Coding perceptual discrimination in the somatosensory thalamus.

The sensory thalamus is classically viewed as a relay station of sensory information to cortex, but recent studies suggest that it is sensitive to cognitive demands. There are, however, few experiments designed to test whether this is so. We addressed this problem by analyzing the responses of single neurons recorded in the somatosensory thalamus while trained monkeys reported a decision based on the comparison of two mechanical vibration frequencies applied sequentially to one fingertip. In this task, monkeys must hold the first stimulus frequency (f1) in working memory and compare it to the current sensory stimulus (f2) and must postpone the decision report until a cue triggers the decision motor report, i.e., whether f2 > f1 or f2 < f1. We found that thalamic somatosensory neurons encoded the stimulus frequency either in their periodicity and firing-rate responses, but only during the stimulus periods and not during the working memory and decision components of this task. Furthermore, correlation analysis between behavior and stimulus coding showed that only the firing rate modulations accounted for the overall psychophysical performance. However, these responses did not predict the animal's decision reports on individual trials. Moreover, the sensitivity to changes in stimulus frequency was similar when the monkeys performed the vibrotactile discrimination task and when they were not required to report discrimination. These results suggest that the somatosensory thalamus behaves as a relay station of sensory information to the cortex and that it is insensitive to the cognitive demands of the task used here.