A comparison of human tactile stimulus velocity discrimination with the ability of S-I cortical neurons in awake rhesus monkeys to signal the same velocity differences before and after non-anesthetic doses of pentobarbital.

Using psychophysical techniques, the ability of human subjects to discriminate the velocity of brush movement across the glabrous skin of the hand was determined. The same stimulus was then used in a series of neurophysiological experiments designed to determine the ability of neurons in the primary somatosensory cortex (S-I) of awake rhesus monkeys to signal differences in the same velocities. Analysis of the data in signal detection theory terms revealed that humans could discriminate two velocities (132 and 182 mm/sec) much better than most of the cortical cells of the monkeys could signal the differences in those same velocities by changes in their stimulus induced mean firing frequency. However, 4 of the 37 cells studied signaled the differences as well as the human observers. Non-anesthetic doses of pentobarbital (1-20 mg/kg) caused a reduction in the firing frequency of the S-I neurons but at the same time enhanced the difference in firing frequency elicited by the two different stimulus velocities. This enhancement in the difference between the firing frequencies was caused by a relatively greater suppression of neuronal activity elicited by the slower velocity. These results not only point out potential problems with the use of barbiturates in neurophysiological studies and by humans who require fine tactile acuity, but they also raise important questions about coding by S-I cells. The reduction in overall neuronal activity accompanied by the enhancement of the difference in firing frequency at the two velocities would suggest that, if firing frequency is coding for stimulus presence or velocity, at 8 mg/kg the monkeys may have a more difficult time detecting the stimulus but an easier time discriminating between different velocities of the stimulus.