Discrimination in the sense of flutter: new psychophysical measurements in monkeys

Humans and monkeys have similar capacities to discriminate the frequencies of mechanical sinusoids delivered to their hands in the range that corresponds to the sense of flutter (10-50 Hz). Previous studies showed that monkeys can discriminate whether comparison stimuli are higher or lower in frequency than a base stimulus that does not vary from trial to trial during an experiment. We verified this result in two monkeys trained in this manner. To confirm that these animals were able to discriminate, we tested them in a variant of the task in which the frequency of the base stimulus changed randomly from trial to trial. The monkeys failed to discriminate in this new testing mode; instead they seemed to categorize the comparison stimuli, ignoring the base stimulus. After further training in the randomized base condition, the two monkeys learned to discriminate accurately. We then explored how the stimulation parameters affected performance. We found that animals could discriminate accurately with stimulus durations as short as 250 msec, with interstimulus intervals as long as 10 sec, with 50% differences between base and comparison stimulus amplitudes or when stimulated on a different finger. Performance did not degrade in these conditions, even though the monkeys had never been trained or tested under them. The results show that monkeys may try to categorize rather than discriminate when the task allows either strategy, although they are capable of performing true discriminations very robustly. These findings have important implications for investigating the neuronal processes underlying sensory discrimination.