Interactions between inputs from adjacent digits in somatosensory thalamus and cortex of the raccoon

Interactions between somatosensory afferents arriving from different points in the periphery play an important role in sensory discrimination and also provide the substrate for plasticity following peripheral injury. To examine the extent and time course of such interactions, extracellular recordings were made from neurons in the primary somatosensory cortex and the ventroposterior lateral thalamus of anesthetized raccoons. Interactions between adjacent digits were studied using the conditioning-test paradigm in which a test pulse was delivered to the digit containing the neuron's receptive field (the on-focus digit) at various intervals following conditioning stimulation of an adjacent, off-focus digit. Off-focus stimulation produced predominantly inhibition of the test response with a maximum effect at 20-40 ms in both cortex and thalamus. The mean inhibition was approximately twice as large in the thalamus as in the cortex. Recordings were made in other animals after unmyelinated C fibers had been destroyed in the on-focus digit by subcutaneous injection of capsaicin. This resulted in a doubling of the responses evoked by the test stimulus in both regions, but the spontaneous discharge rate was not changed. The amount of inhibition produced in the cortex was unchanged by capsaicin treatment, but was reduced in the thalamus compared to control animals. This indicates that capsaicin-sensitive peripheral afferents provide a tonic control over interdigit inhibition in the thalamus.