Stimulus intensity and conditioned suppression magnitude: dependence upon the type of comparison and stage of training.

Three groups of eight rats each were trained in conditioned emotional response (CER) using white noise of 70 dB, 50 dB, or both intensities as conditioned stimuli (CSi) given in unpredictable order. Between-subjects comparisons showed more rapid acquisition to the 70- than to the 50-dB CS. Rats presented with both intensities acquired the CER at a similar rate as those conditioned to the 70 dB value only, and within-subjects comparison did not reveal any CS intensity effects. During the next stage of the experiment all rats were presented with both intensities of white noise. The stimulus intensity effect found in within-subjects comparisons was greatest when the soft CS replaced the more intense value on half of trials. Weakening of the CER after reaching asymptotic conditioning was related to increased response rate at the beginning of CS action with greater suppression of bar presses before shock onset. This decreasing pattern of bar presses within CS action was far more consistent on 70-dB trials, indicating a stimulus intensity effect on the development of the inhibition of delay in CER training.