Unconditional stimulus characteristics in rabbit eyelid conditioning.

Three experiments employed aversive or positive subcortical electrical stimulation in an unconditional stimulus (US) role in rabbit eyelid conditioning. Conditional response (CR) development to a tonal conditional stimulus (CS) was examined when the intracranial stimulation was combined with a conventional cheek-shock US (Experiment 1) or with a lightflash US (Experiment 2). Our findings were consistent with previus observations that the affective impact of the US is important in the development of an overt motor CR. The complex pattern of our results, however, implies that intracranial stimulation has multiple effects on conditioning and argues against any unidimensional interpretation of these outcomes. Our analysis of CR-contingent intracranial stimulation presentation in Experiment 3 indicated that such stimulation does not act as a Thorndikian reward or punishment in rabbit eyelid conditioning.