Toward the Optimal Stimulus to Elicit the Frequency-Following Response

The frequency-following response (FFR) is gaining momentum to investigate central auditory processing in both audiological sciences and cognitive neuroscience. In previous research, FFRs have been most often elicited by the /da/ stimulus. Recently, we introduced the diphthong /oa/, particularly for infant studies, as, in addition to underpinning fundamental frequency encoding, it provides the underpinnings of temporal fine structure encoding. However, both the /da/ and /oa/ stimuli have their limitations: the /da/ stimulus has been shown to be less effective for studying the temporal fine structure, and the /oa/ stimulus yields a smaller response. In this study, we tested the effects of manipulating vowel order and coarticulation of stimuli as factors that may affect FFR amplitudes.

We conducted a multisite study involving 34 normal-hearing adults (aged 18 to 40) at Antwerp University Hospital and the University of Barcelona. Four different stimuli (/dao/, /doa/, /ao/, and /oa/) designed and equated for intensity were presented in three blocks of 1000 trials in randomized block order. The main FFR parameters were the F 0 and F 1 response amplitude for the /o/ and /a/ sections of the stimuli.

The F 0 amplitude was largest for the /ao/ stimulus and smallest for the /oa/ stimulus in both the /a/ and /o/ vowel segments. In contrast, the /oa/ stimulus evoked significantly higher F 1 amplitudes than all other stimuli across both segments, showing an inverse pattern relative to F 0 . The neural response delay was significantly longer for /ao/ compared with the other stimuli. Pre-stimulus root mean square amplitude did not differ across stimuli or sites, indicating consistent baseline neural activity. These amplitude findings were mirrored by significant effects in signal to noise ratio, reinforcing the robustness of the stimulus-driven differences. While a significant Stimulus × Research center interaction was found for F 0 and F 1 amplitudes in the /a/ segment, no interaction effects emerged for the remaining eight FFR parameters, supporting the overall reproducibility and robustness of stimulus effects across centers.

Both amplitude and signal to noise ratio findings confirm that the F 0 response is influenced by the frequency structure of the eliciting vowels and by the preceding vowels and consonants. The choice of stimulus should align with the study's goals: the /ao/ stimulus is ideal for FFR ENV studies, the /oa/ stimulus for FFR TFS studies, and the /dao/ stimulus for studies examining both, offering strong responses for both the FFR ENV and the FFR TFS . The /dao/ stimulus offers the additional advantage that it includes the /da/ stimulus segment, facilitating comparisons with prior research.