Attention Differentially Affects Acoustic and Phonetic Feature Encoding in a Multispeaker Environment.

Humans have the remarkable ability to selectively focus on a single talker in the midst of other competing talkers. The neural mechanisms that underlie this phenomenon remain incompletely understood. In particular, there has been longstanding debate over whether attention operates at an early or late stage in the speech processing hierarchy. One way to better understand this is to examine how attention might differentially affect neurophysiological indices of hierarchical acoustic and linguistic speech representations. In this study, we do this by using encoding models to identify neural correlates of speech processing at various levels of representation. Specifically, we recorded EEG from fourteen human subjects (nine female and five male) during a "cocktail party" attention experiment. Model comparisons based on these data revealed phonetic feature processing for attended, but not unattended speech. Furthermore, we show that attention specifically enhances isolated indices of phonetic feature processing, but that such attention effects are not apparent for isolated measures of acoustic processing. These results provide new insights into the effects of attention on different prelexical representations of speech, insights that complement recent anatomic accounts of the hierarchical encoding of attended speech. Furthermore, our findings support the notion that, for attended speech, phonetic features are processed as a distinct stage, separate from the processing of the speech acoustics.SIGNIFICANCE STATEMENT Humans are very good at paying attention to one speaker in an environment with multiple speakers. However, the details of how attended and unattended speech are processed differently by the brain is not completely clear. Here, we explore how attention affects the processing of the acoustic sounds of speech as well as the mapping of those sounds onto categorical phonetic features. We find evidence of categorical phonetic feature processing for attended, but not unattended speech. Furthermore, we find evidence that categorical phonetic feature processing is enhanced by attention, but acoustic processing is not. These findings add an important new layer in our understanding of how the human brain solves the cocktail party problem.

Prosodic pitch processing is represented in delta-band EEG and is dissociable from the cortical tracking of other acoustic and phonetic features.

Speech is central to communication among humans. Meaning is largely conveyed by the selection of linguistic units such as words, phrases and sentences. However, prosody, that is the variation of acoustic cues that tie linguistic segments together, adds another layer of meaning. There are various features underlying prosody, one of the most important being pitch and how it is modulated. Recent fMRI and ECoG studies have suggested that there are cortical regions for pitch which respond primarily to resolved harmonics and that high-gamma cortical activity encodes intonation as represented by relative pitch. Importantly, this latter result was shown to be independent of the cortical tracking of the acoustic energy of speech, a commonly used measure. Here, we investigate whether we can isolate low-frequency EEG indices of pitch processing of continuous narrative speech from those reflecting the tracking of other acoustic and phonetic features. Harmonic resolvability was found to contain unique predictive power in delta and theta phase, but it was highly correlated with the envelope and tracked even when stimuli were pitch-impoverished. As such, we are circumspect about whether its contribution is truly pitch-specific. Crucially however, we found a unique contribution of relative pitch to EEG delta-phase prediction, and this tracking was absent when subjects listened to pitch-impoverished stimuli. This finding suggests the possibility of a separate processing stream for prosody that might operate in parallel to acoustic-linguistic processing. Furthermore, it provides a novel neural index that could be useful for testing prosodic encoding in populations with speech processing deficits and for improving cognitively controlled hearing aids.

EEG decoding of the target speaker in a cocktail party scenario: considerations regarding dynamic switching of talker location.

OBJECTIVE: It has been shown that attentional selection in a simple dichotic listening paradigm can be decoded offline by reconstructing the stimulus envelope from single-trial neural response data. Here, we test the efficacy of this approach in an environment with non-stationary talkers. We then look beyond the envelope reconstructions themselves and consider whether incorporating the decoder values-which reflect the weightings applied to the multichannel EEG data at different time lags and scalp locations when reconstructing the stimulus envelope-can improve decoding performance. APPROACH: High-density EEG was recorded as subjects attended to one of two talkers. The two speech streams were filtered using HRTFs, and the talkers were alternated between the left and right locations at varying intervals to simulate a dynamic environment. We trained spatio-temporal decoders mapping from EEG data to the attended and unattended stimulus envelopes. We then decoded auditory attention by (1) using the attended decoder to reconstruct the envelope and (2) exploiting the fact that decoder weightings themselves contain signatures of attention, resulting in consistent patterns across subjects that can be classified. MAIN RESULTS: The previously established decoding approach was found to be effective even with non-stationary talkers. Signatures of attentional selection and attended direction were found in the spatio-temporal structure of the decoders and were consistent across subjects. The inclusion of decoder weights into the decoding algorithm resulted in significantly improved decoding accuracies (from 61.07% to 65.31% for 4 s windows). An attempt was made to include alpha power lateralization as another feature to improve decoding, although this was unsuccessful at the single-trial level. SIGNIFICANCE: This work suggests that the spatial-temporal decoder weights can be utilised to improve decoding. More generally, looking beyond envelope reconstruction and incorporating other signatures of attention is an avenue that should be explored to improve selective auditory attention decoding.