The patterns of arborisation of the apical dendrites of different varieties of pyramidal neurons in area 17 differ and are characteristic for each cell type. They appear to serve as a means of collating within one neuron information derived directly from several different laminae. These different patterns of apical dendrite arborisation provide dendritic links which relate closely to the laminar distribution of axons of the spiny stellate neurons as well as the pyramidal neurons themselves. The axons of spiny stellate neurons lying in laminae IVCp and IVA (Lund, '73)which receive information from parvocellular geniculate layersproject heavily to the lower half of lamina 111 (IIIB) and to a narrow zone at the top of lamina V (VA); laminae IIIB and VA are in turn linked by a specific variety of pyramidal neuron, with basal dendritic field in lamina VI, whose apical dendrite has marked lateral branching only in laminae VA and IIIB (where it terminates). Pyramidal neurons with basal dendritic field in laminae VA (with vestigial apical dendrite) or in IIIB have recurrent axon projections to lamina IIIA and above (the descending axon projection of lamina IIIB pyramids is principally to lamina VA itself). The pyramidal neurons of laminae IIIA and above have axons which distribute in the same upper laminae as their dendritic fields and a descending axon projection to lamina VB. Pyramidal neurons with basal dendritic field in lamina

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The morphological maturation of several varieties of neurons of cortical area 17 have been followed in Golgi Rapid preparations from Macaque monkeys ranging in age from fetal day 127 to maturity. A developmental sequence common to all varieties of neuron is described. Maturation occurs at the same rate at all cortical depths and appears to relate to the size of the neuron rather than to factors such as generation time, arrival at a final laminar position or cell type. The characteristic laminar patterns of cell type distribution and the specific axonal and dendritic arborisations seen in the adult are generated in the earliest stages of growth and do not occur as the result of elimination from a wider, less precise, distribution. During the period from birth to postnatal week 8 a marked increase in the numbers of dendritic spines is seen in all varieties of neuron including those which will be spine-free in the adult. Following this period an equally marked reduction in spine numbers occurs, initially rapid but continuing at a slower rate even nine months postnatally. Possible relationships between these postnatal dendritic spine changes and the extreme sensitivity of the system to visual input during the early postnatal weeks are discussed.

The development of gaze-stabilizing systems depends on normal vision during infancy. Monkeys reared with binocular lid suture (BLS) for the first 25-40 days of life have strabismus, optokinetic nystagmus deficits, latent nystagmus, and decreased binocular cells in the visual cortex and nucleus of the optic tract. When BLS is extended to 55 days, pendular and congenital nystagmus also occurs. Eyelids in infant monkeys are hairless and thin, but BLS still degrades sensory fusion, motion, and form perception. To determine to what extent these visual properties are critical in the development of normal gaze stabilization, we examined infant monkeys reared with one opaque contact lens over one eye, alternated to the fellow eye every other day (AMO); and monkeys reared in a 3-Hz strobe environment. Monkeys reared with AMO develop strabismus, but have normal optokinetic nystagmus and no spontaneous nystagmus. Area 17 is monocular, but the medial temporal area and the nucleus of the optic tract are binocular. Monkeys reared in strobe light develop pendular nystagmus but not strabismus. We were puzzled by the results of the AMO monkeys until we examined infant monkeys with BLS that were prevented from seeing form through the lids. This was done by leaving the tarsal plate intact behind the eyelid. They developed similar to the AMO monkeys. These results suggest that disruption of sensory fusion during infancy (BLS, AMO) causes strabismus. If strabismus occurs while the monkeys have some form vision from each eye (BLS without tarsal plate), then the nucleus of the optic tract becomes monocular, which causes optokinetic nystagmus deficits and latent nystagmus. Infant monkeys reared without visual motion develop pendular nystagmus.

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