The mechanisms of extinction were examined by reducing the intensity of the unconditioned stimulus (US) after acquisition training to determine whether such reductions lie on a continuum with CS-alone extinction. The experiments revealed that reductions in US intensity yielded extinction-like effects. Specifically, there were proportional reductions in the daily mean level of responding across sessions. There were also persistent within-session declines and between-session increases of responding analogous to spontaneous recovery. Surprisingly, even when US intensity was held constant, within-session declines and between-session increases were apparent. The results are discussed with respect to possible contributions from unlearning, new learning, generalization decrement, and nonassociative loss, especially CS-specific attentional changes and CR-specific reactive inhibition. EXTINCTION AND US INTENSITY 97the conditioned response (CR) with a 4-mA shock US, each of four groups was presented with a lower US intensity. In addition to CS-alone extinction (no US), three reduced US intensities were used: 0.5, 1, and 2 mA. The latter two values are known to produce reliable CR acquisition in the rabbit nictitating membrane (NM) preparation (Napier, Macrae, & Kehoe, 1992;Smith, 1968). Method

The time course of the conditioned response (CR) in rabbit eyeblink conditioning is highly attuned to the interstimulus interval (lSI) between the onsets of the conditioned stimulus and unconditioned stimulus (US). In the present study, we reexamined data from a large number of experiments in two separate laboratories to determine whether CR timing conformed to a constant ratio analogous to Weber's law. Specifically, the mean and standard deviations (SDs) in the timing of the CR's peak were used to calculate coefficients of variation (CV) for ISIs ranging from 200 to 1,300 msec. Both the mean and standard deviations increased as the lSI was increased, but their ratio (the CV) appeared to remain constant. The average of the estimates was 0.12 (SD = 0.045). The results are discussed with respect to real-time models of classical conditioning and scalar expectancy theory.The time course of the conditioned response (CR) in rabbit eyelid preparations is highly attuned to the interstimu-Ius interval (lSI) between the onset ofthe conditioned stimulus (CS) and the onset of the unconditioned stimulus (US). The rabbit learns not only that the CS is followed by the US, but it also learns when the US will occur. Most notably, the maximal closure of the eyelid CR (the CR peak latency) roughly coincides with the time of US delivery across a wide range of ISIs (see, e.g., Gormezano, Kehoe, & Marshall, 1983;Smith, 1968). The time at which the initiation of the CR occurs is also dynamically linked to the lSI. That is, during acquisition of the CR, its time of initiation decreases to roughly the midpoint of the lSI (Gormezano et aI., 1983).The timing of CR peaks, as well as other features of the CR, has been the focus of "real-time" models of conditioning (e.g.,

Rabbits were given reinforced training of the nictitating membrane (NM) response using separate conditioned stimuli (CSs), which were a tone, light, and/or tactile vibration. Then, two CSs were compounded and given further pairings with the unconditioned stimulus (US). Evidence of both overexpectation and summation effects appeared. That is, responding to the individual CSs declined despite their continued pairing with the US on compound trials (overexpectation), and responding on the compound trials was greater than responding to the individual CSs (summation). The response loss appeared regardless of the testing regime, that is, whether the test presentations of the individual CSs were themselves reinforced (Experiment 2), not reinforced (Experiment 1), or deferred until the end of compound training (Experiment 2). The results are discussed with respect to the roles of excitatory versus inhibitory processes, elemental versus configural processes, and the possible roles of cerebellar and hippocampal pathways.