The effect of food deprivation on within-session patterns of wheel running

In the present study, rats' wheel running was reinforced when they responded at different food deprivations. In experiment 1, fourths of a wheel turn were reinforced on a variable interval (VI) 15- or 60-s schedule during 50-min sessions. Subjects responded at 75, 85 or 95% of their free-feeding body weights, across conditions. Within-session decreases in responding were steepest at subjects' 75% weights for the VI 60-s schedule, but were similar at different weights for the VI 15-s schedule. In experiment 2, subjects responded on a VI 60-s schedule at 75 or 95% of their free-feeding body weights. Reinforcer size was one or four food pellets. Steeper within-session decreases in responding were observed at subjects' 75% weight than at their 95% weight, with no effect of reinforcer amount. The present results cannot disconfirm either leading theory for within-session changes because the terms involved (i.e. habituation and satiation) are not adequately defined. However, the present results seem to pose problems for both explanations.