Lesions in cat lateral suprasylvian cortex affect the perception of complex motion.

We examined the effects of bilateral ibotenic acid lesions of cat lateral suprasylvian (LS) cortex on motion perception. Cats were tested on tasks requiring integration of local directional signals, precise judgements of direction and extraction of structure-from-motion. All animals showed permanent deficits in integrating local motion signals. These deficits were most pronounced in the presence of directional noise and at larger spatial displacements. In addition, LS lesions produced a 2-fold loss in the accuracy of direction discrimination and large deficits in the perception of structure-from-motion. All of these losses were most severe during the first few weeks of testing following the lesion. These findings demonstrate that LS cortex plays an important role in the processing of stimuli requiring integration of motion information and limits the spatial scale over which such integration can proceed. Partial improvements in performance with time and/or training may be indicative of post-operative plastic changes in neurons outside of LS cortex.

A reduction in the number of directionally selective neurons extends the spatial limit for global motion perception.

Dynamic random-dot targets were used to study neural mechanisms underlying motion perception. Performance of cats with severely reduced numbers of cortical directionally selective neurons (reduced DS) was compared to that of normal animals. We assessed the spatial properties of the residual motion mechanism by measuring direction discriminations at various dot displacements. At small displacements, reduced DS cats' motion integration thresholds for opposite direction discrimination were nearly normal. At larger displacements, their thresholds surpassed those of normal cats and their upper displacement limit (dmax) was increased by 0.35 deg. The accuracy of direction discrimination was reduced at small displacements, but at larger displacements direction difference thresholds of reduced DS cats approached or surpassed those of normals. These data were compared to the performance of humans who showed an extension of dmax for peripherally viewed targets. The data support the hypothesis that expansion in spatial scale of the motion mechanism may contribute to extension of dmax. Additional support for this hypothesis is provided by a modified direction discriminating line-element model. The model also suggests that changes in sampling of motion mechanisms in the reduced DS system may play a role.

Responses of neurons in the parietal and temporal visual pathways during a motion task.

The visual cortex of macaque monkeys has been divided into two functional streams that have been characterized in terms of sensory processing (color/form vs motion) and in terms of behavioral goals (object recognition vs spatial orientation). As a step toward unifying these two views of cortical processing, we compared the behavioral modulation of sensory signals across the two streams in monkeys trained to do a visual short-term memory task. We recorded from individual neurons in areas MT, MST, 7a, and V4 while monkeys performed a delayed match-to-sample task using direction of motion as the matching criterion. This task allowed us to determine if sensory responses were modulated by extraretinal signals related to the direction of the remembered sample. We sorted neuronal responses as a function of the remembered direction and calculated a modulation index, MI = (maximum response--minimum response)/(maximum response + minimum response). In the motion pathway, we found virtually no extraretinal signals in MT (average MI = 0.11 +/- 0.01 SE, 66 cells), but progressively stronger extraretinal signals in later stages, that is, MST (average MI = 0.17 +/- 0.01 SE, 57 cells) and 7a (average MI = 0.23 +/- 0.02 SE, 46 cells). In contrast to MT, strong extraretinal signals for direction matching were found in V4 (average MI = 0.28 +/- 0.02 SE, 94 cells), a relatively early stage of the color/form pathway, even though this pathway is not generally viewed as playing a major role in motion processing. Some cells in V4 were also tested while the animals performed a color matching task. These cells showed memory-related modulation of their response when either color or direction was used as the matching criterion. We conclude that extraretinal signals related to the match-to-sample task may be stronger in the temporal (color/form) pathway than in the parietal (motion) pathway, regardless of the stimulus dimension involved. Furthermore, our results indicate that the temporal pathway is capable of making a significant contribution to motion processing in tasks where motion can be considered as a cue for the identification of object attributes.

Evidence for the persistence of visual guidance information.

An earlier study from our laboratory provided initial support for the hypothesis that information facilitating visual guidance persists in the absence of retinal stimulation. The present study supports and extends this hypothesis with three experiments in which visually occluded subjects positioned a point of light at the location of a previously viewed target and also walked in the direction of a previously viewed path. In both tasks, performance was possible following occlusion, and in all cases, performance slowly and significantly decreased with longer durations of occlusion. This decay in performance was gradual and had a "half-life" of greater than 15 sec. Absolute performance was correlated across tasks. The effect of occlusion on absolute error in the localization performance was relatively stable within individuals over a 3-week period. The biological utility of guidance information persistence is discussed along with implications for space constancy, illusions of motion, and problems of disorientation.