Animal Learning & Behavior

In Experiment 1, a group of rats were runway trained on each of two reward series for 32 days. The two series consisted of three runs, the first two of which were, respectively, rewarded and nonrewarded; the third run was rewarded in one series but nonrewarded in the other. A 40-min interval separated the two series; the first and second runs within the series were separated by a 10-min interval, whereas the second and third runs were separated by a 30-sec interval. The reward (and nonreward) events and temporal cues of the two series are designated R-NR/R-NN. A second group was similarly trained, with the exception that the 10-min interval separated the second and third runs (RN-R/RN-N). Both groups developed appropriate differential running on the third run of the two series, and the RN-R/RN-N animals ran appropriately (slowly) on the second run of both series. Appropriate Run 2 performance appeared in one half of the R-NR/R-NN animals (depending upon order of series presentation); the remaining half ran faster on Run 2 of the R-NR series than on the same run of the R-NN series, an effect currently termed interevent anticipation. A cue shift phase in which all within-series intervals were 30 sec showed that the temporal intervals were controlling performance before the shift. Experiment 2 showed that interevent anticipation appears when all within-series intervals are either 10 min or 30 sec from the beginning of training, suggesting that the elimination of interevent anticipation in Experiment 1 was due to the differential cuing of runs by the temporal intervals rather than the particular interval duration. The overall findings suggest that the similarity of Run 2 and Run 3 performance termed interevent anticipation may be due to a failure to discriminate the ordinal position of runs within a series.

Undeprived rats on a choice between saccharin mixed with food and food alone prefer the saccharin-food mixture at the lower concentrations, .19% and .38%, and prefer food alone at concentrations exceeding .75%. However, whether hungry or undeprived, rats tested on single conditions eat food and saccharin-food equally. These data parallel those of studies using saccharin solutions and are interpreted in terms of sensory stimulation.

The effect of differential outcome expectancies on memory for temporal and nontemporal information was examined. Pigeons were trained to match short (2-sec) and long (8-sec) sample durations to red and green comparison stimuli, and vertical and horizontal lines to vertical and horizontal comparison stimuli. In Experiment 1, one differential outcome (DO) group received food for correct choices on short-sample trials, whereas another received food for correct choices on long-sample trials. On line-orientation trials, half of each DO group received food for correct responses following vertical samples, whereas the other half received food for correct responses following horizontal samples. Overall retention was greater in the DO groups than in a nondifferential (NDO) group that received either food or no food for correct responses on a random half of all trials. Furthermore, although the NDO group displayed a choose-short bias for temporal samples, both DO groups displayed equivalent biases to select the comparison stimulus associated with food. In Experiment 2, differential outcome expectancies were extinguished off-baseline. Subsequently, in the first nondifferential outcome test session, the. DO groups performed less, accurately than the NDO group. These findings indicate that temporal samples are not retrospectively and analogically coded when they are differentially associated with food and no food. Instead, they are remembered in terms of the corresponding outcome expectancies.