Functions and interactions of brain areas hMT+ and hV4 in the human visual system

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The objective is to investigate the modularity of the human brain for the visual attributes of color, form and motion perception. Although these fundamental aspects of the visual image appear perceptually integrated, accumulated evidence suggests that they are processed separately within different, specialized areas of the brain. Two key brain areas are thought to be fundamental for the processing of these attributes: hMT+ and hV4. The human MT complex (hMT+) is traditionally considered to be part of the dorsal pathway, despite its more lateral location, and is known to be involved in motion perception, whereas hV4 is part of the ventral pathway and is thought to be involved in the processing of shape and color. Much of what we know about these two areas has been acquired from work done on non-human primates. Here we aim to test hypotheses about their functions in human vision by selectively and temporarily impairing the responses of each using a brain stimulation technique, rTMS. If stimulation of the targeted brain area results in a consistent impairment (or improvement) of a specific visual task, it is likely that the brain area is functionally involved. After localizing the relevant brain areas using fMRI, we determine the effect of brain stimulation of each area on the performance of specific visual tasks. Each task is designed to test a specialized visual function relating to motion, form or color perception. The proposed program aims to provide a comprehensive investigation of the functional roles of areas hMT+ and hV4 in vision, but also to determine the functional overlap and interactions between the areas. The program addresses core questions about the modularity of processing of visual information in the human brain. The proposal is high impact since very little known about the role of hV4 in the human brain. This area has rarely investigated with rTMS, yet the approach opens up important opportunities for understanding its role in human vision. Furthermore, the experiments specifically address the overlap of function between two key areas with the aim of understanding the common functions of the dorsal and ventral pathways rather than simply how visual areas operate as separate modules. This program represents a new direction of research rather than a simple continuation of my previously supported NSERC program. The TMS approach combines the understanding of color, form and motion processing that I have developed in my lab over the past decade from both psychophysical and fMRI experiments into a program of research that directly addresses the function of two key visual areas of the human brain.