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  Findings from the Behaving Monkey Studies
  • "Wide-dynamic-range" neurons, not pain-specific cells, are most active in discriminations of changes in stimuli intensity
  • Behaviorally relevant stimuli trigger "a neuronal burst" of activity, independent of stimuli intensity
  • Detection of temperature shifts, in monkeys and humans, is an index of pain sensation

1) The physiological studies had identified two types of nociceptive neurons, neurons which responded to painful stimuli, in the trigeminal system and the spinal cord. "Nociceptive-specific" neurons responded with high levels of activity; but "wide-dynamic-range" (WDR) neurons responded with graded sensitivities, depending on the intensity of the stimulus. Even light touch applied to the center of the WDR neuron's receptive field would trigger a response, but stronger pressure was required in the surrounding area; and at the perimeter of the field, only a painful stimulus such as pinch would cause the neurons to fire.

Behavioral recordings from these different neurons during the awake behaving monkey studies showed that the WDR neurons, which were active at both innocuous and noxious stimulus levels, were those most involved in the monkey's perception of temperature shifts.

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Both Nociceptive-Specific neurons and Wide-Dynamic-Range neurons increase their rates of firing as the intensity of the pain stimulus (the hot pot) increases. Wide-dynamic-range neurons, however, show an accelerated increase in firing rate; these neurons are the most sensitive to changes in stimulus intensity. Drawing and Graph by Donald Bliss.

2) As the experimenters began offering well-trained monkeys more choices and decisions to make, they established that non-behaviorally significant stimuli -- an change in temperature that was irrelevant to the monkey's reward-seeking actions, for example -- led to only low levels of response from the WDR neurons. But light or heat stimuli important to the monkey's choices and behavior triggered very high, rapid levels of firing. These variations in nerve response were independent of the actual level of stimulus intensity. Thus, the nerve activity was the product of both sensory input and behavioral state.

These observations supported the concept of pain as dynamic and experiential: while the neurons involved in the monkey's behavior were highly specific in type and function, they were capable of different perceptual and behavioral responses to the same stimuli, depending on other environmental and internal events. This flexibility suggested that the nervous system is highly plastic and adaptive as an organism learns from and behaves within the outside environment.

3) In a subsequent series of experiments, Dan Kenshalo asked human volunteers to detect small shifts in painful stimuli and to estimate the intensity; correlation of "detection speed" with the monkeys' showed that "the monkeys and humans perceive, in terms of their detection speeds, about the same thing, and…they're all an index of the measure of pain sensation, pain intensity." 7

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7 Interview with Daniel Kenshalo, Jr., 2000.

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