The effects of a second stimulus on the auditory steady state response (ASSR) from the inferior colliculus of the chinchilla.

The auditory steady-state response (ASSR) is an auditory evoked potential which follows the envelope of the stimulus. One of the advantages of the ASSR is that multiple stimulation frequencies can be tested simultaneously. In experiment 1, we evaluated the effects of simultaneously presenting two separate stimuli on ASSR response amplitude. In experiment 2, we evaluated the effects of presenting two ASSR-generating stimuli monotically vs. dichotically, either ipsilaterally or contralaterally to the recording electrode. Recordings were made from the chinchilla inferior colliculi, in response to tonebursts, two-tones, or sinusoidally-amplitude modulated tones. We found that the addition of a second stimulus resulted in a reduction in ASSR response amplitude at moderate to high stimulus levels. The amount of amplitude reduction was typically larger in the monotic (e.g. approximately 50%) vs. dichotic condition (e.g. approximately 10-20%), regardless of whether the responses were recorded ipsilaterally or contralaterally to the ear of stimulus presentation. In conclusion, central as well as peripheral interactions contribute to the reduction in ASSR amplitude in response to the simultaneous presentation of multiple stimuli.