Texture discrimination and unit recordings in the rat whisker/barrel system

We have developed a semi-automated technique for acquiring neurophysiological data during whisker-based tactile discriminative behavior. Water-deprived, blindfolded rats are tethered by means of a harness vest that permits them to contact a rough (250 micrometer grooves) or smooth discriminandum with only their vibrissae. Discriminanda are mounted on a motor-driven carousel, and the rat indicates its choice (rough, smooth) by licking either a right or left water port located near the carousel. A narrow light beam detects general proximity of the animal's nose to the discriminandum, although actual whisker contact is monitored by a SuperVHS camera and measured offline using field-by-field videographic analysis. Rats can be trained within 3-6 weeks at which time they perform 100-150 trials/day at a level of 80% correct. Unit recording from the somatosensory cortex reveals that neurons increase their firing upon whisker contact of a discriminandum and that firing remains elevated during several hundred milliseconds of ongoing contact, even with the smooth surface. Nevertheless, despite the animal's ability to distinguish the rough and smooth surfaces, overall neuronal firing rates were indistinguishable for the two surfaces. In some cases, temporal firing patterns differed, although not in a consistent way across recording sites.