Th. Bullock et al., EEG COHERENCE HAS STRUCTURE IN THE MILLIMETER DOMAIN - SUBDURAL AND HIPPOCAMPAL RECORDINGS FROM EPILEPTIC PATIENTS, Electroencephalography and clinical neurophysiology, 95(3), 1995, pp. 161-177
Subdural recordings from 8 patients and depth recordings from 3 patien
ts via rows of electrodes with 5-10 mm spacing were searched for signs
of significant local differentiation of coherence calculated between
all possible pairs of loci. EEG samples of 2-4 min were taken during 4
states: alertness, stage 2-3 sleep, light surgical anesthesia permitt
ing the patient to respond to questions and electrical seizures. Coher
ence was computed for all frequencies from 1 to 50 Hz or 0.3-100 Hz; f
or comparisons the mean coherence over each of 6 or 7 narrower bands b
etween 2 and 70 Hz was used. Whereas the literature supports the view
that EEG coherence is usually substantial over many centimeters, the h
ypothesis here tested and found to be well above stochastic expectatio
ns - is that significant structure occurs in the millimeter domain for
EEG recorded subdurally or within the brain. In both the subdural sur
face samples and those from temporal lobe depth electrode arrays coher
ence declines with distance between electrodes of the pair, on the ave
rage quite severely in millimeters. This is nearly the same for all fr
equency bands. For middle bands like 8-13 and 13-20 Hz, mean coherence
typically declines most steeply in the first 10 mm, from values indis
tinguishable from 1.0 at < 0.5 mm distance to 0.5 at 5-10 mm and to 0.
25 in another 10-20 mm in the subdural surface data. Temporal lobe dep
th estimates decline about half as fast; coherence greater than or equ
al to 0.5 extends for 9-20 mm and greater than or equal to 0.25 for an
other 20-35 mm. Low frequency bands (1-5, 5-8 Hz) usually fall slightl
y more slowly than high frequency bands (20-35, 35-50 Hz) but the diff
erence is small and variance large. The steepness of decline with dist
ance in humans is significantly but only slightly smaller than that we
reported earlier for the rabbit and rat, averaging less than one half
. Local coherence, for individual pairs of loci, shows differentiation
in the millimeter range, i.e., nearest neighbor pairs may be locally
well above or below average and this is sustained over minutes. Local
highs and lows tend to be similar for widely different frequency bands
. Coherence varies quite independently of power, although they are som
etimes correlated. Regional differentiation is statistically significa
nt in average coherence among pairs of loci on temporal vs. frontal co
rtex or lateral frontal vs. subfrontal strips in the same patient, but
such differences are usually small. We could not test how consistent
they are over hours or between patients. Differences between left and
right hemispheres, whether symmetrical pairs or pooled from two or mor
e lobes on each side, can be quite large; in our patients the right si
de is usually higher, especially in the waking state. Brain state has
a large influence. Slow wave sleep usually shows slightly more coheren
ce at each distance, in all bands, compared to the waking EEG, but not
consistently. Coherence at a given distance or its rate of decline wi
th distance is a more direct measure of synchrony than naked-eye ''syn
chronization'' which is dominated by the power spectrum. Among the ran
ge of EEG states classified as seizures, coherence varies widely but i
t averages higher by 0.05-0.2 than in pre-ictal states, usually in all
frequencies when computed over the whole seizure but much more in the
higher bands during the height of the electrical paroxysm. The findin
gs point to still finer structure and more variance with closer spacin
g of electrodes. They could not predict the known large scale coherenc
e between scalp electrodes, but are not in conflict with them. Scalp r
ecording blurs the finer spatial structure, but reveals macrostructure
missed by subdural and depth recording with limited numbers of channe
ls. The strong tendency for correlated fluctuations across frequency b
ands is contrary to expectation from the common model of independent o
scillators.