THE EFFECTS OF FRONTAL CORTICAL-LESIONS ON REMEMBERING DEPEND ON THE PROCEDURAL DEMANDS OF TASKS PERFORMED IN THE RADIAL-ARM MAZE

We trained 24 rats to perform an eight-arm radial maze task and then a ssigned them with a matching procedure to one of three treatments: sha m surgery or lesions of the projection areas of the mediodorsal thalam ic nucleus (MDn) in the medial wall (MW) or in both the MW and rhinal sulcal (RS) areas of frontal cortex. After recovery we trained the rat s to perform six tasks, beginning with the standard eight-aim task, fo llowed by two versions of a four forced choice procedure, and then thr ee versions of a two-choice delayed-nonmatching-to-sample (DNMTS) task . The two lesion groups performed comparably on all tasks, showing tha t impairments were not exacerbated by extension of the MW lesion to in clude all cortical areas innervated by MDn. As in previous studies, fr ontal animals performed the radial maze task poorly immediately after surgery but improved with subsequent training. Controlling the order o f the arm entries by opening the first four gates in a random sequence had little effect on performance, although frontal animals were impai red when lengthy delays (5 or 15 min) were imposed after the last of t he four forced entries. Frontal animals were not impaired on two-choic e DNMTS when the arms used for training were selected at random from t he eight alternatives on a trial by trial basis, even when visual cues were eliminated by darkening the room and covering the maze. Frontal animals were significantly impaired when the selection of sample and c hoice arms was limited to the same two alternatives on every trial. Th is finding may explain the reported sensitivity of DNMTS to the effect s of frontal lesions when training is carried out in operant chambers. (C) 1997 Elsevier Science B.V.