Art.-Hadjikhani (R)

235 In Old World primates such as macaque monkeys and humans, visual information about color is processed in anatomically segregated columns, layers, channels or areas. It is important to know to what extent color is processed in separate versus convergent visual information pathways, because the added dimension of color is so rich in visual information. For example, we can discriminate about fifteen hundred different levels of luminance1, whereas we can make several million discriminations if we also consider variations in color2. It is likely that this glut of color information is incorporated into the labeled lines of the neural architecture in some organized way. In macaque monkeys, an anatomical segregation between chromatic-opponent versus achromatic-opponent cells has been reported as early as the lateral geniculate nucleus. Color-specific anatomical segregation has also been described in primary (V1) and secondary (V2) visual cortex. In V1, prominent populations of color-selective cells have been reported in specific layers3–5 and in the cytochromeoxidase blobs4–6, though the latter claim has been disputed7,8. Similar (and equally controversial) claims have been made about the prominence of color-opponent cells in the ‘thin’ stripes in area V2, to which the V1 blobs project (ref. 9,10, but see 11). However, the most prominent controversy about the anatomical segregation of color-selective neurons occurs at a higher level, in cortical area V4. According to different reports, a high percentage of color-selective cells is either present12–15 or absent16 in the largest and best-studied portion of that area, dorsal V4 (V4d). A high percentage of color-selective cells has not been reported in the smaller, ventral subdivision of V4 (V4v). More recent evidence suggests that brain mechanisms critical for color selectivity are located not in macaque V4, but rather in areas anterior to it (ref. 17–19, Vanduffel et al. Soc. Neurosci. Abstr. 23, 845, 1997). This controversy about color selectivity in V4 has now been extended to human visual cortex. Based on human neuroimaging studies, a small patch of color-selective activity near the middle of the collateral sulcus has been named ‘V4’ (ref. 20–22). This choice of name presupposes that (1) an area homologous to macaque V4 exists in humans, (2) V4 is color-selective, and (3) this region in or near the collateral sulcus is the macaque V4 homolog. However, in humans, the location of this color-selective region has not yet been compared with the map of retinotopic areas, to see whether color selectivity is really is in a retinotopically defined human area V4. Furthermore, the degree of color selectivity in macaque V4 is itself controversial17–19. This issue is not just of academic interest. In an intriguing clinical syndrome (‘achromatopsia’), human patients report that the visual world becomes colorless following damage to a cortical region that apparently includes this color-selective area in the collateral sulcus23–25. This suggests that the conscious percept of ‘color’ involves that area, although it is known that physical wavelength-dependent differences are coded throughout prior levels of the visual system as well. If we can define better which area this is in humans, we can learn something about where conscious perceptions of color arise. Accurate localization in humans should also make it possible to study the homologous area in macaques using more incisive (but invasive) classical neurobiological techniques. We have attempted to clarify these issues in humans using functional magnetic resonance imaging (fMRI). Technical details were similar to those described elsewhere26, except that here we manipulated the color content of the visual stimuli. We also used a high-field MRI scanner and other improvements to substantially increase the sensitivity of the retinotopic maps (Methods).