Receiving grooming as a reinforcer for the monkey.

The present study was intended to evaluate whether receiving grooming, given to a monkey by an experimenter, can be used as a positive reinforcer in operant conditioning. When the monkey touched the surface of the correct pattern in a visual discrimination task after a tone cue, the experimenter groomed the monkey's face, neck, and head with his hand. To test whether the discrimination behavior depended on the shape of the stimuli or on the position of the pattern, these experimental parameters were changed in the different tasks. When the square pattern was assigned as correct and presented on the animal's left side, the average score for correct discrimination was 90% in the last 10 sessions out of 30 sessions, and this was statistically significant at a confidence level of p < 0.005 (Grant's table). Correct discrimination was statistically significant when the position of the square was randomly changed to the right and left side of the monkey, and also when the correct pattern was reversed from the square to the cross and its position was again randomly changed. Therefore, it was concluded that the grooming that an experimenter gives to a monkey can be applied as a positive reinforcer in operant conditioning. This experimental paradigm is considered to be useful for neurophysiological analysis of brain mechanisms underlying reward derived from somatosensory input in nonhuman primates.