Neurophysiologic activity recorded from arrays of electrodes implanted over human cerebral cortex provides a unique opportunity to study the macroscopic functional organization of the brain. Such arrays (5-10 mm electrode spacing) are implanted for up to two weeks, according to the needs of the patients, in order to locate and characterize seizure foci in cases of intractable epilepsy. We have focused on the analysis of lateral coherence patterns over the cortical surface, which are related to several variables: 1) inter-electrode distance—coherence generally decreases with increasing distance, 2) frequency band—coherence is inversely related to frequency, being particularly low in the gamma band, 3) cortical anatomy—coherence is reduced when recorded across sulci, especially the Rolandic and Sylvian fissures, 4) pathological processes—neoplastic and epileptic cortical tissue reveal altered coherence patterns that are partially isolated from neurophysiologic signals arising from normal tissue, and 5) cognitive and behavioral manipulations—a subset of electrodes reveal changes in coherence when higher order cognitive tasks are performed. When calculations are made over 1-2 hour-long samples, the day-to-day variability of coherence is quite low, averaging about 2%. Calculations over shorter periods are more variable, and are dynamic in all frequency bands during the onset and evolution of a seizure. We have been especially interested in examining the phase relationships between different cortical areas in hopes of (a) characterizing relationships between multiple epileptic foci, and (b) detecting the organization of information flow in normal brain. When recorded using a distant reference, coherence tends to cluster around 0_ or 180_, suggesting the influence of volume conduction. Signals with 180_ phase relationships likely arise from within the depths of sulci. When the grand mean of the recordings is utilized as a reference, coherence values tend to approximate a normal distribution around 0_ phase, over a low coherence background, which appears uniform over the entire phase scale. When integrated with intraoperative photographs, CT scans or MRI volume reconstructions of cortex, coherence patterns can provide information about brain function and dysfunction, which can be relevant when planning cortical resections.