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Monkeys were trained in several complex tasks. In an initial task, the monkeys responded to a perceived shift in temperature, by pressing a panel to receive "a liquid reward" (water or juice). A second model allowed the monkey to make "informed choices" and forced him to discriminate between behaviorally-relevant and non-relevant stimuli.

DiagramImage ModifiedIllustration of experimentImage Modified
From Task-related responses of monkey medullary dorsal horn neurons

The monkey could choose to initiate a trial at any time by pressing a panel and could terminate any painful stimulus by releasing the panel. When he activated the stimulus, the thermode presented sequences of low and high-temperature heat pulses in a "quasi-random" sequence. If the monkey released the panel within two seconds after he detected a temperature shift downward, he received his reward. Noxious stimuli (45 oC or higher) were presented for only a few seconds and could be immediately terminated by the monkey. He would receive no reward, however, unless he waited for the downward shift. A third type of task required the monkey to earn his reward by responding to a visual cue (light), while ignoring the temperature shifts of the thermode pulse (although again, noxious stimuli could be terminated immediately.)

Once a monkey had learned the tasks, the researchers began recording trigeminal neuron activity, using a microelectrode method developed by Ed Evarts at the National Institute of Mental Health (NIMH) . The key data recorded were the time delay before the nerve responded to a stimulus and the frequency with which the nerve "fired". These were analyzed in conjunction with electrophysiological recordings from the same neurons in anesthetized animals and with anatomical and cytological observations under electron microscopy.


Apparatus built by Fred Brown to administer thermal stimuli and record nerve impulses.
Apparatus built by Fred Brown to administer thermal stimuli and record nerve impulses