Neuronal activity in the midbrain reticular formation related to behavioral habituation and sensitization

Single units of the midbrain reticular formation (MRF) were recorded in the active rat (N = 10) during the presentation of repetitive trains of clicks or flashes. The movement and orienting behavior elicited by the stimuli, measured by direct observation and in terms of electrical disturbances in the recording cable, showed habituation in 14 cases, sensitization in six cases, and no decrement in nine cases. Of the 18 MRF units recorded during at least one of these behavioral conditions, seven were classed as congruent, i.e., they showed activity that paralleled the behavioral response. The majority of MRF units were behaviorally incongruent; in the most frequently observed case, their activity failed to change reliably when behavior habituated. Although the activity of congruent units generally showed moderate correlations with behavior that followed stimulation, disassociations between the two responses were commonly found. While the data are consistent with a role for the MRF in behavioral habituation and sensitization, they provide little evidence that the region is closely linked to the systems in direct control of behavior.