Guttman (1953) has reported that hungry rats, exposed in a .skinner box to changing sucrose concentrations (4, 8,16, and 32%) responded in a monotonically increasing fashion to the increasing concentrations. Tombaugh & Marx (1965) reported that thirsty rats on the other hand yield a rather different function relating bar pressing and sucrose concentrations (4,8, 32, and 64%) with less response strength at 32 and 64% than at 4 or 8%. The former study presented the different concentrations in counterbalanced order. The latter study, however, used either increasing or decreasing modes of presentation. Ison & Rosen (1965) have reported that the performance of rats for sucrose rewards appears to interact somewhat with their motivational state (hunger or thirst). It was decided, therefore, to replicate the Guttman procedure using counterbalanced presentation with thirsty rats to determine the generality of the relationship between sucrose concentration and bar pressing.
METHOD Subjects and ApparatusThe Ss were four male, albino rats of the Sprague-Dawley strain, approximately 102 days old at the start of the experiment, obtained from Holtzman suppliers, Madison, Wisconsin. The experimental test chambers were four Plexiglas cages which were 9 x 8 x 8 in. with a 2-in. x ~-in. bar protruding ~ in. into the chamber 4 in. from the top of the cage and 3 in. from the sides of the cage. Procedure Upon arrival Ss were marked, handled and assigned to individual cages. The Ss were maintained on ad lib Purina Chow. Water was removed from the individual cages 23 h before the beginning of each day of training and on the days preceeding the four experimental sessions. The Ss were magazine trained and put on a continuous reinforcement schedule with water as reinforcement for two weeks prior to the initial experimental session. Twenty-four hours before each test session four concentrations of sucrose (table sugar and water), 4%, 8%, 16%, and 32%, were prepared (gm/ml). Four orders of presentation of the concentrations were selected in a counterbalanced design. The orders were (a) 4%-160/0-320/0-8%, (b) 80/0-40/0-160/0-32%, (c) 160/0-320/0-80/0-4%, and (d) 320/0-80/0-40/0-16%. On the test days the S was placed in the test chamber for 15 min and permitted to bar press for the first solution of a given order. The S was then removed from the chamber and held for 5 min during which time the second solution in that order was substituted for the first and the dipper was cleaned. This procedure was carried out for the four concentrations. Each S received the four orders in different sequences across the four test days. Number of reinforcements obtained in each 15 min period was recorded. Each reinforcement was approximately .10 mt. The inter-testday interval ranged from two to seven days. Figure I shows the mean number of reinforcements obtained by the Ss at each sucrose concentration over the four IS-min intervals. Analysis of variance with IS-min intervals, orders of presentation, and solutions as within-S variables demonstrated that...