THE ENVELOPE FOLLOWING RESPONSE (EFR) IN THE MONGOLIAN GERBIL TO SINUSOIDALLY AMPLITUDE-MODULATED SIGNALS IN THE PRESENCE OF SIMULTANEOUSLY GATED PURE-TONES

The envelope following response (EFR) is an auditory-evoked potential recorded from the scalp which is elicited by long duration, amplitude-modulated stimuli. In this paper, the results of a series of experiments exploring the behavior of the EFR elicited with sinusoidally amplitude modulated (SAM) tones in the presence of simultaneously gated, continuous, pure-tone interfering signals of varying intensity are reported. Probe stimuli consisted of SAM tones with carriers ranging in frequency from 800 Hz-4 kHz, modulated at frequencies between 30-150 Hz. Probe signals were presented at intensities between 50 and 75 dB pSPL. Pure-tone interfering signals consisted of frequencies between 100 Hz and 10 kHz and ranged in intensity from -10 to +20 dB re: the probe. In these experiments a maximum reduction in the response to the probe tone, measured at the probe modulation frequency, appeared as a sharp peak within a narrow frequency band above the frequency of the probe carrier and a broader region of reduced response extending to higher frequencies. This reduction in response was asymmetrical, spreading more to high than to low frequencies. With an increase in the intensity of the interfering signal the maximum reduction of the response increased in a saturating, monotonic fashion with a concomitant broadening of the frequency region affected. The obtained interference response pattern may be attributable to both ''synchrony capture'' (i.e., capture of the EFR of the system by envelope components arising due to the interaction of probe and interfering signals) and ''synchrony suppression'' (i.e., a reduction in the synchronized response from neurons excited by the probe in the presence of the added interfering tone). It appears that the EFR to SAM stimuli of low to moderate intensity arose primarily from neuronal populations tuned to frequencies at or above the probe f(c). The results of the present study suggest that at low intensity levels SAM signals are indeed relatively frequency specific and warrant further study for audiometric applications.