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Speech-in-noise (SPIN) perception involves neural encoding of temporal acoustic cues. Cues include temporal fine structure (TFS) and envelopes that modulate at syllable (Slow-rate ENV) and fundamental frequency (F-rate ENV) rates. Here the relationship between speech-evoked neural responses to these cues and SPIN perception was investigated in older adults. Theta-band phase-locking values (PLVs) that reflect cortical sensitivity to Slow-rate ENV and peripheral/brainstem frequency-following responses phase-locked to F-rate ENV (FFR) and TFS (FFR) were measured from scalp-electroencephalography responses to a repeated speech syllable in steady-state speech-shaped noise (SpN) and 16-speaker babble noise (BbN). The results showed that (1) SPIN performance and PLVs were significantly higher under SpN than BbN, implying differential cortical encoding may serve as the neural mechanism of SPIN performance that varies as a function of noise types; (2) PLVs and FFR at resolved harmonics were significantly related to good SPIN performance, supporting the importance of phase-locked neural encoding of Slow-rate ENV and TFS of resolved harmonics during SPIN perception; (3) FFR was not associated to SPIN performance until audiometric threshold was controlled for, indicating that hearing loss should be carefully controlled when studying the role of neural encoding of F-rate ENV. Implications are drawn with respect to fitting auditory prostheses.

Phase-locked responses are vital for auditory perception and they may vary with participants' arousal state and age. Two phase-locked neural responses that reflect fine-grained acoustic properties of speech were examined in the current study: the frequency-following response (FFR) to the speech fundamental frequency (F0), which originates primarily from the auditory brainstem, and the theta-band phase-locked response (θ-PLV) to the speech envelope that originates from the auditory cortices. The ways these responses were affected by arousal in adults across a wide age-range (19 ~ 75 years) were examined. Extracts from electroencephalographic (EEG) responses to repeated syllables were classified into either high or low arousal state based on the occurrence of sleep spindles. The magnitudes of both FFRs and θ-PLVs were statistically greater in the high, than in the low, arousal state. The difference in θ-PLV between the two arousal states was significantly associated with sleep spindle density in the young, but not the older, adults. The results show that (1) arousal affects phase-locked processing of speech at cortical/sub-cortical sensory levels; and that (2) there is an interplay between aging and arousal state which indicates that sleep spindles have an age-dependent neuro-regulatory role on cortical processes. The results lay the grounds for studying how cognitive states affect early-stage neural activity in the auditory system across the lifespan.

26Neural entrainment of acoustic envelopes is important for speech intelligibility in spoken 27 language processing. However, it is unclear how it contributes to processing at different 28 linguistic hierarchical levels. The present EEG study investigated this issue when participants 29 responded to stimuli that dissociated phonological and semantic processing (real-word, 30 pseudo-word and backward utterances). Multivariate Temporal Response Function (mTRF) 31 model was adopted to map speech envelopes from multiple spectral bands onto EEG signals, 32 providing a direct approach to measure neural entrainment. We tested the hypothesis that 33 entrainment at delta (supra-syllabic) and theta (syllabic and sub-syllabic) bands take distinct 34 roles at different hierarchical levels. Results showed that both types of entrainment involve 35 speech-specific processing, but their underlying mechanisms were different. Theta-band 36 entrainment was modulated by phonological but not semantic contents, reflecting the possible 37 mechanism of tracking syllabic-and sub-syllabic patterns during phonological processing. 38Delta-band entrainment, on the other hand, was modulated by semantic information, indexing 39 more attention-demanding, effortful phonological encoding when higher-level (semantic) 40 information is deficient. Interestingly, we further demonstrated that the statistical capacity of 41 mTRFs at the delta band and theta band to classify utterances is affected by their semantic 42 (real-word vs. pseudo-word) and phonological (real-word and pseudo-word vs. backward) 43 contents, respectively. Moreover, analyses on the response weighting of mTRFs showed that 44 delta-band entrainment sustained across neural processing stages up to higher-order timescales 45 (~ 300 ms), while theta-band entrainment occurred mainly at early, perceptual processing 46 stages (< 160 ms). This indicates that, compared to theta-band entrainment, delta-band 47 entrainment may reflect increased involvement of higher-order cognitive functions during 48 interactions between phonological and semantic processing. As such, we conclude that neural 49 entrainment is not only associated with speech intelligibility, but also with the hierarchy of 50 linguistic (phonological and semantic) content. The present study thus provide a new insight 51 into cognitive mechanisms of neural entrainment for spoken language processing.52 Keywords: Delta-and theta-band neural entrainment, EEG, mTRF, speech envelopes, 53 phonological processing, semantic processing 54 55 Abbreviations: EEG, electroencephalography; MEG, magnetoencephalography; TRF, 56 temporal response function; mTRF, multivariate temporal response function; tACS, 57 transcranial alternative current stimulation; SUS, semantically unpredictable sentences; AM, 58 amplitude-modulated 59 60 61 62 63 Highlights 64  Low-frequency neural entrainment was examined via mTRF models in EEG during 65 phonological and semantic processing. 66  Delta entrainment take roles in effortful listening for ph...

