Time-frequency analyses of responses to standard and deviant tones
ERSP analysis assessed dynamic changes in spectral bands after stimulus onset. The single-trial epochs used in the ERP analyses were inputted into the ERSP function ‘newtimef’ implemented with EEGLAB. The time limits of the trial epochs were [−100, 500] ms referenced to the stimulus onset, with sampling rate 250 Hz. The frequency limits were set as [4, 25] Hz with 0.5 Hz frequency resolution. Baseline spectra were calculated from the pre-stimulus interval [−50, 0] ms. Each epoch was divided into overlapping Hanning windows—range 128 ms (32 samples), step-size 4 ms. Spectra of each window were normalized with reference to the baseline and assigned to the center point of the window. Time limits of the output ERSP were [−36, 436] ms referenced to the stimulus onset. For each stimulus condition, the normalized transforms of single-trial epochs were then averaged and recorded as log values in a time-by-frequency ERSP matrix. Grand average ERSP under each stimulus condition was then calculated by averaging the time-by-frequency ERSP matrix across participants grouped by the ASD/TD and placebo/drug conditions.Grand average ERSP outcomes of participants grouped by the placebo/drug condition were calculated by averaging ERSP matrices in each group. The main event-related perturbations at the grand average level occurred within the theta and the alpha band [4, 12] Hz. Therefore, for each participant visit, ERSP values in the [4, 12] Hz band were summed up to obtain the evoked power waveform. The mean of the evoked power waveform within the specified time window was extracted as a single-point measurement of event-related spectral dynamics. The measurement windows used for the four stimulus conditions were the same as the N1 settings in the ERP analyses—([200, 300] ms for the duration deviant, [100, 200] ms for others). The ERP and ERSP procedures were applied for post hoc comparisons between responses to pre-deviant standards and post-deviant standards (see 