Motion Integration for Ocular Pursuit Does Not Hinder Perceptual Segregation of Moving Objects

When confronted with a complex moving stimulus, the brain can integrate local element velocities to obtain a single motion signal, or segregate the elements to maintain awareness of their identities. The integrated motion signal can drive smooth-pursuit eye movements (Heinen and Watamaniuk, 1998), whereas the segregated signal guides attentive tracking of individual elements in multiple-object tracking tasks (MOT; Pylyshyn and Storm, 1988). It is evident that these processes can occur simultaneously, because we can effortlessly pursue ambulating creatures while inspecting disjoint moving features, such as arms and legs, but the underlying mechanism is unknown. Here, we provide evidence that separate neural circuits perform the mathematically opposed operations of integration and segregation, by demonstrating with a dual-task paradigm that the two processes do not share attentional resources. Human observers attentively tracked a subset of target elements composing a small MOT stimulus, while pursuing it ocularly as it translated across a computer display. Integration of the multidot stimulus yielded optimal pursuit. Importantly, performing MOT while pursuing the stimulus did not degrade performance on either task compared with when each was performed alone, indicating that they did not share attention. A control experiment showed that pursuit was not driven by integration of only the nontargets, leaving the MOT targets free for segregation. Nor was a predictive strategy used to pursue the stimulus, because sudden changes in its global velocity were accurately followed. The results suggest that separate neural mechanisms can simultaneously segregate and integrate the same motion signals

Speed discrimination of motion-in-depth using binocular cues

AbstractAlthough it is well known that motion-in-depth can be detected using binocular cues, it is not known whether those cues can be used to judge the speed of an object moving in depth. There are at least t two possible binocular cues that could be used by the visual system to calculate three dimensional (3-D) speed: the rate of change of binocular disparity, or a comparison of the speeds of motion in the two eyes. We tested which of these cues is used to discriminate the speed of motion-in-depth. First, speed discrimination was measured for a dot moving away from the observer in depth (along the z-axis) and for a random dot stereogram in which a central square moved away from the observer in depth. These stimuli contained both disparity and monocular motion cues. Speed discrimination thresholds were as good for 3-D motion as for monocular sideways motion. Second, a dynamic random dot stereogram (in which the random dot pattern was replaced by a new dot pattern every frame) was used to remove consistent monocular cues. 3-D speed discrimination was now very poor, suggesting that the rate of change of disparity is not a good cue for 3-D speed. Finally, we tested whether observers were able to use the monocular motion cue from one eye to perform the speed discrimination task, or whether there had to be a comparison of the two eyes' monocular cues. By adding a small x-axis velocity component (with random direction) to the z-axis motion, it was possible to disrupt the monocular motion signals without altering the speed of the motion in 3-D. This manipulation did not disrupt the observers' performance, suggesting that monocular speed cues were not being used independently but that there was a comparison of monocular motion signals from the two eyes

Poor speed discrimination suggests that there is no specialized speed mechanism for cyclopean motion

AbstractLuminance-defined and stereo-defined (cyclopean) motion share some common properties, suggesting that the two forms of motion may be detected by similar mechanisms. For luminance-defined motion there are at least two levels of processing: direction is detected and then speed is thought to be extracted by a specialized processing mechanism at a higher level. Here, we tested whether there is also a specialized speed processing mechanism for stereo-defined motion. Speed discrimination thresholds were compared for stimuli containing only stereo-defined motion, and stimuli that contained both stereo-defined and luminance-defined motion. When the stimulus contained luminance-defined motion, increment thresholds were around 0.05–0.1. For stereo-defined motion, increment thresholds were never better than 0.3. By careful analysis, it was possible to test what cues were being used to solve the speed discrimination task. Results were consistent with observers responding to distance cues rather than to speed for stereo-defined motion, suggesting that there is no specialized mechanism for processing the speed of stereo-defined motion. Copyright © 1996 Elsevier Science Ltd

Visual search for motion-in-depth:Stereomotion does not 'pop out' from disparity noise

In a visual search task, targets defined by motion or binocular disparity stand out effortlessly from stationary distractors (\u27pop-out\u27), suggesting that target and distractors are processed by different neural mechanisms. The authors used pop-out to explore whether motion directly toward or away from the observer (z-motion) is detected using binocular motion cues. A target moving laterally (x-motion) popped out amid stationary distractors with binocular disparity, but z-motion did not pop out. However, a small x-motion added to the target\u27s z-motion caused it to pop out. The authors suggest that the visual system may not be specifically sensitive to binocular motion differences

Is stereopsis effective in breaking camouflage for moving targets?

Investigated whether stereopsis is less effective in breaking camouflage for moving targets than for static ones. Observers were asked to detect a single dot moving on a straight trajectory amidst identical noise dots in random motion. In the 3-dimensional (3-D) condition, the noise dots filled a cylindrical volume 5.7 cm in height and diameter; the trajectory signal dot moved on an oblique 3-D trajectory through the center of the cylinder. In the 2-dimensional (2-D) control condition, observers viewed one half-image of the 3-D cylinder binocularly. Trajectory detection in the 3-D condition was only slightly better than in the 2-D condition. Stereoscopic tuning for motion detection was also measured with a novel target configuration. As the disparity between the noise planes and the fixation plane was increased, trajectory detection improved, and then declining to the 2-D level at larger disparities. Similar tuning measurements were made for detecting a static pattern. Adding disparity to the noise planes produced a greater improvement in static detection than in motion detection. It is speculated that the temporal characteristics of the stereo system are not well suited for responding to moving targets, with the result that stereo does not greatly enhance motion detection in noise