A multi-channel correlation method detects traveling gamma-waves in monkey visual cortex

Correlations among simultaneously recorded signals are mostly analyzed pairwise and include temporal averaging. However, pairwise methods are not suitable for characterizing relationships among multiple channels for signals which vary temporally in an unpredictable way. Here we develop a time-resolved spatio-temporal correlation (STC) measure among simultaneously recorded signals. We demonstrate the capabilities of the method with artificial data sets and with multiple-channel recordings from striate cortex of awake monkeys. We concentrate on correlations in the gamma-frequency range (gamma: 30-90 Hz) because they were prominent in the analyzed recordings and gained high interest in the recent years due to their assumed role in associative processing, including perceptual binding. Former analyses of gamma-activities in visual cortex, using pairwise correlation methods, mostly revealed zero-delay correlation, indicating synchrony. In cat and monkey visual cortex this gamma-synchrony is restricted to 1.5-3.0 mm (half-height decline). However, our spatio-temporal correlation (STC)-method demonstrates for striate cortex from awake monkeys that gamma-synchrony is a local phenomenon of more global traveling plane waves that appear stimulus-induced at randomly varying orientations. These gamma-waves are coupled over much larger cortical distances (approximately 7 mm half-height decline) than the gamma-synchrony ranges obtained by pairwise correlation analyses from the same data. Our STC-method therefore suggests that the previously reported results of short-range and zero-delay correlations were often due to temporal averaging of traveling gamma-waves.