From Modulated Noise to Natural Speech: the Effect of Stimulus Parameters on the Frequency Following Response

Frequency following responses (FFRs) can be evoked by a wide range of auditory stimuli, but for many stimulus parameters the effect on FFR strength is not fully understood. This complicates the comparison of earlier studies and the design of new studies. Furthermore, the most optimal stimulus parameters are unknown. To help resolve this issue, we investigated the effects of four important stimulus parameters and their interactions on the FFR. FFRs were measured in 16 normal hearing subjects evoked by stimuli with four levels of stimulus complexity (amplitude modulated noise, artificial vowels, natural vowels and nonsense words), three frequencies (around 105 Hz, 185 Hz and 245 Hz), three frequency contours (upward sweeping, downward sweeping and flat) and three vowels (Flemish /a:/, /u:/, and /i:/). We found that FFRs evoked by artificial vowels were on average 4 to 6 dB SNR larger than responses evoked by the other stimulus complexities, probably because of (unnaturally) strong higher harmonics. Moreover, response amplitude decreased with stimulus frequency but response SNR did not. Thirdly, frequency variation within the stimulus did not impact FFR strength, but only when rate of change remained low (e.g. not the case for sweeping natural vowels). Finally, the vowel /i:/ appeared to evoke larger response amplitudes compared to /a:/ and /u:/, but analysis power was too small to confirm this statistically. Differences in response strength between evoking vowels have been suggested to stem from destructive interference between response components. We show how a model of the auditory periphery can simulate these interference patterns and predict response strength. Altogether, the results of this study can guide stimulus choice for future FFR research and practical applications.