LARGE- AND SMALL-SCALE FUNCTIONAL ORGANIZATION OF VISUAL FIELD REPRESENTATION IN THE HUMAN VISUAL CORTEX

A fundamental characteristic of the human visual cortex is its retinotopic organization. Taking advantage of the systematic association between cortical position and visual field position, many important aspects of visual processing have been revealed by functional brain imaging. We have investigated, visualized, and characterized retinotopic organization using fMRI , in conjunction with several novel methods of analysis. In this chapter, we describe the methodology used and present findings on the basic functional organization of the visual cortex from two interlocking largeand small-scale perspectives. By larger-scale analyses of retinotopic organization, we have been able to delineate hierarchically organized visual areas (Vl, V2, V3, V3A, V3B, V4v, V8, LOc, and MT+) for ten hemispheres and investigated their individual variability in size and location using a probabilistic approach, in which probability maps of the visual areas were created. With smaller-scale analyses of retinotopy, we obtained two basic factors of visual field representation within each area (cortical magnification factor and average receptive field size), and with these factors estimated the cortical point spread of fMRI activity. We found that point spread is nearly constant across eccentricities and • Correspondence to: Dr. Hiroki Yamamoto. Department of Human Coexistence, Graduate School of Human and Environmental Studies, Kyoto University, Yoshida Nihonmatsu-cho, Sakyo-ku, Kyoto 606-8501 , Japan, E-mail: yamamoto@cv.jinkan.kyoto-u.ac.jp, Tel: +81-75-753-2978 ; Fax: +81-75-753-6574 196 Hiraki Yamamoto, Hiroshi Ban, Masaki Fukunaga et a!. increases as one ascends the visual cortical hierarchy. Knowledge of retinotopic organization is important not only in itself; it also provides essential information for analysis and interpretation of functional activity in visual cortex. As representative examples, we present our recent findings on visual functions involving contextual effects. The present findings on the largeand small-scale functional organization of the human visual cortex shed new light on the relationship between functional segregation and cortical processing hierarchy in the visual system.