Auditory oddball paradigm
On each visit, a total of 1,400 auditory stimulus trials were presented in an oddball paradigm. The stimulus train comprised sinusoidal tones: 82% standard trials (1000 Hz, 50 ms), 6% frequency deviants (1200 Hz, 50 ms), 6% duration deviants (1000 Hz, 100 ms), and 6% combined frequency-duration deviants (1200 Hz, 100 ms) in random order generated by the random function in MATLAB 9.2.0. The inter-stimulus interval (ISI) was randomized between 500 and 600 ms for each trial of the standard and the deviant tones to prevent anticipation. The regularity imposed by the standards therefore was expected to mainly reflect features of standard tones [