This project correlates behavioral responses with responses of neurons in the medullary dorsal horn produced by noxious thermal stimuli on the face of behaving monkeys. Medullary dorsal horn neurons encode noxious thermal information used by the monkey to perform a thermal detection task. Two populations of nociceptive neurons are found in the medullary dorsal horn: nociceptive-specific (NS) and wide-dynamic-range (WDR) neurons. Nociceptive specific neurons respond only to intense mechanical and thermal stimulation, whereas wide dynamic range neurons respond to low-threshold mechanical stimuli, but their largest response is to noxious mechanical and thermal stimulation. We found that a subpopulation of wide-dynamic-range neurons encoded the intensity of noxious thermal stimulation and could account for the monkey's ability to detect noxious thermal stimulation. The discharge of nociceptive-specific neurons could not account for the monkey's ability to detect noxious thermal stimulation. An argument that has been used against the role of wide-dynamic-range neurons in pain sensation is that the large size of the receptive fields would not allow for localization of a noxious stimulus. However, we found that there was a strong correlation between neuronal discharge and the monkey's ability to detect noxious thermal stimulation only when the stimulus was located in the central, most sensitive portion of the receptive field. When the stimulus was located in the peripheral, less sensitive portion of the receptive field, a correlation was not found between neuronal discharge and the monkey's ability to detect noxious thermal stimulation. These data suggest that information from only the central portion of the wide- dynamic-range receptive field is important for pain sensation and the localization of a painful stimulus. Information from the peripheral portion of the receptive field does not appear to be important for oral-facial pain sensation.
