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 IV beta, 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 IV alpha, 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.(ABSTRACT TRUNCATED AT 400 WORDS)

The main objective of this study was to examine the role of neural activity in the development of cell layers in the dorsal lateral geniculate nucleus (LGN). We studied this relationship in postnatal tree shrews either by completely blocking retinal ganglion cell activity with TTX or by selectively blocking activity to the developing ON-center LGN layers (1 and 2) with 2-amino-4-phosphonobutyric acid (APB), using unilateral and bilateral eye injections. All manipulations were carried out from birth (P0), when no LGN cell layers are evident, to or past the point when layers are recognizable (i.e., 1-2 weeks). Nissl-stained and cytochrome oxidase (CO)-reacted material was examined for all cases. Our results show that in the absence of activity produced by bilateral TTX injections, interlaminar spaces between cell layers do begin to develop. Retinal afferents, which are segregated at birth, remain segregated, and differential CO staining between matched sets of LGN layers is evident. The normal pace of LGN development, however, is slowed significantly: LGN cells are smaller and interlaminar spaces are narrower than are seen in age-matched controls. Unilateral TTX injections produce similar, but more dramatic and asymmetric, effects on LGN cells, perhaps because cells are at a competitive disadvantage relative to their normally innervated counterparts in cortex. Combining unilateral eye enucleation at P0 with subsequent TTX treatment of the other eye clearly demonstrates that axons from the remaining eye are capable of producing their normal complement of LGN layers. The development of the LGN ON-center layers, 1 and 2, and the interlaminar space between them are more affected by TTX treatment than are the other layers. By contrast, APB eye injections do not selectively affect the development of the ON-center layers, but do result in some slowing of overall LGN development. Taken together, these results suggest that activity of retinal afferents is not essential for initiating interlaminar space formation, but is important for the normal pace of maturation of LGN cell layers.

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In this paper we examine mechanisms that could explain how layers form within the lateral geniculate nucleus (LGN). Analysis of normal LGN development and development following experimental or genetic perturbation, together, suggest that binocular competitive interactions alone cannot account for either the segregation of retinogeniculate axons or subsequent formation of cell layers. Instead, it appears more likely that initial sorting of axons results from an activity-dependent interaction between populations of axons with different identities and differential affinities for postsynaptic LGN targets. Competitive interactions, however, may aid in sorting axons of like type or in refining topography. We also propose that the subsequent steps in LGN layer formation, such as the formation of interlaminar spaces, depend upon a sequence of interactions between retinal axons, extraretinal axons and outgrowth of developing dendrites of LGN cells.

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