Shock intensity and duration interactions on free-operant avoidance behavior.

Shock intensities (1 to 4 mA) and shock durations (0.3 to 0.75 sec) were concurrently varied over a range commonly used in free-operant avoidance studies using a lever-press response. Response rates were a positive linear function of the log of the product of intensity times duration. Shock rates were a negative linear function of that log. The increase in response rates was primarily due to a selective increase in the conditional probability of making responses with long interresponse times. The disproportionality of receiving shocks early in the session (warm-up) was also a linear function of the log of the intensity-duration product, with increasing disproportionality as the value of the intensity-duration product was increased. Thus, with all measures of the avoidance performance, shock intensity and shock duration combine in a multiplicative fashion to determine the avoidance performance.