THE ROLE OF THE PEDUNCULOPONTINE TEGMENTAL NUCLEUS IN RELATION TO CONDITIONED MOTOR-PERFORMANCE IN THE CAT-I - CONTEXT-DEPENDENT AND REINFORCEMENT-RELATED SINGLE-UNIT ACTIVITY

The activity of the pedunculopontine tegmental nucleus (PPTg) neurons was recorded in three unrestrained cats operantly conditioned to perfo rm a lever-release movement. The movement had to be initiated either r apidly after a (click) stimulus in a simple reaction-time paradigm or had to be delayed after the same stimulus in trials identified by a to ne cue. Successful trials were rewarded by a food pellet. A total of 1 07 neurons were recorded with microelectrodes. Brief spike neurons (me an duration: 0.7 ms) and broad spike neurons (mean duration: 2 ms) pre sumed to be cholinergic were detected. Of the 73 neurons localized in the PPTg area, 53 had brief spikes and 20 broad spikes. Changes in act ivity most commonly occurred very early after the stimulus or during t he reinforcement process. Most neurons with brief spikes exhibited ver y early excitation after the stimulus and reinforcement-related activi ty. These neurons had a mean activity of 23.7 impulses/s in the period preceding the stimulus. The onset of activation after the stimulus ha d a latency of 8.6 +/- 6.9 ms (mean +/- SD), with a range of 4-35 ms. In trials where the movement had to be delayed after the stimulus, the early activation disappeared or was considerably reduced, showing tha t it was context-dependent. A small proportion of neurons with brief s pikes initially decreased activity after the stimulus, but with a late ncy >9 ms. All the neurons with broad spikes, except one, had reinforc ement-related activity. Half of them showed exclusively reinforcement- related activity, the other half also early activation after the stimu lus. These neurons were about half as active in the period preceding t he stimulus occurrence than the neurons with brief spikes. The early c ontext-dependent activation is discussed in relation to the excitatory projection of PPTg neurons an the subthalamic nucleus. The reinforcem ent-related activity, preferentially evidenced in broad spike neurons presumed to be cholinergic, is speculated to be associated with cholin ergic projection of PPTg neurons to the dopaminergic neurons of the su bstantia nigra. Finally, the role of PPTg in the ongoing control of mo tor performance and reinforcement processes is discussed in relation t o the basal ganglia circuitry.