Fig. 1.

Photograph and schematic drawings illustrating the apparatus and behavior in the odor discrimination task.A, Photograph of the Plexiglas insert removed from the operant chamber showing the odor port and the fluid delivery well. The opening of the odor port was ∼2.5 cm in diameter (white circle). Behind the opening was a hemicylinder into which odors were delivered by a computer-controlled system of solenoids and flow meters. Odors were isolated and precisely controlled to provide an onset latency of <40 msec from activation of the solenoid valve controlling delivery. Air flow from the training chamber and into the odor sampling port was maintained at a rate of at least 0.5 l/min to prevent any diffusion of odors from the port into the chamber. The fluid well consisted of a conical depression (black circle) in a 1-inch-deep (front–back) polycarbonate ledge. The depression could easily hold a 0.05 ml bolus of fluid. Four concealed lines entered a central opening in the bottom of the depression to allow the delivery of the two fluid reinforcers, water to flush the well, and attachment of a vacuum-assisted drain line. Fluid delivery and the vacuum drain were controlled by activation of solenoid valves. Infrared photodetectors mounted in the opening to the odor port and in the blocks on either side of the fluid well signaled behavioral responses. B, Schematic drawings illustrating the sequence of behaviors in the go, no-go olfactory discrimination task. On each trial, the rat had to sample an odor presented to an enclosed hemicylinder behind an odor port (Odor Sampling). Nose-poke into the odor port triggered odor delivery. Based on the identity of that odor, the rat then had to decide whether to respond (Go Response) at a nearby fluid well. A go response resulted in delivery of a rewarding sucrose solution, after presentation of a “positive” odor, or an aversive quinine solution, after presentation of a “negative” odor. Novel odors were presented each day, and the rats began each session by responding rapidly after sampling of each odor. Learning was evident in changes in the rat's latency to respond at the fluid well and also in the shift to an adaptive strategy of only responding on positive trails and of withholding responses on negative trials (No-Go). These two measures of learning emerged at different rates (see Fig. 8). Figure adapted from Schoenbaum et al. (1999).