Lateral coherence of the electrocorticogram: a new measure of brain synchrony

As one test of the idea that compound field potentials in higher centers have a fine structure, the horizontal extent of coherence (C) was studied on the brain surface, with many closely spaced semimicroelectrodes in rabbits and rats. On the average C tends to fall with distance (D) in the 0.5-10 mm range; apart from driven rhythms, C usually falls to noise level at D greater than 10 mm. A useful measure is D (mm) where C has fallen to 0.5 (DC = 0.5); for most F bands within the range 1-50 Hz this is usually 2.5-5 mm, averaging over the neocortex in both species. Synchrony for neural tissue should mean a degree of congruence in a population (not a 2-point correlation); decline of C with D can measure synchrony by reflecting the volume at or above a specified C. Sleeping and waking mammals, an invertebrate (Aplysia), a ray, and a reptile were compared in degrees of synchrony; this cannot be judged by eye and is found sometimes hardly different between high-voltage-slow and low-voltage-fast states. Aplysia has negligible synchrony; the ray and lizard may be intermediate. C maps show patchiness superimposed on the general decline with D; no obvious pattern between parts of the cortex is consistent among individuals. Factors influencing variance, repeatability and extent of significant C are assessed. Brain size, passive spread, electrode size (at least 1-100 microns) and closeness of contact with pia mater rarely contribute materially, even within 1 mm. C commonly falls moderately with frequency (F) from a maximum between 1 and 8 Hz, usually without consistent peaks except for special cases of driving rhythms, such as theta. Intracortically the distribution of C is more local, both radially and horizontally. Although it was not possible to say when the two electrodes were in the same lamina, most laminae are highly coherent with all others. One or two sharp radial discontinuities in C are common, often but not consistently in the middle layers. C shows no simple relation to distance. In spite of the prevalent high coherence between laminae, radially, C varies widely horizontally from low to high in the 0.1-1 mm range. C is regarded as one aspect of cooperativity in a cellular dynamic system with fine structure in the fractional millimeter and second range; so far we are observing it with severely distorting smoothing procedures.