Lack of collateral thalamocortical projections to fields of the first somatic sensory cortex in monkeys.

Retrograde axoplasmic transport of differently colored fluorescent dyes was used to determine the distributions and relative proportions of cells in the ventrobasal complex of the monkey thalamus that project to each of the architectonic fields of the first somatic sensory cortex. Fast blue was injected into portions of area 3a identified by first recording short latency, multiunit responses to electrical stimulation of Group I afferents in a muscle nerve of the forelimb or hindlimb. Nuclear Yellow was later injected into a part of area 2 responding to maximal electrical stimulation of the same nerve. In experiments that served as controls, Fast Blue was injected into area 3b and Nuclear Yellow into area 1. The results confirm the division of the ventrobasal complex into: (i) a central core with an inner part projecting to area 3b and a surrounding part projecting area 1; (ii) a peripheral shell projecting to areas 3a and 2 (Jones et al. 1982). A far greater proportion of VB cells projects to areas 3b or 3a than to areas 1 or 2. In portions of the ventrobasal complex projecting to two areas, no cells were double labelled provided that the injections of blue and yellow dyes did not overlap. The results, thus, show a lack of collateral thalamocortical projections to the fields of the postcentral gyrus and, when taken in conjunction with other data, imply the independent relay of modality specific information through the thalamus.