motion coherence detection at different luminance levels

We studied effects of dark adaptation on spatial and temporal tuning for motion coherence detection. We compared tuning for step size and delay for moving random pixel arrays (RPAs) at two adaptation levels, one light adapted (50 cd/m(2)) and the other relatively dark adapted (0.05 cd/m(2)). To study coherence detection rather than contrast detection, RPAs were scaled for equal contrast detection at each luminance level, and a signal-to-noise ratio paradigm was used in which the RPA is always at a fixed, supra-threshold contrast level. The noise consists of a spatio-temporally incoherent RPA added to the moving RPA on a pixel-by-pixel basis. Spatial and temporal limits for coherence detection were measured using a single step pattern lifetime stimulus, in which patterns on alternate frames make a coherent step and are being refreshed. Therefore, the stimulus contains coherent motion at a single combination of step size and delay only. The main effect of dark adaptation is a large shift in step size, slightly less than the adjustment of spatial scale required for maintaining equal contrast sensitivity. A similar change of preferred step size occurs also for scaled stimuli at a light-adapted level, indicating that the spatial effect is not directly linked to dark adaptation, but more generally related to changes in the available low-level spatial information. Dark-adaptation shifts temporal tuning by about a factor of 2. Long delays are more effective at low luminance levels, whereas short delays no longer support motion coherence detection. Luminance-invariant velocity tuning curves, as reported previously [Lankheet, M.J.M., van Doorn, A.J., Bouman, M.A., & van de Grind, W.A. (2000) Motion coherence detection as a function of luminance in human central vision. Vision Research, 40, 3599-3611], result from recruitment of different sets of motion detectors, and an adjustment of their temporal properties.

Light adaptation in motion direction judgments

We examined the time course of light adaptation in the visual motion system. Subjects judged the direction of a two-frame apparent-motion display, with the two frames separated by a 50-ms interstimulus interval of the same mean luminance. The phase of the first frame was randomly determined on each trial. The grating presented in the second frame was phase shifted either leftward or rightward by pi/2 with respect to the grating in the first frame. At some variable point during the first frame, the mean luminance of the pattern increased or decreased by 1-3 log units. Mean luminance levels varied from scotopic or low mesopic to photopic levels. We found that the perceived direction of motion depended jointly on the luminance level of the first frame grating and the time at which the shift in average luminance occurs. When the average luminance increases from scotopic or mesopic to photopic levels at least 0.5 s before the offset of the first frame, motion in the 3pi/2 direction is perceived. When average luminance decreases to low mesopic or scotopic levels, motion in the pi/2 direction is perceived if the change occurs 1.0 s or more before first frame offset, depending on the size of the luminance step. Thus light adaptation in the visual motion system is essentially complete within 1 s. This suggests a rapid change in the shape (biphasic or monophasic) of the temporal impulse response functions that feed into a first-order motion mechanism.

Velocity discrimination in scotopic vision

To characterize scotopic motion mechanisms, we examined how variation in average luminance affects the ability to discriminate velocity. Stimuli were drifting horizontal sine-wave gratings (0.25, 1.0 and 2.0 c/deg) viewed through a 2 mm artificial pupil and neutral density filters to produce mean adapting levels from 2.5 to -1.5 log photopic trolands. Drift temporal frequency varied from 0.5 to 36.0 Hz. Grating contrasts were either three or five times direction discrimination threshold contrasts at each adaptation level. Following 30 min adaptation, two drifting gratings were presented sequentially at the fovea. Subjects were asked to indicate which interval contained the faster moving stimulus. The Weber fraction for each base temporal frequency was determined using a staircase method. As previously reported, velocity discrimination performance was most acute at temporal frequencies of about 8.0 Hz and greater than 20.0 Hz (though there are individual differences), and fell off at both higher and lower temporal frequencies under photopic conditions. As adaptation level decreased, discrimination of high temporal frequencies in the central retina became increasingly worse, while discrimination of low temporal frequencies remained largely unaltered. The overall scotopic discrimination performance was best at about 3.0 Hz. These results can be explained by a motion mechanism comprising both low-pass and band-pass temporal filters whose peak and temporal cut-off shifts to lower temporal frequencies under scotopic conditions.

Motion perception at scotopic light levels

Although the spatial and temporal properties of rod-mediated vision have been extensively characterized, little is known about scotopic motion perception. To provide such information, we determined thresholds for the detection and identification of the direction of motion of sinusoidal grating patches moving at speeds from 1 to 32 deg/s, under scotopic light levels, in four different types of observers: three normals, a rod monochromat (who lacks all cone vision), an S-cone monochromat (who lacks M- and L-cone vision), and four deuteranopes (who lack M-cone vision). The deuteranopes, whose motion perception does not differ from that of normals, allowed us to measure rod and L-cone thresholds under silent substitution conditions and to compare directly the perceived velocity for moving stimuli detected by either rod or cone vision at the same light level. We find, for rod as for cone vision, that the direction of motion can be reliably identified very near to detection threshold. In contrast, the perceived velocity of rod-mediated stimuli is reduced by approximately 20% relative to cone-mediated stimuli at temporal frequencies below 4 Hz and at all intensity levels investigated (0.92 to -1.12 log cd m(-2)). Most likely, the difference in velocity perception is distal in origin because rod and cone signals converge in the retina and further processing of their combined signals in the visual cortex is presumably identical. To account for the difference, we propose a model of velocity, in which the greater temporal averaging of rod signals in the retina leads to an attenuation of the motion signal in the detectors tuned to high velocities.