The somatosensory intercollicular nucleus of the cat's mesencephalon.

1. Unitary, neural activity was sampled with tungsten electrodes in the mesencephalic, intercollicular region of cats anaesthetized with chloralose. The units' stereotaxic co-ordinates were noted and they were tested for activation by adequate tactile, visual and acoustic stimuli as well as electrical nerve stimulation. The units' geometrical gradations were afterwards translated into morphological terms by means of unbiased, cytoarchitectonic identifications of those structures which had been penetrated by the electrodes. 2. The primary objectives were the conventional, somatosensory units, which had reliable, low-threshold, tactile receptive fields and could not be activated by the two other types of adequate stimulation. There were 139 such units from a total sample of 495. 3. These somatosensory units were found to occupy many of the region's structures, notably the intercollicular nucleus (INC), the nucleus of the brachium of the inferior colliculus, the stratum griseum intermedium and the stratum griseum profundum of the superior colliculus. INC was the only structure which had an exclusively somatosensory input. 4. The units of INC had tactile receptive fields varying between one and several hundred square centimetres. Convergence of afferent input from outside these fields could sometimes be demonstrated by nerve stimulation. The latencies of activation from the contralateral sciatic nerve were, on average, shorter for units of INC than for the somatosensory units of the other intercollicular structures, and INC units could also follow higher stimulation frequencies. 5. The findings support the assumption that INC may constitute a distinct mesencephalic centre for somatosensory function.