Attenuation of overshadowing as a function of nondifferential compound conditioning trials

Using a classical conditioning procedure, four groups of eight rats each were given 50 to 4,050 conditioning trials with a simultaneous tone-light compound stimulus paired with non-contingent delivery of a water dipper. The anticipatory response of entering the magazine during the CS served as the conditioned response. Following training, each group was tested in extinction such that one-third of the test trials were to the tone-light compound, one-third to the tone alone, and one-third to the light alone. Rats that had received 50 or 250 conditioning trials showed very little resistance to extinction to the light alone, whereas resistance to the tone was substantial and similar to that for the tone-light compound. After 1,250 and 4,050 conditioning trials, responding to the light increased to match the high levels of responding elicited by the tone and tone-light compound. The results present difficulties for both “continuity” and “noncontinuity” theories of stimulus selection.