Unit analysis of nociceptive mechanisms in the thalamus of the awake squirrel monkey.

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Abstract
The activity of units recorded from the medial-intralaminar and posterolateral thalamus was analyzed with the aid of a digital computer. Noxious stimuli were defined as those which always produced withdrawal of the stimulated part. The animal''s level of wakefulness, established according to behavioral and physiological criteria, was designated S1, S2 or S3, in order of increasing arousal. Of 142 units responding to somatic stimuli, none responded exclusively to noxious stimuli. Those neurons which responded only to noxious stimuli in S1 or S3, responded to innocuous stimuli in S2. Thirty-four units, responding to widespread somatic stimuli, were sufficiently stable to allow a comparison of the responses to noxious and innocuous somatic stimuli delivered during S2. For a significant proportion of these units, noxious stimuli produced the greater shift in discharge frequency. The results suggest that pain sensation is associated with distinctive changes in the activity of certain neurons, rather than with the presence of activity in exclusively nociceptive elements.