A new study authored by Laura Jacxsens, Natàlia Gorina-Careta, Alejandro Mondéjar-Segovia, and colleagues from the University of Antwerp and led by Carles Escera at the Brainlab (University of Barcelona) has just been published in Ear & Hearing.

The authors tested four simple speech sounds to answer the key question: which stimulus best captures how the brain encodes speech? Their results showed that each sound highlights a different aspect of the frequency-following response (FFR):

• /ao/ produces the strongest response for tracking the voice pitch (F0),
• /oa/ works best for capturing fine acoustic details (TFS),
• /dao/ offers a good balance and links well with previous studies using the classic /da/ sound.

These findings provide practical guidance for researchers who use the FFR to test auditory processing in adults, infants, and clinical populations.

Fig. 2. Results for the FFRENV for /a/ section. A, Grand-averaged time-domain waveform of the FFRENV elicited by the four different stimuli. B, FFRENV frequency spectra extracted from the equivalent /a/ analyzed section of the neural response. C, Boxplots of the mean F0 spectral amplitude from the equivalent /a/ analyzed section of the neural response for each stimulus.

Fig. 4. Results for the FFRTFS for /a/ section. A, Grand-averaged time-domain waveform of the FFRTFS elicited by the four different stimuli. B, FFRTFS frequency spectra extracted from the equivalent /a/ analyzed section of the neural response. C, Boxplots of the mean F1 spectral amplitude from the equivalent /a/ analyzed section of the neural response for each stimulus.

The full abstract reads as follows:

Objectives. 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 of 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 shown to be less effective to study 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. Design. We conducted a multi-site study involving 34 normal-hearing adults (aged 18-40) 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 F₀ and F₁ response amplitude for the /o/ and /a/ sections of the stimuli. Results. The F₀ 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 F₁ amplitudes than all other stimuli across both segments, showing an inverse pattern relative to F₀. The neural response delay was significantly longer for /ao/ compared to 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 SNR, reinforcing the robustness of the stimulus-driven differences. While a significant stimulus × research center interaction was found for F₀ and F₁ 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. Conclusions. Both amplitude and SNR findings confirm that the F₀ 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.

Reference:

Jacxsens, L., Gorina Careta, N., Mondéjar-Segovia, A., Arenillas Alcón, S, Biot, L., Cardon, E., Van Rompaey, V., De Hertogh, W., Lammers, J.W., Escera, C. (2025). Towards the Optimal Stimulus to Elicit the Frequency-Following Response. Ear & Hearing, 10.1097.

Permanent DOI: 10.1097/AUD.0000000000001764