Christian Walther

Neural correlates of adaptation to voice identity

Apart from speech content, the human voice also carries paralinguistic information about speaker identity. Voice identification and its neural correlates have received little scientific attention up to now. Here we use event-related potentials (ERPs) in an adaptation paradigm, in order to investigate the neural representation and the time course of vocal identity processing. Participants adapted to repeated utterances of vowel-consonant-vowel (VCV) of one personally familiar speaker (either A or B), before classifying a subsequent test voice varying on an identity continuum between these two speakers. Following adaptation to speaker A, test voices were more likely perceived as speaker B and vice versa, and these contrastive voice identity aftereffects (VIAEs) were much more pronounced when the same syllable, rather than a different syllable, was used as adaptor. Adaptation induced amplitude reductions of the frontocentral N1-P2 complex and a prominent reduction of the parietal P3 component, for test voices preceded by identity-corresponding adaptors. Importantly, only the P3 modulation remained clear for across-syllable combinations of adaptor and test stimuli. Our results suggest that voice identity is contrastively processed by specialized neurons in auditory cortex within ∼250 ms after stimulus onset, with identity processing becoming less dependent on speech content after ∼300 ms.

Dissociating the neural bases of repetition-priming and adaptation in the human brain for faces.

The repetition of a given stimulus leads to the attenuation of the functional magnetic resonance imaging (fMRI) signal compared with unrepeated stimuli, a phenomenon called fMRI adaptation or repetition suppression (RS). Previous studies have related RS of the fMRI signal behaviorally both to improved performance for the repeated stimulus (priming) and to shifts of perception away from the first stimulus (adaptation-related aftereffects). Here we used identical task (sex discrimination), trial structure [stimulus 1 (S1): 3,000 ms, interstimulus interval: 600 ms, stimulus 2 (S2): 300 ms], and S2 stimuli (androgynous faces) to test how RS of the face-specific areas of the occipito-temporal cortex relates to priming and aftereffects. By varying S1, we could induce priming (significantly faster reaction times when S1 and S2 were identical compared with different images) as well as sex-specific aftereffect [an increased ratio of male responses if S1 was a female face compared with ambiguous faces or to Fourier-randomized noise (FOU) images]. Presenting any face as S1 led to significant RS of the blood oxygen level-dependent signal in the fusiform and occipital face areas as well as in the lateral occipital cortex of both hemispheres compared with FOU, reflecting stimulus category-specific encoding. Additionally, while sex-specific adaptation effects were only observed in occipital face areas, primed trials led to a signal reduction in both face-selective regions. Altogether, these results suggest the differential neural mechanisms of adaptation and repetition priming.

Adaptor Identity Modulates Adaptation Effects in Familiar Face Identification and Their Neural Correlates

Adaptation-related aftereffects (AEs) show how face perception can be altered by recent perceptual experiences. Along with contrastive behavioural biases, modulations of the early event-related potentials (ERPs) were typically reported on categorical levels. Nevertheless, the role of the adaptor stimulus per se for face identity-specific AEs is not completely understood and was therefore investigated in the present study. Participants were adapted to faces (S1s) varying systematically on a morphing continuum between pairs of famous identities (identities A and B), or to Fourier phase-randomized faces, and had to match the subsequently presented ambiguous faces (S2s; 50/50% identity A/B) to one of the respective original faces. We found that S1s identical with or near to the original identities led to strong contrastive biases with more identity B responses following A adaptation and vice versa. In addition, the closer S1s were to the 50/50% S2 on the morphing continuum, the smaller the magnitude of the AE was. The relation between S1s and AE was, however, not linear. Additionally, stronger AEs were accompanied by faster reaction times. Analyses of the simultaneously recorded ERPs revealed categorical adaptation effects starting at 100 ms post-stimulus onset, that were most pronounced at around 125–240 ms for occipito-temporal sites over both hemispheres. S1-specific amplitude modulations were found at around 300–400 ms. Response-specific analyses of ERPs showed reduced voltages starting at around 125 ms when the S1 biased perception in a contrastive way as compared to when it did not. Our results suggest that face identity AEs do not only depend on physical differences between S1 and S2, but also on perceptual factors, such as the ambiguity of S1. Furthermore, short-term plasticity of face identity processing might work in parallel to object-category processing, and is reflected in the first 400 ms of the ERP.