Viscerosomatic interactions in the thalamic ventral posterolateral nucleus (VPL) of the squirrel monkey

In anesthetized squirrel monkeys single cell recordings were performed using tungsten microelectrodes. The responses of 29 viscerosomatoceptive and somatoceptive VPL neurons to noxious distension of the urinary bladder, the lower esophagus and the distal colon and to innocuous and noxious somatic stimuli were assessed when these stimuli were presented separately or together. Neuronal responses were defined as additive or interactive depending on the relative changes in responses to individual somatic or visceral stimuli, and on their responses during conditioning (somatic and visceral stimuli applied concurrently). In 13 neurons interactions between the somatosensory and visceral inputs could be demonstrated. The dominant interactive effect was inhibition, although facilitatory effects were seen as well (2 of 13). The magnitude or direction of the interactions seemed independent of the location of the somatic and visceral receptive fields. The mean population response of the neurons showing interactions was 4.66 spikes/s to somatic stimulation, and 0.07 spikes/s to visceral stimulation. During conditioning the mean interactive effect was -62% of the calculated additive effect. This implies that overall the somatic responses are halved during a coincident visceral stimulus. In a subgroup of the VPL neurons, which were classified as pure somatic responsive (n=14) due to their unresponsiveness during visceral stimulation alone, a third (n=5) still exhibited visceral convergence during conditioning. The latter neurons, therefore, receive visceral inputs, which function in a purely interactive (modulatory) manner. It is concluded that part of the described effects is due to competition (cross modality suppression) between the visceral and somatic inputs. We further conclude that the suppression of somatic information by noxious visceral stimuli may contribute to a more effective processing of the discriminatory aspects of nociceptive visceral information previously demonstrated in VPL.