Neural Dynamics in Speech Processing: Properties, Specificity and Open Questions

Abstract

Human speech is undoubtedly a particularly relevant acoustic stimulus for our species, due to its role of information transmission during communication. Previous research has aimed to map speech processing onto macro-anatomical brain structures specifically dedicated to this purpose, an endeavour that turned out to be challenging. A more recent line of research addressed the fact that speech is a dynamic signal, and instead focused on similarly dynamic neural activity. We here focus on the question of whether and how speech is special for such neural dynamics. We review findings that suggest that this is indeed the case: First, properties of human speech are strikingly similar to those found in neural dynamics, indicating that the two co-evolved. Second, neural dynamics produced by human speech have distinct, characteristic properties that are not only an amplified version of those observed during other, non-speech sounds. Although open questions remain, speech seems special for neural dynamics in humans.

Rhythmic Entrainment Echoes in Auditory Perception

“…Our findings are therefore compatible with the system’s tuning to process speech. The fact that they are located at the upper limit of this range is in line with the suggestions that eigenfrequencies decrease along the auditory hierarchy: As we used tones, not speech, to entrain neural oscillations, perceptual outcomes might have been determined by activity in an earlier cortical stage of cortical processing, where eigenfrequencies are likely to be higher than in regions processing more complex linguistic structure 44,45,52 .…”

“…Preferred rates ( eigenfrequencies ) for auditory neural or perceptual responses have been described before (for review, see ref 44 ). Humans are most sensitive to acoustic spectro-temporal modulations between ~2 and 5 Hz 45 .…”

“…Neural activity measured with electro- or magnetoencephalography (EEG/MEG) follow such modulations most reliably if they occur in the theta (~4-7 Hz) or gamma (~30-45 Hz) range, but not in-between 46,47 . It has been speculated that these eigenfrequency ranges reflect audition’s specialisation to process speech 44,48 , which contains amplitude modulations and changes in linguistic patterns at similar rates 49 . However, most studies have almost exclusively focused on responses during the rhythmic stimulus, when endogenous oscillations are difficult to identify 6 .…”

What is a Rhythm for the Brain? The Impact of Contextual Temporal Variability on Auditory Perception

“…Speech acoustic signals in particular presents complex statistical temporal regularities (Singh et al, 2003;Cummins, 2012;Varnet et al, 2017) that are supposedly used to form temporal expectations and influence language comprehension (Tillmann, 2012;Jadoul et al, 2016;Kösem & Van Wassenhove, 2017;Kösem et al, 2018;Aubanel & Schwartz, 2020). How temporal predictions occur in non-fully predictable temporal contexts such as speech and music and how they influence auditory perception is still under debate (Jadoul et al, 2016;Herbst & Obleser, 2017, Zoefel & Kösem, 2022. During speech listening in particular, temporal prediction mechanisms are put forward as an important mechanism that would contribute to acoustic segmentation and enhanced processing of relevant auditory information (Giraud & Poeppel, 2012;Meyer et al, 2019;Peelle & Davis, 2012;Zoefel & Kösem, 2022).…”

“…How temporal predictions occur in non-fully predictable temporal contexts such as speech and music and how they influence auditory perception is still under debate (Jadoul et al, 2016;Herbst & Obleser, 2017, Zoefel & Kösem, 2022. During speech listening in particular, temporal prediction mechanisms are put forward as an important mechanism that would contribute to acoustic segmentation and enhanced processing of relevant auditory information (Giraud & Poeppel, 2012;Meyer et al, 2019;Peelle & Davis, 2012;Zoefel & Kösem, 2022). In line with this, temporal predictability based on rhythmic cues present in the signal, specifically on the average speech rate, influences speech perception (Dilley & Pitt, 2010;Kösem et al, 2018).…”