Speech-evoked envelope-following response (EFR) reflects brain encoding of speech periodicity that serves as a biomarker for pitch and speech perception and various auditory and language disorders. Although EFR is thought to originate from the subcortex, recent research illustrated a right-hemispheric cortical contribution to EFR. However, it is unclear whether this contribution is causal. This study aimed to establish this causality by combining transcranial direct current stimulation (tDCS) and measurement of EFR (pre- and post-tDCS) via scalp-recorded electroencephalography. We applied tDCS over the left and right auditory cortices in right-handed normal-hearing participants and examined whether altering cortical excitability via tDCS causes changes in EFR during monaural listening to speech syllables. We showed significant changes in EFR magnitude when tDCS was applied over the right auditory cortex compared with sham stimulation for the listening ear contralateral to the stimulation site. No such effect was found when tDCS was applied over the left auditory cortex. Crucially, we further observed a hemispheric laterality where aftereffect was significantly greater for tDCS applied over the right than the left auditory cortex in the contralateral ear condition. Our finding thus provides the first evidence that validates the causal relationship between the right auditory cortex and EFR.

25Speech-evoked frequency-following response (FFR) reflects the neural encoding of speech 26 periodic information in the human auditory systems. FFR is of fundamental importance for 27 pitch and speech perception and serves as clinical biomarkers for various auditory and 28 language disorders. While it is suggested that the main neural source of FFR is in the auditory 29 brainstem, recent studies have shown a cortical contribution to FFR predominantly in the right 30 hemisphere. However, it is still unclear whether auditory cortex and FFR are causally related. 31The aim of this study was to establish this causal relationship using a combination of 32 transcranial direct current stimulation (tDCS) and scalp-recorded electroencephalography 33 (EEG). We applied tDCS over the left and right auditory cortices in right-handed normal-34 hearing participants and examined the after-effects of tDCS on FFR using EEG during 35 monaural listening to a repeatedly-presented speech syllable. Our results showed that: (1) 36 before tDCS was applied, participants had greater FFR magnitude when they listened to speech 37 from the left than the right ear, illustrating right-lateralized hemispheric asymmetry for FFR; 38(2) anodal and cathodal tDCS applied over the right, but not left, auditory cortex significantly 39 changed FFR magnitudes compared to the sham stimulation; specifically, such after-effects 40 occurred only when participants listened to speech from the left ear, emphasizing the right 41 auditory cortical contributions along the contralateral pathway. The current finding thus 42 provides the first causal evidence that validates the relationship between the right auditory 43 cortex and speech-evoked FFR and should significantly extend our understanding of speech 44 encoding in the brain. 45 46 Significance Statement 47 Speech-evoked frequency-following response (FFR) is a neural activity that reflects the brain's 48 encoding of speech periodic features. The FFR has great fundamental and clinical importance 49 for auditory processing. Whilst convention maintains that FFR derives mainly from the 50 brainstem, it has been argued recently that there are additional contributions to FFR from the 51 auditory cortex. Using a combination of tDCS, that altered neural excitability of auditory 52 cortices, and EEG recording, the present study provided the first evidence to validate a causal 53 relationship between the right auditory cortex and speech-evoked FFR. The finding supports 54 the right-asymmetric auditory cortical contributions to processing of speech periodicity and 55 3 advances our understanding of how speech signals are encoded and analysed along the central 56 auditory pathways. 57 58 82 White-Schwoch et al., 2015), dyslexia (Hornickel et al., 2013) and autism (Russo et al., 2008).83 It is argued that the fundamental and clinical importance of FFR is linked to the neural 84 fidelity of speech in the inferior colliculus at the brainstem, which has been proposed as the 85 4 main neural origin of FFR (Chandrasekaran and K...

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