A temporal parameter influencing choice between signalled and unsignalled shock schedules

The present study investigated whether choice of a signalled variable-time shock schedule over an unsignalled one was influenced by the average intershock interval. Eight rats were given a choice between signalled and unsignalled shock schedules in a series of conditions with average intershock intervals of 510, 270, 150, 90, 60, and 45 sec. Each test condition was preceded by a training-baseline condition, and schedule values were arranged in an ascending (four subjects) or descending (four subjects) order. Choice of the signalled conditions was directly related to the average intershock interval of the variable-time schedule for six of the eight subjects. The per cent of time in the signalled condition was highest when the average intershock interval was 150 sec or longer and lowest when the average intershock interval was 45 sec. The findings were interpreted as being due to changes in the safety features of the signalled schedule, rather than to changes in the average intershock interval per se.