Differential thalamic relationships of sensory‐motor and parietal cortical fields in monkeys

The thalamic connectivity of areas 3b, 1 and 2 of the first somatic sensory cortex (SI) and of the adjacent areas 4, 3a and 5 has been studied in monkeys with anterograde and retrograde labeling techniques. Anteroposterior sectors of the SI cortex are represented in the thalamic ventrobasal complex by curved lamellae of thalamocortical relay cells extending through the dorso‐ventral and anteroposterior dimensions of the ventrobasal complex. Within such a lamella there are clustered aggregations of cells each projecting to a punctate zone of SI. Such cortical zones are less than 1 mm in circumference and are interpreted as comparable to the “columns” of electrophysiological studies. Each clustered aggregation in the ventrobasal complex is of limited mediolateral and dorsoventral extent but extends through much of the anteroposterior dimension of the ventrobasal complex. Punctate zones lying adjacent to one another in the mediolateral dimension of the SI cortex are connected with aggregations of cells lying in adjacent lamellae of the ventrobasal complex. Punctate zones lying anterior or posterior to one another in the anteroposterior dimension of SI are connected with aggregations of cells lying ventral or dorsal to one another in a lamella of the ventrobasal complex. Sectors of SI extending from posterior to anterior across areas 2 and 1 and others extending from posterior to anterior across area 3b, are each represented systematically across the full dorsal to ventral dimension of the ventrobasal complex. This implies at least two separate representations of the body surface: one in areas 2 and 1, and another in area 3b. Within a lamella of the ventrobasal complex, aggregations of cells projecting to areas 2 and 1 are mingled with those projecting to area 3b. Measurements of the sizes of retrogradely labeled cells in brains in which areas 2 and 1 or area 3b were separately injected showed no distinction between cells projecting to the three areas on the basis of size. Experiments combining retrograde cell degeneration due to ablation of area 3b with retrograde labeling after injection of areas 1 and 2 indicated little possibility of collateral projections to the three areas from the same cell. No part of SI, as defined by the most liberal anatomical criteria, is connected with any thalamic nucleus outside the confines of the ventrobasal complex (the caudal division of the ventroposterolateral nucleus and the large‐celled part of the ventroposteromedial nucleus) or of the intralaminar complex. Area 3a, as traditionally defined, has connectional relationships that strongly suggest it is a part of the motor cortex, area 4. However, short latency Group I evoked potentials could be elicited from a small part of area 3a lacking layer V giant cells and lying adjacent to area 3b. This part receives its thalamic input from the ventrobasal complex. The data indicate that area 4 is connected with the oral division of the ventroposterolateral nucleus and with the caudal nucleus of the ventral lateral complex. These cellular groupings, however, are only artificially separated and appear to form part of the same thalamic relay nucleus. Experiments on area 5 not only suggest that the posterior boundary of SI should be placed further posterior than is customary, but also suggest that area 5 can be divided into an anterior field related to the anterior nucleus of the pulvinar and a posterior field related to the lateral posterior nucleus.