Factors That Can Influence the Acquisition of the CR

1. Interstimulus Interval

Every type of conditioning paradigm has an optimal interstimulus interval (ISI). The optimal ISI depends on the particular paradigm, but it is clear that ISIs that are very short or very long and result in little or no CR acquisition. For example, in the rabbit classically conditioned NM response paradigm, the optimal ISI

for delay conditioning is 250 msec. An ISI of 100 msec or less results in poor conditioning, as does increasing the ISI to longer than 250 msec. In this paradigm, an ISI of 2000 msec (i.e., 2 sec) will not result in any CR acquisition.

2. Intertrial Interval

It is a general finding from many conditioning paradigms that increasing the ITI will tend to produce more rapid CR acquisition, although this influence is not robust in most cases. Reducing the ITI to less than several seconds can drastically impair acquisition of the CR. The typical ITI ranges from 30 to 60 sec.

3. Temporal Arrangement and Spacing of the CS and the US

As noted previously, delay conditioning generally results in the most rapid rate of CR acquisition followed by trace conditioning. Simultaneous conditioning generally results in much poorer CR acquisition. In many cases simultaneous conditioning (ISI=0) does not produce CRs. Backward conditioning results in CRs in only a few paradigms and generally does not produce any CR acquisition.

4. CS Intensity

It was initially believed that the CS intensity did not influence that rate of CR acquisition, but recent studies indicate that more intense CSs tend to result in slightly more rapid CR acquisition.

5. US Intensity

An increase in the acquisition rate of the CR has been a consistent finding with increases in the intensity of the US.

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