Anterograde and retrograde transport methods were used to study the corticocortical connectivity of areas 3a, 3b, 1, 2, 5, 4 and 6 of the monkey cerebral cortex. Fields were identified by cytoarchitectonic features and by thalamic connectivity in the same brains. Area 3a was identified by first recording a short latency group I afferent evoked potential. Attempts were made to analyze the data in terms of: (1) routes whereby somatic sensory input might influence the performance of motor cortex neurons; (2) possible multiple representations of the body surface in the component fields of the first somatic sensory area (SI). Apart from vertical interlaminar connections, two types of intracortical connectivity are recognized. The first, regarded as "non-specific," consists of axons spreading out in layers I, III and V-VI from all sides of an injection of isotope; these cross architectonic borders indiscrimininately. They are not unique to the regions studied. The second is formed by axons entering the white matter and re-entering other fields. In these, they terminate in layers I-IV in one or more mediolaterally oriented strips of fairly constant width (0.5--1 mm) and separated by gaps of comparable size. Though there is a broadly systematic topography in these projections, the strips are probably best regarded as representing some feature other than receptive field position. Separate representations are nevertheless implied in area 3b, in areas 1 and 2 (together), in areas 3a and 4 (together) and in area 5; with, in each case, the representations of the digits pointed at the central sulcus. Area 3b is not connected with areas 3a or 4, but projects to a combined areas 1 and 2. Area 1 is reciprocally connected with area 3a and area 2 reciprocally with area 4. The connectivity of area 3a, as conventionally identified, is such that it is probably best regarded not as an entity, but as a part of area 4. Areas identified by others as area 3a should probably be regraded as parts of area 3b. Parts of area 5 that should be more properly considered as area 2, and other parts that receive thalamic input not from the ventrobasal complex but from the lateral nuclear complex and anterior pulvinar, are also interconnected with area 4. More posterior parts of area 5 are connected with laterally placed parts of area 6. A more medial part of area 6, the supplementary motor area, occupies a pivotal position in the sensory-motor cortex, for it receives fibers from areas 3a, 4, 1, 2 and 5 (all parts), and projects back to areas 3a, 4 and 5.

We thank Dr. Clark Riley, Ms. Carol Davenport, and Ms. Jani ne Ptak for synthetic oligonucleotides; Dr. Keith Fry for the gift of mAb ABS; Ms. Cathy Blizzard for cat eyes; Dr. Mark Molliver for donating fixed monkey eyes; Drs. Masafumi Tanaka and Wi nshi p Herr for Oct.1 cDNA; Mr. Hao Zhou for the GST-Ott-1 POU domain protein; Dr. Y.-W. Peng for rabbit retinas and Dr. Hua-Shun Li for chicken retinas; Dr. Elio Ravi ol a for advice on histologic techniques; and Drs. Robert Rodieck, Jen-Chi h Hsieh, and King-wai Yau for hel pful comments on the manuscript.

The number and proportion of neurons displaying GABA immunoreactivity were determined for 50-micron-wide columns through the thickness of 10 areas of monkey cerebral cortex, including the precentral motor area (area 4), 3 cytoarchitectonic fields of the first somatic sensory area (areas 3b, 1, and 2), 2 areas of parietal association cortex (areas 5 and 7), the first and second visual areas (areas 17 and 18), area 21 of the temporal lobe, and areas of the orbital and lateral frontal cortex. Methods of fixation and immunocytochemical processing were designed to maximize the number of stained cells in 15-micron-thick frozen sections and 1-micron-thick plastic sections. In 8 of the 10 areas the number and proportion of GABA-immunoreactive neurons per 50-micron-wide column were found to be the same (34-43 cells/column; 25% of the total neuronal population). Areas 17 and 3b differed. Area 17 contained 50% more GABA-immunoreactive neurons (52-66 cells/column) but more than twice the total number of neurons, so that the GABA cells made up less than 20% of the total. In 3 monkeys, the number and proportion of GABA-positive neurons per 50-micron-wide column in area 3b were smaller than in adjacent areas of sensorimotor cortex (26-42 cells/column; 19-22%). In 2 other monkeys, the number and proportion (34-43 cells/column; 24-26%) were the same as in adjacent areas. Despite the similarity among most areas of monkey cortex, within some areas, the number of GABA-positive neurons per 50-micron-wide column varied as much as 30%. These variations form a significant, repeating pattern only in area 18, where narrow bands (150-200 micron wide) of relatively few stained cells alternated with either narrow or wide bands (600-700 micron wide) in which columns contained more cells. The GABA-immunoreactive neurons were unevenly distributed across layers, with every area containing large numbers and proportions of stained cells in layer II, and every area but area 4 displaying a second concentration in the principal thalamocortical recipient layers. In area 4, the number of GABA-positive neurons declined sharply from layer II to layer III and remained low through layer VI. For areas displaying the greatest intra-areal variability, only 1 or 2 layers contributed significantly to that variability (layer IV in area 3b, layers III and V in area 18, and layers II and III in area 17).(ABSTRACT TRUNCATED AT 400 WORDS)

A neurochemically distinct population of koniocellular (K) neurons makes up a third functional channel in primate lateral geniculate nucleus. As part of a general pattern, K neurons form robust layers through the full representation of the visual hemifield. Similar in physiology and connectivity to W cells in cat lateral geniculate nucleus, K cells form three pairs of layers in macaques. The middle pair relays input from short-wavelength cones to the cytochrome-oxidase blobs of primay visual cortex (V1), the dorsal-most pair relays low-acuity visual information to layer I of V1, and the ventral-most pair appears closely tied to the function of the superior colliculus. Throughout each K layer are neurons that innervate extrastriate cortex and that are likely to sustain some visual behaviors in the absence of V1. These data show that several pathways exist from retina to V1 that are likely to process different aspects of the visual scene along lines that may remain parallel well into V1.