THALAMOCORTICAL PROCESSING OF VIBRISSAL INFORMATION IN THE RAT .1. INTRACORTICAL ORIGINS OF SURROUND BUT NOT CENTER-RECEPTIVE FIELDS OF LAYER-IV NEURONS IN THE RAT S1 BARREL FIELD CORTEX

The receptive fields of cells restricted to the D1 cortical barrel territory in the S1 cortex of the rat were examined before and after substantial lesions of the D2 barrel. We tested 131 cells (N = 62, unlesioned controls; N = 69, lesioned animals) for modal latency and response magnitude to standard vibrissal deflections of 1.14 degrees. Lesions ranged in size to encompass 22-95% of the volume of the D2 barrel hollow and 5-75% of its neighbouring septal region, as calculated from cytochrome oxidase and Nissl staining of alternate sections. Negligible loss (mean 1.1%) of other barrel hollows and their septal regions (6.3%) occurred. A mean loss of 58% of the D2 barrel hollow and 27% of its accompanying septa was paralleled by a highly significant deficit in response magnitude (57.3%); p < 0.005) of D1 barrel cells to D2 vibrissal stimulation, when compared with controls. The best-fit relationship between deficit and volumetric loss of the D2 barrel hollow was linear (regression coefficient -0.91). In the extreme case where 95% loss of D2 barrel hollow occurred, there was a 92% deficit in response of D1 barrel cells to the D2 input. No significant loss in response magnitude to other vibrissae, including the principal D1 input, occurred. Postlesioned animals exhibited some increase in excitability to the D1 vibrissa, and to vibrissae whose principal barrel territories were undamaged (delta, gamma, C1). Lesioning of the D2 barrel caused a highly significant mean increase (60%) in latency of residual responses to stimulation of the D2 vibrissal input (15.2 ms controls; 24.3 ms experimentals). No significant changes in response latency to other vibrissae compared to controls occurred. These results suggest that an intact D2 barrel is essential for the generation of responses of D1 barrel cells by the D2 vibrissa, and further imply that surround receptive fields of layer IV barrel cells are largely generated intracortically by barrel-to-barrel relay. The implications of these findings to cortical processing of tactile information and plasticity in the somatosensory system are discussed.