Time-frequency analysis of the EEG gamma-activity was based on Morlet wavelets using the freely distributed FieldTrip (http://fieldtrip.fcdonders.nl/) software in Matlab (http://www.mathworks.com/products/matlab/), as in our prior work (Roach and Mathalon, 2008). The Morlet wavelet has a Gaussian shape that is defined by a ratio (σf = f/C) and a wavelet duration (6σt), where f is the center frequency and σt = 1/(2πσf). In a classic wavelet analysis, C is a constant (e.g., 7), ensuring an equal number of cycles in the mother wavelet for each frequency. In this approach, as the frequency (f) increases, the spectral bandwidth (6σf) increases. This method was used to decompose single trial time-frequency values between 20 and 60 Hz for the central nine electrodes.
