ORGANIZATION OF CYTOCHROME-OXIDASE STAINING IN THE VISUAL-CORTEX OF NOCTURNAL PRIMATES (GALAGO-CRASSICAUDATUS AND GALAGO-SENEGALENSIS) .1. ADULT PATTERNS

The distribution and differential staining patterns of cytochrome oxidase (CO) activity in visual cortical areas have provided useful anatomical markers for the modular organization of area 17 (striate cortex) and area 18 in primates. In macaque and squirrel monkeys, previous studies have shown that the majority of cells that lie in areas of high CO activity are color selective, are nonoriented, and project to adjacent zones of high CO activity in area 17 and to stripes of high CO activity in area 18. By contrast, most cells in zones with weak CO activity in area 17 have relatively narrow orientation tuning and are not color selective (Livingstone and Hubel:J. Neurosci. 4:309‐356, 2830‐2835, '84; 7:3371‐3377, '87). The periodic organization of CO activity in area 17, the „blobs,” and the stripe‐like organization in area 18 thus seem to define visual cortical processing modules and/or channels in primates. We have investigated the organization of CO activity in areas 17 and 18 in two species of nocturnal prosimian primates[Galago crassicaudatus (GCC) and Galago senegalensis (GSS)] in order to evaluate CO staining patterns in primates that have been reported to possess almost exclusively rod retinae and no color vision. In area 17 of both species, our results show that, as in diurnal and nocturnal simian primates, the darkest CO staining occurs in layers III and IV, with clear periodicity in layer III (i.e., CO blobs) and homogeneous staining in layer IVβ, the cortical recipient sublayer of the geniculate parvocellular layers. In GCC, individual blobs in layer III appear to be larger and less frequent than has been reported for the macaque monkey. Unlike simian primates, both galago species exhibit clear CO periodicities within layer IVα, the cortical recipient sublayer of the magnocellular geniculate layers. In addition, faint CO periodicities are apparent in layer VI and scattered large darkly CO stained pyramidal cells are visible throughout layer V. Quantitative analysis suggests that CO periodicities are more frequent in GSS than in GCC, suggesting that there may be evolutionary pressure to maintain the same number of CO modules within the smaller striate cortex of the lesser galago, although this is not the trend found across distantly related species. CO activity in area 18 is less well‐developed than reported in other primates. In fact, we could not reliably identify discontinuities in CO staining in area 18 of GSS. Instead, where present, discontinuities in CO staining in area 18 of GCC appear as faint regularly spaced patches. Taken together, these results suggest underlying similarities in the basic features of primate cortical organization as revealed by CO staining. There are nonetheless some differences in the distribution of CO activity in visual cortex acros primate species. Thus, given species variation in visual niche requirements including differential dependence on color vision, it seems unlikely that CO blobs have evolved as a strictly color channel in all primates. Copyright © 1990 Wiley‐Liss, Inc.