Electrocorticographic Coherence Patterns

Abstract

The availability of implantable subdural electrode arrays has made systematic studies of electrocorticographic (ECoG) coherence possible. Studies of coherence patterns recorded directly from human cortex are reviewed along with the presentation of original human clinical data, which reveal reliable and characteristic patterns of coherence. A data-driven technique for discriminating between reliable and unreliable coherence and phase values is described and used to reveal the relationship between coherence and cortical anatomy, such as in the region of the central sulcus, where low phase coherence declines and high phase-shifted coherence increases. Analysis of coherence magnitude and phase makes it possible to determine which signals likely arise from the cortical surface, and which arise from the depths of a sulcus. Alterations in coherence patterns caused by tumors or epilepsy are described and may be used to identify normal and pathological functional relationships between distant cortical areas. Some electrophysiologic/pathologic correlations indicate at least two types of epileptic abnormality, implying a sequence in breakdown of epileptic tissue. The relationship between coherence patterns and behavior and cognition is introduced and compared to similar studies of single-unit binding in animals.

Cortical abnormalities in epilepsy revealed by local EEG synchrony

“…In those patients in whom the epileptogenic zone was covered by the subdural grid, there appeared to be an association between the epileptogenic zone and LH regions, however they were more likely to be adjacent than concordant, as demonstrated by Patients 1, 2, 4, 6, and 8. These findings are consistent with prior observations of elevated local coherence over tumors and epileptogenic areas observed during intraoperative corticography (Towle, Syed et al;Towle, Carder et al 1999), waveletbased synchrony in chronic ICEEG recordings of a patient with a large frontal tumor (Le Van Quyen, Soss et al 2005) and cross correlation of microelectrode recordings using a non-lesional cat model of seizures (Valentine, Teskey et al 2004). In the cases in which there was concordance between the epileptogenic zone and an LH region (Patients 5, 6, and 9), the epileptogenic zone (or the portion that was recorded from the subdural grid) was determined primarily from interictal activity rather than from seizure onsets.…”

“…Accordingly, abnormally enhanced synchrony may be expected to be apparent in the background EEG, apart from spike discharges, as it reflects a fundamental property of the underlying cortex. Indeed, coherence, a normalized frequency-dependent measure of correlation, has been shown to be increased over tumors and regions thought to be part of the epileptogenic zone (Towle, Syed et al 1998;Towle, Carder et al 1999;Zaveri, Williams et al 1999). Similarly, changes in phase synchrony, a related measured that has been used to detect EEG changes before seizure onset, also appear to be most prominent proximate to the epileptogenic zone (Lehnertz and Elger 1995;Le Van Quyen, Martinerie et al 2001;Chavez, Le Van Quyen et al 2003;Mormann, Andrzejak et al 2003;Le Van Quyen, Soss et al 2005),…”

Detecting and localizing the foci in human epileptic seizures

“…Previous observations involving different linear and non-linear measures [13,14,21,22,23] indicate that the values of the corresponding measures are altered over tumors and regions thought to be part of the epileptogenic zone. The regions of such a locally enhanced or suppressed activity are stable and have well defined edges.…”

The contribution of EEG coherence to the investigation of language

“…However, a general acceptance and increasing interest in EEG coherence analysis as a method to monitor frequency dependent large-scale synchronization during human intact and disturbed information processing was not established before the 1990s, and the number of studies on EEG coherence and cognitive information processing in healthy humans has exponentially increased during the past four years. Detailed reviews on methodical aspects of EEG coherence analysis are given in Shaw (1984), Challis and Kitney (1991), Schack, Grieszbach, Arnold, and Bolten (1995), Nunez et al (1997), ; on clinical aspects in Leoncani and Comi (1999), Towle, Carder, Khorasani, and Lindberg (1999) and on cognitive aspects in French and Beaumont (1984) and Petsche and Etlinger (1998).…”