Abstract
Cerebral visual impairment (CVI) is a brain based visual impairment characterized by deficits in visual function and higher order visual perception. Individuals with CVI report feeling overwhelmed in cluttered environments and have difficulties recognizing familiar individuals in a crowd and following moving traffic. We investigated how visual performance in CVI is affected by perceptual load in a dynamic visual environment using eye tracking and fMRI methodologies. We developed a virtual reality based naturalistic visual search task (a school corridor) in participants were required to find and track the principal and three load conditions (low, medium, high) were tested by manipulating the number of people walking through the corridor. In the eye-tracking experiment, success rate, reaction time, and gaze error were compared across CVI (N=7) and controls (N=9). In the fMRI experiment, activations in visual areas within early visual cortex, dorsal and ventral visual streams were compared (CVI: N=4; controls: N=4). Eye tracking revealed that individuals with CVI were more likely to miss the target, took longer to find the target, and had higher gaze errors. Qualitatively, CVI appeared to be more sensitive to increasing task load. In both groups, fMRI results showed comparable response profiles with respect to increasing load within early visual areas (including V1, V2, V3). In contrast, visual areas associated with the dorsal stream (including hMT+, V3a&V3b and IPS subdivisions) showed overall lower responses in CVI and less sensitivity to task load. Ventral visual areas (including hV4, VO1&VO2) showed comparable responses in both groups with respect to load. Interestingly, certain areas including PHC1&PHC2 and V6 showed the opposite profile, with CVI having higher neural responses than controls, but again more sensitivity to increasing task demands. Overall, these results show a complex response profile in higher level visual areas in CVI with respect to a dynamic search task.