Figure 15. Local and long-distance synchronization in the gamma frequency range.
(A) Under certain conditions in vitro LFP gamma may exhibit a sharp peak in the power spectrum. In this case, gamma rhythm was induced pharmacologically by metabotropic gluatamte receptor activation in a rat hippocampal slice where fast synaptic excitations mediated by AMPA and NMDA receptors were blocked. (B) By contrast, in vivo LFP oscillations are typically manifested by small peaks superimposed on high powers at low frequencies (reflecting slow fluctuations or drifts over time). Data were obtained in a single mice during wheel running (red), REM sleep (blue) and slow-wave sleep (SWS, black). (C) Power spectra of intracellularly recorded synaptic potentials recorded from a pyramidal neuron of ferret in vivo. Membrane fluctuations are dominated by inhibitory postsynaptic potentials near 0 mV (blue), and by excitatory synaptic potentials near −80 mV (red). The peak in the gamma band is more significant at 0 mV, suggesting a prominent role for synaptic inhibition in rhythmic fluctuations. (D) Coherence between two cortical areas of alert monkeys in cross-modal information processing. LFPs were measured from the auditory cortex and the superior temporal sulcus (STS), an association area involved with face processing and cross-modal integration. Voice: trials when a naturalistic sound (monkey’s vocalization) was played; Face: trials when the corresponding mouth movement was shown visually; face+voice: trials when both stimuli were presented concurrently. Left: LFP power spectrum displays a peak in the gamma frequency range for both the auditory cortex and STS (red) in the face+voice condition, but there is no significant gamma-band peak in the auditory cortex with voice alone (green). Middle: inter-areal coherence displays a maximum in the gamma-band correlated with cross-modal integration (red), but not when face (blue) or voice (green) was presented alone. Right: Time-frequency cross-areal coherence as a function of time in the face+voice condition, showing coherence at gamma interspersed with theta transiently (for about 300 ms). (E) Coherence in V4 during sustained stimulation from behaving monkeys in an attention task. Red: attended condition; grey: non-attended condition. Left: LFP power spectrum from area V4. Middle: spike-to-field coherence (between multiunit activity and LFP). Right: spike-to-spike coherence between two simultaneously recorded multiunit spike trains. SSC (defined between 0 and 1) is less than 5%. (A) reproduced with permission from [1053], (B) from [148], (C) from [423], (D) from [349] (with left panel kindly produced by Dr. A Ghazanfar), (E) from [315].