Contemporary explanations of the trial-spacing effect (TSE) were evaluated. Experiment 1 revealed, in rats given tone-footshock trials with 15-, 60-, or 900-s intertrial intervals (ITIs), a direct relationship between freezing to the tone and ITI (the TSE) but an inverted U-shaped relationship between freezing to the training context and ITI. In Experiment 2, footshock preexposure eliminated the TSE that otherwise occurs across 15- to 60-s ITIs but had no effect on the TSE that occurs across 60- to 900-s ITIs. In Experiments 3 and 4, neither (a) increasing posttraining exposure to the training context in rats trained with 60-s ITIs nor (b) reducing between-trial exposure to this context in rats trained with 900-s ITIs influenced freezing to the tone. These findings suggest that the TSE obtained in this research is due to more than 1 mechanism: 1 responsible for the TSE that occurs with ITIs less than approximately 60 s and another responsible for the TSE that occurs with ITIs greater than this. Although the perceptual-defensive-recuperative model may correctly describe the former mechanism, none of the theories tested seems to correctly describe the latter.

Summary.-A preparation for the study of classical fear conditioning in vertebrates is described. Its unique features are that it is inexpensive and easy to construct and operate. The following classical conditioning phenomena are demonstrated using this preparation: excitatory conditioning, extinction, contextual conditioning, blocking, a conditioned inhibition discrimination, and latent inhibition.

Barela (Psychological Reports, 110: 173-186, 2012) presented a novel preparation for the experimental study of classical fear conditioning in goldfish, and successfully employed it in the demonstration of several basic conditioning principles. The present research addressed some basic methodological questions using a modified version of this preparation. Results indicated that the modification made to the visual CS increased its efficacy, and that conditioning to this stimulus was directly related to the number of CS-US trials given, US intensity, and intertrial interval. In addition, conditioning was detected with interstimulus intervals both shorter and longer than those used previously, as well as with an auditory CS. Methodological implications for the future employment of this preparation are discussed.

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