The effect of lesions in the mesencephalic reticular formation upon conditioned avoidance responses in rat: I. Lesions of the nucleus commissurae posterioris.

After bilateral symmetric lesions of the n. commissurae posterioris (NCP) hooded rats of the Long-Evans strain did not acquire the conditioned avoidance response (CAR) in a jumping test. The NCP rats were rather unable to find out the escape possibility on the rod hanging in the middle of the apparatus. They did not learn the context between the conditioned stimulus (CS), the unconditioned stimulus (UCS) and the jump. In an Y-maze formation of CAR was even better in NCP rats than in sham-operated controls because of their shorter reaction times and their higher running speed. Their brightness discrimination, on the other hand, was significantly impaired and erroneous runs were only reduced to a low insignificant amount after more than 100 trials. Lesions in the NCP had no severe effects upon preoperatively learnt CARs, but brightness discrimination was still impaired. From these results it may be concluded that NCP plays a role in the formation of goal-directed behaviour depending on external cues and in actual visually guided behaviour.