Vibrissal roughness discrimination is barrelcortex-dependent

We have investigated the contribution of the neocortical vibrissal representation within the posterior medial barrel subfield (PMBSF) to the high performance levels obtained by rats in a complex roughness discrimination task mediated by vibrissal inputs. Nine binocularly occluded rats were trained in a two-choice roughness discrimination until they obtained the 85% correct response criteria. Subsequently, the PMBSF was localized by electrophysiological recordings and bilaterally ablated. The locus and extent of the cortical lesions were confirmed by histological analysis after additional training and testing. There was no evidence of task retention after the cortical lesion and barrelless rats were unable to obtain prelesion discriminative performance levels when stimulation was restricted solely to vibrissal cues. After extensive postlesion training, four of these rats were allowed to palpate the discriminanda with their forepaws. Under these conditions rats rapidly reached the 85% correct criterion once again. The present results indicate that the PMBSF is essential for complex tactile discrimination when sensory information is obtained through the vibrissae by active palpation. This deficit is specific for the vibrissal system, the PMBSF is not essential to solve the same tactile discrimination task when the source of the somatosensory information is provided by other non-vibrissal cutaneous sensory receptors.