DISCRIMINATION-LEARNING USING DIFFERENT CS-US INTERVALS IN CLASSICAL-CONDITIONING OF THE RABBITS NICTITATING-MEMBRANE RESPONSE

Differential conditioning of the rabbit's nictitating membrane (NM) re sponse was examined when two CSs each signaled the US, but at differen t CS-US intervals (e.g., 200 and 600 msec). For each subject, one CS w as a 600-Hz tone, and the other CS was either a 2100-, 1000-, or 660-H z tone. This task took advantage of the distinctive temporal character of the NM conditioned response (CR); namely, the NM closure reaches i ts peak near the point of US delivery. The distribution of CR peaks ar ound the points of US delivery varied as a function of the difference between the tones. For the differences that were ''easy'' (600 vs. 210 0 Hz) and ''medium'' (600 vs. 1000 Hz), the distributions overlapped b ut conformed to their CS-US intervals. For the ''hard'' difference (60 0 vs. 660 Hz), the distributions overlapped entirely. In an analysis o f individual subjects using signal detection measures, half the subjec ts showed very high degrees of sensitivity to the easy and medium tone differences. The remaining subjects in those two groups showed high r esponse biases that obscured any sensitivity. For the hard tonal diffe rence, there was no evidence of sensitivity, and variation between sub jects reflected only variation in response bias. The results suggest t hat this task can help illuminate the neural basis of encoding for the sensory and temporal differences among CSs in a useful way.