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Jonathan Winawer from Stanford University
Seeing around the veins: Mapping ventral occipital cortex
Visual field maps in human V1, V2 and V3 are well established and routinely measured. Measuring maps on the ventral cortical surface has proven more difficult. In particular, the structure of the V4 map (hV4) has been unclear. Some groups have claimed that hV4 occupies a contiguous region of ventral cortex adjacent to ventral V3, and that this region represents the entire contralateral visual hemifield (Brewer et al, 2005; Larsson and Heeger, 2006). Others claim that hV4 is split into a ventral (upper field) and dorsal (lower field) portion (Hansen et al, 2007). We sought to clarify this issue using functional magnetic resonance imaging. We used new, model-based receptive field mapping (Dumoulin & Wandell, 2008).
In analyzing the data from individual subjects, we discovered that a significant problem is in the raw measurements. In most hemispheres we can see the effect of the transverse sinus (TS) from inspection of the BOLD data. In many cases, the effect of the TS is to mask the responses in the region of cortex that can decide between these models at the lateral edge of hV4 on the ventral surface. We believe that this artifact explains a considerable part of the variation in maps seen across observers, as well as some of the differences in conclusions drawn by different labs. By taking advantage of individual variability in the alignment of the TS and the hV4 map, we attempt to 'see around the vein'.
I will discuss the relevant methodological issues in the study and present our preliminary conclusions about the hV4 map. I will also discuss the implications of the TS artifact on mapping foveal representations in early visual cortex and object selective areas in more anterior regions of ventral occipital cortex.