Cross-modal sensitivities to auditory and visual stimulations in the first-order somatosensory thalamic nucleus.

The ventral posterolateral nucleus (VPL), being categorized as the first-order thalamic nucleus, is considered to be dedicated to uni-modal somatosensory processing. Cross-modal sensory interactions on thalamic reticular nucleus cells projecting to the VPL, on the other hand, suggest that VPL cells are subject to cross-modal sensory influences. To test this possibility, the effects of auditory or visual stimulation on VPL cell activities were examined in anaesthetized rats, using juxta-cellular recording and labelling techniques. Recordings were obtained from 70 VPL cells, including 65 cells responsive to cutaneous electrical stimulation of the hindpaw. Auditory or visual alone stimulation did not elicit cell activity except in three bi-modal cells and one auditory cell. Cross-modal alterations of somatosensory response by auditory and/or visual stimulation were recognized in 61 cells with regard to the response magnitude, latency (time and jitter) and/or burst spiking properties. Both early (onset) and late responses were either suppressed or facilitated, and de novo cell activity was also induced. Cross-modal alterations took place depending on the temporal interval between the preceding counterpart and somatosensory stimulations, the intensity and frequency of sound. Alterations were observed mostly at short intervals (< 200 ms) and up to 800 ms intervals. Sounds of higher intensities and lower frequencies were more effective for modulation. The susceptibility to cross-modal influences was related to cell location and/or morphology. These and previously reported similar findings in the auditory and visual thalamic nuclei suggest that cross-modal sensory interactions pervasively take place in the first-order sensory thalamic nuclei.