Oscillatory activity in primate reticular formation

Abstract

Background Piper rhythmicity (35–60 Hz) has been reported in EMGs since 1907. These oscillations are often assumed to originate in the cortex, as they are coherent with sensorimotor EEG (Brown et al., 1998). However, we have recently seen similar oscillations in a slice preparation of primate brainstem reticular formation. These oscillations were decreased by the GABA-A antagonist bicuculline, and completely suppressed by the GAP junction blocker carbenoxolone, suggesting generation by a complex intrinsic network. Given our recent work showing the importance of the reticular formation for recovery after corticospinal tract lesion (Zaaimi et al., 2012); we were interested in whether such oscillations might become more visible during functional recovery. Methods Macaque monkeys were trained to pull a lever, which opened a door allowing them to retrieve a food reward. In two animals, after training we made a unilateral lesion of the pyramidal tract by thermocoagulation, and allowed recovery, which reached plateau after ∼ 4 months. Results The recovery differed between the two lesioned monkeys: one was moderately impaired and could still take the food with the paretic hand, whilst the second lost the ability to perform precision grip but was still able to pull a lever. Recordings were then made of local field potential (LFP) from the reticular formation and EMG. Recordings from one unlesioned animal served as a control. Conclusions In the control and in the less impaired monkeys reticulomuscular coherence was observed in the beta band (20–30 Hz); this was not seen in the more impaired monkey. By contrast, the more impaired animal showed ∼ 70 Hz oscillations in reticular formation LFP around the time of movement. These preliminary results suggest that intrinsic rhythmicity around 40–60 Hz is present in the reticular formation and could be unmasked after lesion of pyramidal tract.