Temporal fluctuations in coherence of brain waves.

As a measure of dynamical structure, short-term fluctuations of coherence between 0.3 and 100 Hz in the electroencephalogram (EEG) of humans were studied from recordings made by chronic subdural macroelectrodes 5-10 mm apart, on temporal, frontal, and parietal lobes, and from intracranial probes deep in the temporal lobe, including the hippocampus, during sleep, alert, and seizure states. The time series of coherence between adjacent sites calculated every second or less often varies widely in stability over time; sometimes it is stable for half a minute or more. Within 2-min samples, coherence commonly fluctuates by a factor up to 2-3, in all bands, within the time scale of seconds to tens of seconds. The power spectrum of the time series of these fluctuations is broad, extending to 0.02 Hz or slower, and is weighted toward the slower frequencies; little power is faster than 0.5 Hz. Some records show conspicuous swings with a preferred duration of 5-15s, either irregularly or quasirhythmically with a broad peak around 0.1 Hz. Periodicity is not statistically significant in most records. In our sampling, we have not found a consistent difference between lobes of the brain, subdural and depth electrodes, or sleeping and waking states. Seizures generally raise the mean coherence in all frequencies and may reduce the fluctuations by a ceiling effect. The coherence time series of different bands is positively correlated (0.45 overall); significant nonindependence extends for at least two octaves. Coherence fluctuations are quite local; the time series of adjacent electrodes is correlated with that of the nearest neighbor pairs (10 mm) to a coefficient averaging approximately 0.4, falling to approximately 0.2 for neighbors-but-one (20 mm) and to < 0.1 for neighbors-but-two (30 mm). The evidence indicates fine structure in time and space, a dynamic and local determination of this measure of cooperativity. Widely separated frequencies tending to fluctuate together exclude independent oscillators as the general or usual basis of the EEG, although a few rhythms are well known under special conditions. Broad-band events may be the more usual generators. Loci only a few millimeters apart can fluctuate widely in seconds, either in parallel or independently. Scalp EEG coherence cannot be predicted from subdural or deep recordings, or vice versa, and intracortical microelectrodes show still greater coherence fluctuation in space and time. Widely used computations of chaos and dimensionality made upon data from scalp or even subdural or depth electrodes, even when reproducible in successive samples, cannot be considered representative of the brain or the given structure or brain state but only of the scale or view (receptive field) of the electrodes used. Relevant to the evolution of more complex brains, which is an outstanding fact of animal evolution, we believe that measures of cooperativity are likely to be among the dynamic features by which major evolutionary grades of brains differ. IMAGES:

本研究以动态结构度量为目标，对人类脑电图（electroencephalogram, EEG）中0.3~100 Hz频段内的相干性短期波动展开分析。数据采集采用间距5~10 mm的慢性硬膜下宏电极，覆盖颞叶、额叶与顶叶；同时使用置入颞叶深部（包括海马体）的颅内探针，采集睡眠、清醒及癫痫发作状态下的脑电信号。
相邻采集位点间的相干性时间序列以每秒或更短的间隔计算，其稳定性随时间呈现显著波动；部分时段该序列可维持半分钟以上的稳定状态。在2分钟的采样窗口内，全频段相干性通常会在数秒至数十秒的时间尺度内出现2~3倍的波动。
此类波动时间序列的功率谱分布较宽，下限可至0.02 Hz甚至更低，且整体偏向低频段；高于0.5 Hz的功率成分占比极低。部分记录可观察到显著的波动周期，其偏好时长为5~15秒，或以准节律形式呈现不规则波动，在0.1 Hz附近存在宽峰。多数记录的周期性未达到统计学显著性水平。
本次采样未发现脑叶、硬膜下与深部电极，或睡眠与清醒状态下的相干性波动存在一致性差异。癫痫发作通常会提升全频段的平均相干性，并可能通过上限效应降低波动幅度。
不同频段的相干性时间序列呈正相关关系（整体相关系数为0.45）；显著的非独立性至少覆盖两个倍频程范围。相干性波动具有显著的局域性：间距10 mm的相邻电极的相干性时间序列平均相关系数约为0.4；间距20 mm（间隔一个电极）的序列相关系数降至约0.2，间距30 mm（间隔两个电极）的序列相关系数则低于0.1。
上述结果表明，该协同性度量在时间与空间维度上均存在精细结构，其动态特性具有局域性。尽管在特定条件下存在少数已知脑电节律，但不同频率成分倾向于同步波动的现象，表明独立振荡器并非脑电图产生的普遍基础，宽频段事件或为更常见的信号来源。
间距仅数毫米的采集位点可在数秒内出现显著波动，且波动模式可呈同步或独立状态。无法通过硬膜下或深部电极的记录预测头皮脑电图的相干性，反之亦然；而皮层内微电极的记录则显示出更强的时空相干性波动。
目前广泛应用于头皮、甚至硬膜下或深部电极脑电数据的混沌与维度分析，即便在重复采样中可复现，也仅能反映所用电极的观测尺度或感受野（receptive field），无法代表全脑、特定脑区或脑状态的真实特性。
动物演化的一个显著特征是大脑复杂度的逐步提升，本研究认为，协同性度量或为区分不同进化层级大脑的动态特征之一。IMAGES: