Effects of Frequency Modulation on Auditory Averaged Evoked Response

Frequency modulated signals with linear up-ramps of various durations were used to evoke electroencephalic responses in two normal hearing Ss. Stimuli were presented randomly to either the right Ni peak latency ear, the left ear, or binaurally. The Ss were seated in a sound attenuated chamber in the light with eyes open. Electroencephalic activity was recorded from an electrode on the vertex referred to the left mastoid with a ground at the forehead midline. The N1-P2 amplitude was the most prominent and it decreased as the ramp durations increased from 25 to 2000 msec. It became progressively smaller as the frequency region was increased from 0.5 kHz to 2.0 kHz and as the intensity was decreased from 60 dB to 40 dB SL. The N1 peak latency increased with an increase in ramp duration and a change from binaural to monaural stimulation. The view that transitions between two frequencies activate additional neural units was maintained. The similarity of the stimuli to speech formant transitions makes the AER evoked by such ramps a useful approach to objective speech audiometry.