High frequency EEG ripples in primate entorhinal cortex.
1. ARL NSMA
2. CNPRC
Recordings of EEG and unit activity were made, using chronically implanted 12-tetrode "hyperdrives", from the posterior entorhinal cortex (EC) of two rhesus macaques, which corresponds to the medial EC of rats. In one monkey the recording locations were verified histologically; in the other electrode placements are tentative pending histology. In both monkeys, high-frequency EEG oscillations, with most of the spectral power above 200 Hz, lasting usually 300-500 msec, were recorded at multiple EC locations. These bore a strong resemblance to "high frequency ripples" (HFRs) described in the EC of epileptic human patients, and theorized to be associated with epilepsy [Bragin et al., Epilepsia 40:127, 1999]; but there were no obvious signs of seizure-related pathology in the monkeys. As in the human patients, the monkey EC-HFRs were not associated with hippocampal sharp waves. The monkey HFRs had strong behavioral correlates, appearing rarely when the monkeys were sleeping or passive, but frequently when they were actively engaged in behavioral tasks. HFRs were particularly associated with reward, and, when tested in one monkey, were reliably evoked by giving unexpected and unsignaled Tang via a sipper tube, appearing 1-2 sec after reward delivery. HFRs appeared to be specific to the EC, and were not seen in neighboring cortical regions, including TL and TH. HFRs always occurred simultaneously on all electrodes that showed them. Many EC neurons showed very strong activation during the HFRs. In some cases the unit responses were oscillatory with a frequency of about 4 Hz, and in a few cases the unit activity commenced several hundred msec before the HFR appeared in the EEG. These observations, taken together, suggest that the primate posterior EC undergoes very robust global activation, in a consistent pattern, shortly after "notable" events occur. Such activation might reflect a contribution to the encoding of events in the hippocampal entorhinal system. It is curious, however, that similar phenomena have not been described in the rat EC, except in cases where seizure-inducing drugs had previously been administered.
Supported by: AG003376
Keywords: hippocampus, rhythm, oscillations, memory, epilepsy