Effects of Stimulus Complexity of the Auditory Evoked Response

Abstract

Sixty-four subjects were run in an experiment to determine the effect of stimulus complexity on the auditory evoked (Vertex) response. The 760-msec stimulus consisted of 10 bursts of pure tone with 40-msec burst durations and interburst intervals. Stimulus complexity was varied by utilizing tone bursts of different frequencies within the stimulus. Four levels of complexity (1, 2, 5, and 10 frequencies per stimulus) were used. Tone order in the multifrequency conditions was quasirandom, with each frequency represented an equal number of times per stimulus. The frequencies were balanced for loudness. In each condition, 500 stimuli were presented with an interstimulus interval of 1.2 sec. Recording of the responses was by electrodes placed at vertex and mastoid, with a forehead ground. The 500 responses were broken down in a signal averager into 20 averages of 25 responses each. These 20 waveforms were then averaged across subjects within conditions. Results indicate that increasing stimulus complexity produces an exaggerated negative excursion at about 350 msec after stimulus onset. A more rapid habituation of the N1−P2 complex is also present as compared to simpler stimuli.