THE ENVELOPE FOLLOWING RESPONSE (EFR) IN THE MONGOLIAN GERBIL TO SINUSOIDALLY AMPLITUDE-MODULATED SIGNALS IN THE PRESENCE OF SIMULTANEOUSLYGATED 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 experimen ts exploring the behavior of the EFR elicited with sinusoidally amplit ude modulated (SAM) tones in the presence of simultaneously gated, con tinuous, pure-tone interfering signals of varying intensity are report ed. Probe stimuli consisted of SAM tones with carriers ranging in freq uency from 800 Hz-4 kHz, modulated at frequencies between 30-150 Hz. P robe 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. I n these experiments a maximum reduction in the response to the probe t one, measured at the probe modulation frequency, appeared as a sharp p eak within a narrow frequency band above the frequency of the probe ca rrier and a broader region of reduced response extending to higher fre quencies. 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 t he frequency region affected. The obtained interference response patte rn may be attributable to both ''synchrony capture'' (i.e., capture of the EFR of the system by envelope components arising due to the inter action 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 appear s that the EFR to SAM stimuli of low to moderate intensity arose prima rily from neuronal populations tuned to frequencies at or above the pr obe f(c). The results of the present study suggest that at low intensi ty levels SAM signals are indeed relatively frequency specific and war rant further study for audiometric applications.