1995 Volume 37 Issue 4 Pages 247-251
The purpose of this experiment was to investigate whether overshadowing occurred between two contextual stimuli (light and tone) in Pavlovian conditioning of rats. Thirty-two albino rats, randomly assigned to four groups, were individually placed in a chamber, in which light, tone or both served as contextual stimuli. A single electric shock (1mA, 2-s) was delivered to the floor during once-a-day 5 min sessions for 5 consecutive days (conditioning phase). The day after finishing-the conditioning phase, each was placed for 5 min in the chamber, and light or tone was presented as a contextual stimulus (test phase). The group of rats exposed to both stimuli during the conditioning sessions but to light only during the test session (LT/L) froze less frequently in the test phase than in the conditioning phase. The other three groups (LT/T, T/T, and L/L) froze in the test phase as frequently as in the conditioning phase. These results suggest that the tone overshadowed a light when both of them were presented together as contextual stimuli of conditioning.