Coherence of compound field potentials reveals discontinuities in the CA1-subiculum of the hippocampus in freely-moving rats

Abstract

The ongoing micro-electroencephalogram was recorded with a chronically implanted comb-like array of 16 tungsten semi-microelectrodes 0.2 or 0.25 mm apart, spanning CA1 strata oriens, pyramidale and radiatum and into subiculum, in four behavioral states: walking, standing still, paradoxical and slow wave sleep and under scopolamine. Power, phase and coherence spectra were computed, the latter two for each of the 120 pairs, in frequency bands from 1 to 64 Hz. (1) Coherence is high for all frequencies within the same subfield, e.g. stratum radiatum, but falls with distance. Theta frequency (8 Hz), when prominent and widespread (during “theta states” walking and paradoxical sleep), shows the most widespread synchrony: coherence falls slowly, from 1.0 at 0.2mm to 0.7 at c. 2 mm longitudinally within stratum radiatum; all other frequencies fall two or three times faster. (2) An abrupt drop in coherence occurs across field borders (CA1-subiculum) and between stratum oriens and radiatum, across a line just under stratum pyramidale, between high coherence regions on each side of the coherence discontinuity. A less extreme drop occurs in stratum radiatum 0.4 mm from the subiculum border, without obvious histological correlate. The discontinuities in coherence are stable through all four behavioral states as well as under scopolamine. (3) Phase profiles diagonally across CA1 and into subiculum show abrupt, local shifts of phase (up to 125°) at these same levels. No gradual shift reaching 180° (phase reversal) occurs in the span of loci examined. (4) The theta power peak in theta states is not necessarily due to additional energy in that band; in some conditions it is mainly due to reduced power in other frequencies. Root mean square voltage is generally less in the high theta (“synchronized”) than in the non-theta states. Only the theta peak correlates with a peak in coherence. (5) Significant microstructure in the dynamics of neuronal cooperativity distinguishes behavioral states and regions of the hippocampal cortex.