Neural entrainment to continuous speech and language processing in the early years of life

This thesis aimed to explore the neural mechanisms of language processing in infants under 12 months of age by using EEG measures of speech processing. More specifically, I wanted to investigate if infants are able to engage in the auditory neural tracking of continuous speech and how this processing can be modulated by infant attention and different linguistic environments. Limited research has investigated this phenomenon of neural tracking in infants and the potential effects that this may have on later language development. Experiment 1 set the groundwork for the thesis by establishing a reliable method to measure cortical entrainment by 36 infants to the amplitude envelope of continuous speech. The results demonstrated that infants have entrainment to speech much like has been found in adults. Additionally, infants show a reliable elicitation of the Acoustic Change Complex (ACC). Follow up language assessments were conducted with these infants approximately two years later; however, no significant predictors of coherence on later language outcomes were found. The aim of Experiment 2 was to discover how neural entrainment can be modulated by infant attention. Twenty infants were measured on their ability to selectively attend to a target speaker while in the presence of a distractor of matching acoustic intensity. Coherence values were found for the target, the distractor and for the dual signal (both target and distractor together). Thus, it seems that infant attention may be fluctuating between the two speech signals leading to them entraining to both simultaneously. However, the results were not clear. Thus, Experiment 3 expanded on from Experiment 2. However, now EEG was recorded from 30 infants who listened to speech with no acoustic interference and speech-in-noise with a signal-to-noise ratio of 10dB. Additionally, it was investigated whether bilingualism has any potential effects on this process. Similar coherence values were observed when infants listened to speech in both conditions (quiet and noise), suggesting that infants successfully inhibited the disruptive effects of the masker. No effects of bilingualism on neural entrainment were present. For the fourth study we wanted to continue investigating infant auditory-neural entrainment when exposed to more varying levels of background noise. However, due to the COVID-19 pandemic all testing was moved online. Thus, for Experiment 4 we developed a piece of online software (the memory card game) that could be used remotely. Seventy three children ranging from 4 to 12 years old participated in the online experiment in order to explore how the demands of a speech recognition task interact with masker type and language and how this changes with age during childhood. Results showed that performance on the memory card game improved with age but was not affected by masker type or language background. This improvement with age is most likely a result of improved speech perception capabilities. Overall, this thesis provides a reliable methodology for measuring neural entrainment in infants and a greater understanding of the mechanisms of speech processing in infancy and beyond

Decoding auditory attention and neural language processing in adverse conditions and different listener groups

This thesis investigated subjective, behavioural and neurophysiological (EEG) measures of speech processing in various adverse conditions and with different listener groups. In particular, this thesis focused on different neural processing stages and their relationship with auditory attention, effort, and measures of speech intelligibility. Study 1 set the groundwork by establishing a toolbox of various neural measures to investigate online speech processing, from the frequency following response (FFR) and cortical measures of speech processing, to the N400, a measure of lexico-semantic processing. Results showed that peripheral processing is heavily influenced by stimulus characteristics such as degradation, whereas central processing units are more closely linked to higher-order phenomena such as speech intelligibility. In Study 2, a similar experimental paradigm was used to investigate differences in neural processing between a hearing-impaired and a normal-hearing group. Subjects were presented with short stories in different levels of multi-talker babble noise, and with different settings on their hearing aids. Findings indicate that, particularly at lower noise levels, the hearing-impaired group showed much higher cortical entrainment than the normal- hearing group, despite similar levels of speech recognition. Intersubject correlation, another global neural measure of auditory attention, however, was similarly affected by noise levels in both the hearing-impaired and the normal-hearing group. This finding indicates extra processing in the hearing-impaired group only on the level of the auditory cortex. Study 3, in contrast to Studies 1 and 2 (which both investigated the effects of bottom-up factors on neural processing), examined the links between entrainment and top-down factors, specifically motivation; as well as reasons for the 5 higher entrainment found in hearing-impaired subjects in Study 2. Results indicated that, while behaviourally there was no difference between incentive and non-incentive conditions, neurophysiological measures of attention such as intersubject correlation were affected by the presence of an incentive to perform better. Moreover, using a specific degradation type resulted in subjects’ increased cortical entrainment under degraded conditions. These findings support the hypothesis that top-down factors such as motivation influence neurophysiological measures; and that higher entrainment to degraded speech might be triggered specifically by the reduced availability of spectral detail contained in speech

Semantic radical consistency and character transparency effects in Chinese: an ERP study

BACKGROUND: This event-related potential (ERP) study aims to investigate the representation and temporal dynamics of Chinese orthography-to-semantics mappings by simultaneously manipulating character transparency and semantic radical consistency. Character components, referred to as radicals, make up the building blocks used dur...postprin

Psychophysiological indices of recognition memory

It has recently been found that during recognition memory tests participants’ pupils dilate more when they view old items compared to novel items. This thesis sought to replicate this novel ‘‘Pupil Old/New Effect’’ (PONE) and to determine its relationship to implicit and explicit mnemonic processes, the veracity of participants’ responses, and the analogous Event-Related Potential (ERP) old/new effect. Across 9 experiments, pupil-size was measured with a video-based eye-tracker during a variety of recognition tasks, and, in the case of Experiment 8, with concurrent Electroencephalography (EEG). The main findings of this thesis are that: - the PONE occurs in a standard explicit test of recognition memory but not in “implicit” tests of either perceptual fluency or artificial grammar learning; - the PONE is present even when participants are asked to give false behavioural answers in a malingering task, or are asked not to respond at all; - the PONE is present when attention is divided both at learning and during recognition; - the PONE is accompanied by a posterior ERP old/new effect; - the PONE does not occur when participants are asked to read previously encountered words without making a recognition decision; - the PONE does not occur if participants preload an “old/new” response; - the PONE is not enhanced by repetition during learning. These findings are discussed in the context of current models of recognition memory and other psychophysiological indices of mnemonic processes. It is argued that together these findings suggest that the increase in pupil-size which occurs when participants encounter previously studied items is not under conscious control and may reflect primarily recollective processes associated with recognition memory