The effect of lesions in the mesencephalic reticular formation upon conditioned avoidance responses in rat. III. Lesions of the area subcuneiformis.

Two groups of 6 month old male hooded rats of the Long-Evans strain with bilateral symmetrical lesions in the anterior part of the mesencephalic area subcuneiformis (CUS) were compared with two sham-operated control groups (each group n = 8). CUS rats were absolutely unable to acquire a conditioned avoidance response (CAR) in a jump test; they even did not find out the escape possibility. Postoperative CAR acquisition of CUS rats in a horizontal alley was not different from controls, but their running speed was decreased (p less than 0.005). Lesioned rat's postoperative acquisition of brightness-discrimination was evidently disturbed and their error rate was still enhanced after 120 trials (p less than 0.005). Their reaction times of leaving the start box, however, were significantly lowered. The postoperative retention of preoperatively learnt CAR was very low. Relearning was possible in the Y-maze at lower level, but impossible in the jump test. The error rate decreased very slowly in the postoperative testing, whereas controls performed the brightness discrimination without errors as before. The unconditioned escape, however, was performed as correctly by CUS rats in the jump test as in the preoperative trials but rather slowly. The pattern of the behavioural syndrome after CUS lesions and the deficits in avoidance learning are possibly caused by a loss in sensory-motor coupling and a weakened ability to select visual, olfactory or painful stimuli which are important for the regulation of certain forms of behaviour.