The effects of combined perirhinal and postrhinal damage on complex discrimination tasks

Abstract

Rats with combined lesions of the perirhinal (PER) and postrhinal (POR) cortices were trained on a complex discrimination in the simultaneous feature-positive and feature-negative discrimination task. In this task, a panel light (L) paired with an auditory stimulus determined whether a tone (T) or white noise (N) would be rewarded (+) or not rewarded (-). Thus, the light feature determined whether the target auditory stimuli were rewarded or not. In each session, trial types were LT+, T-, N+, and LN-. We had hypothesized that damage to the target regions would impair performance on this task. Acquisition was altered in the lesioned rats, but not in the predicted direction. Instead, lesioned rats exhibited significantly enhanced acquisition of the discrimination. Manipulation of intertrial intervals indicated that reduction of proactive interference did not explain the enhancement. Lesioned rats were not different from controls on a multiple-cued interval timing task, providing evidence that the enhancement does not extend to all types of discriminations and is not due to a deficit in timing. Other research shows that rats with PER lesions are impaired on similar tasks, thus the enhancement is likely due to the effects of POR damage. Normally in this task, context is thought to accrue inhibitory control over other cues. Without this inhibitory control, animals might be expected to learn the task more efficiently. Our conclusion is that deficits in processing contextual information underlie the enhanced acquisition observed in rats with combined PER and POR damage on this complex discrimination task.

Figure 1

Combined perirhinal (PER) and postrhinal (POR) lesions. Schematics of coronal sections through the rat brain are shown with the largest lesion in gray and the smallest in black. Lesions were relatively small but distributed along the rostrocaudal extent of the PER and POR.

Figure 2

FPFN task results. A. Feature-positive (excitatory) component of the discrimination. B. Feature-negative (inhibitory) component of the discrimination. C. Discrimination index (reinforced response rate – nonreinforced response rate). Data are shown in responses per minute (rpm). Control and lesioned rats learned both the excitatory and the inhibitory components of the discrimination, but the lesioned rats showed significantly enhanced learning of both components. Moreover, by the end of testing the lesioned rats had learned the more difficult inhibitory component as well as they learned the excitatory component.

Figure 3

FPFN task results of light only trials. Responses to the unreinforced light only trials did not differ between control and lesioned groups, suggesting that the light component of the compound cues was processed similarly in both groups.

Figure 4

Response rates for the MCI task plotted against time. Rates are shown for the control (A) and lesioned (B) groups. There were no differences between groups in the ability to associate a discrete cue with a duration in this concurrent discrimination task using cues similar to those in the FPFN task except that there were no compound cues.