The mesencephalic reticular formation as a link in the cortical control of exploratory and goal-directed behaviour.

The general accepted concept about the MRF as an unspecific ascending activating system concerns only one of its multiple functions. Investigations on more than 100 hooded rats of the Long-Evans strain with small bilateral symmetric lesions in dorsal, central and ventral subnuclei of the MRF brought out the following results pointing to further important functions: 1. Each lesion type produced a different syndrome of parameter changes of the spontaneous open field behavior with some common tendencies of reduced ambulatory and exploratory activities. 2. Visual placing responses were strongly reduced or totally abolished after lesions without tendencies of recovery. 3. Changes of locomotion and muscular tonus were quite different or even opposite in dorsal, central and ventral types of lesions. 4. In four tasks of postoperative active avoidance acquisition or retention and performance of preoperatively learned tasks the impairments were different related to the lesion type including a different loss in brightness discrimination. The results support the hypothesis that MRF subdivisions participate differently in information selection, tuning and coupling information with goal directed movements of different type. Lesions severely disturb the proper use of some information for a cue, especially visual cues when they are in the anterior part of the mesencephalon.