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要旨:
Much of human ventral occipital cortex is retinotopically organized, but the specific visual field maps beyond the upper visual field representations of areas V1/2/3 remain to be worked out. One particular point of interest has been a search for a human area homologous to monkey V4.
We compared human and monkey visual areas using fMRI and stimuli designed to measure the eccentric and angular dimensions of retinotopy. Using flattened views of visual cortex, reversals in the 2D retinotopic phase maps were identified to define the locations of visual areas.
Monkey V4 was clearly identifiable. In human, there is a representation of the entire contralateral hemifield adjacent to ventral V3. This map spans 4 cm2 of cortex and includes a foveal representation that is confluent with that of areas V1/2/3. This hemifield does not correspond to V8, whose definition depends on V4 representing a quarter-field (although a large, separate foveal representation can also be seen anterior and lateral to this hemifield).
Due to its position, we propose calling this hemifield representation human V4 (hV4). The homology of this area to macaque V4 is uncertain because the human map does not extend onto the dorsal surface nor does it surround V1, as in the monkey activation maps. Thus, the spatial patterns of human and macaque visual field maps differ beyond area V3.
The full hemifield representation adjacent to V3-ventral represents a clear difference in the organization of human and monkey extrastriate visual areas. It is uncertain whether these differences are simply a matter of location or they represent differences in computational functions as well.
