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Letters to Editor
Nature Neuroscience 1, 335 (1998)
doi:10.1038/1537
Has a new color area been discovered?
S. Zeki, D.J. McKeefry, A. Bartels R.S.J. Frackowiak
Wellcome Department of Cognitive Neurology, Institute of Neurology, University College London, London WC1E 6BT UK
To the editor − A recent paper in Nature Neuroscience1 claims to show "a previously undifferentiated cortical area that we call V8" in the human fusiform gyrus. This claim has given hopes to some2 that the cortical area responsible for the conscious perception of colors in humans has at last been found. However, the Talairach coordinates for this 'new' area "V8" (ref. 1) are identical to those that we had published for V4 (ref. 3). The authors have therefore not found a new area; instead they have rediscovered and tried to rename area V4. Furthermore, their report1 states, in reference to our paper3, that "a prior study also concluded that this human color-selective region included a representation of upper and lower visual fields". How, then, can they state that colors activate "area V8 but not V4" (ref. 1)?
The answer is simple: it hinges on the use of the letter v, enabling one to write of V4 or V4v. To understand how a single letter can lead to such confusion, one has to retrace the history of the subject briefly. In 1995, Sereno and his colleagues, including Roger Tootell, co-author of ref. 1, reported the results of their mapping experiments in human visual cortex4. Many of the areas described had maps similar to ones found earlier in the macaque. Their map of what they supposed to be human V4 was not so straightforward. They distinguished a ventral V4v, located in the fusiform gyrus, from a dorsal V4d, located dorsolaterally, the two separated from each other by a relatively large expanse of cortex. V4v was clearly shown in the diagrams, but not V4d. This separation was unlike the V4 map in the monkey, where the two subdivisions, representing lower and upper visual fields respectively, are continuous with each other5.
This made us suspicious, because the clinical evidence shows that lesions in the fusiform gyrus, where we had located V4 (Refs 3,6) can result in total hemi-achromatopsias7, 8 that include both upper and lower quadrants of the visual hemifield. We therefore undertook a mapping experiment3 and found, unlike the Sereno report4, that both quadrants are mapped within the color center (area V4) in the fusiform gyrus of each hemisphere. Human V4, like monkey V4, therefore contains a complete map of the visual hemifield in each hemisphere. It is this crucial finding that Hadjikhani et al.1 have now confirmed. Not surprisingly, the Talairach coordinates of their 'new' area are identical to those of V4 (plusminus26, -67, -9 for our V4 and plusminus33, -65, -14 for the 'new' color area) but differ significantly from the coordinates of the more posterior V4v, at plusminus32, -87, -16 (Fig. 1).
