Mapping of functional organization in human visual cortex

Objectives: To investigate the pattern of functional organization in the human visual cortex through electrical cortical stimulation. Methods: Electrical cortical stimulation was applied to the occipital cortex and adjacent cortices using subdural grid electrodes in 23 epilepsy patients. Diverse visual responses were recorded. These responses were divided into different categories according to the specific response modalities, such as form, color, and motion. Form visual responses were further subdivided into simple, intermediate, and complex responses. The cortical localization of subdural electrodes was identified using MRI-CT coregistration. The cortical distribution of different visual responses was projected into three-dimensional surface renderings of the brain. The distribution and frequency of subdural electrodes showing different visual responses were quantified by calculating the percentage of the number of electrodes showing one specific type of visual response at the corresponding anatomic region to the total number of electrodes in all brain regions that produced the same response. Results: Simple form responses were obtained mostly at the occipital pole and the inferior occipital gyrus (47.4%) and the striate cortex (42.4%). Intermediate form responses occurred mainly on the peristriate cortex (52.5%) and the lateral occipital (28.0%) and fusiform gyri (19.5%). Complex forms were produced by stimulation of the basal temporo-occipital region (57.6%) and the lateral temporal or lateral temporo-occipital junctional region (42.4%). Color responses occurred on the basal occipital area, mostly at the fusiform (40.0%) and lingual gyri (36.0%). Moving sensations were evoked by stimulation of the basal temporo-occipital (28.4%) and the mesial parieto-occipital or temporo-parieto-occipital junctional regions (23.9%). Conclusions: Different modalities of vision, such as form, color, and moving sensation, appeared to be distributed and organized in different areas of the human visual cortex.

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