Suppression of γ-Oscillations in the Dorsolateral Prefrontal Cortex following Long Interval Cortical Inhibition: A TMS-EEG Study

Gamma (gamma)-oscillations (30-50 Hz) represent important electrophysiological measures, which are generated through the execution of higher order cognitive tasks (eg, working memory) in the dorsolateral prefrontal cortex (DLPFC). By contrast, cortical inhibition (CI) refers to a neurophysiological process in which GABAergic inhibitory interneurons selectively suppress the activation of other neurons in the cortex. Recently, abnormalities in both CI and gamma-oscillations have been associated with various neuropsychiatric disorders including schizophrenia. Animal research suggests that suppression of gamma-oscillations is, in part, mediated through GABAergic inhibitory neurotransmission. However, no such evidence has been demonstrated in human, largely because of technological limitations. Recently, we reported on novel methods permitting the recording of CI from the DLPFC through transcranial magnetic stimulation (TMS) combined with electroencephalography (EEG). The aim of this study was to examine the effects of GABAergic inhibitory neurotransmission on gamma-oscillations by combining TMS with EEG. Long interval cortical inhibition (LICI), a paired TMS paradigm, was used to index GABA(B) receptor mediated inhibitory neurotransmission in the motor cortex and DLPFC of healthy individuals. g- Oscillations were significantly inhibited by LICI (38.1 +/- 26.5%; p <= 0.013) in the DLPFC but not in the motor cortex. These results provide neurophysiological evidence to demonstrate gamma-oscillations are inhibited by LICI in the DLPFC but not in the motor cortex. Such specificity suggests that the modulation of gamma-oscillations may represent an important neurophysiological process that may, in part, be responsible for optimal DLPFC functioning in healthy human subjects.