[Localization and response characteristics of thalamic corneal units in the cat]

Units in the thalamus responsive to mechanical stimulation of the cornea (thalamic corneal units) were studied in urethane-chloralose anesthetized cats. Four different classes of corneal units were found in the nucleus ventralis posteromedialis (VPM) and intralaminar nuclei. They corresponded to 4 different classes of corneal units in the trigeminal subnucleus caudalis and adjacent bulbar lateral reticular formation reported by Nishida (1987). Low threshold corneal specific (LTCS) units, whose mechanical threshold was well below the pain threshold in patients with cataract, were located in the dorsolateral part of the VPM proper. High threshold corneal specific (HTCS) units and wide dynamic range (WDR) units, whose mechanical threshold was well above the pain threshold of the human cornea, were located in the shell region of caudal VPM. Each class of these three corneal units was incorporated in the somatotopic organization of low threshold mechanoreceptive, nociceptive specific, or WDR units having a cutaneous receptive field in the contralateral trigeminal nerve territory. Units having receptive fields in the head similar to those of subnucleus reticularis ventralis (SRV) units were found in the intralaminar nuclei. Their corneal mechanical thresholds were much the same as those of HTCS units and WDR units. These results supported the suggestion that both nonpainful sensation and pain can be evoked in the cornea.