Inner Speech's Relationship With Overt Speech in Poststroke Aphasia

Learning to operate an imagined speech Brain-Computer Interface involves the spatial and frequency tuning of neural activity

Brain-Computer Interfaces (BCI) will revolutionize the way people with severe impairment of speech production can communicate. While current efforts focus on training classifiers on vast amounts of neurophysiological signals to decode imagined speech, much less attention has been given to users' ability to adapt their neural activity to improve BCI-control. To address whether BCI-control improves with training and characterize the underlying neural dynamics, we trained 15 healthy participants to operate a binary BCI system based on electroencephalography (EEG) signals through syllable imagery for five consecutive days. Despite considerable interindividual variability in performance and learning, a significant improvement in BCI-control was globally observed. Using a control experiment, we show that a continuous feedback about the decoded activity is necessary for learning to occur. Performance improvement was associated with a broad EEG power increase in frontal theta activity and focal enhancement in temporal low-gamma activity, showing that learning to operate an imagined-speech BCI involves dynamic changes in neural features at different spectral scales. These findings demonstrate that combining machine and human learning is a successful strategy to enhance BCI controllability.

Evidence from aphasia suggests a bidirectional relationship between inner speech and executive function

Research over the past several decades has revealed that non-linguistic cognitive impairments can appear alongside language deficits in individuals with aphasia. One vulnerable cognitive domain is executive function, an umbrella term for the higher-level cognitive processes that allow us to direct our behavior towards a goal. Studies in healthy adults reveal that executive function abilities are supported by inner speech, the ability to use language silently in one's head. Therefore, inner speech may mediate the connection between language and executive function deficits in individuals with aphasia. Here, we investigated whether inner speech ability may link language and cognitive impairments in 59 adults with chronic, post-stroke aphasia. We used two approaches to measure inner speech: one based on internal retrieval of words and one based on internal retrieval plus silent manipulation of the retrieved phonological forms. Then, we examined relationships between these two approaches to measuring inner speech and five aspects of executive function ability: response inhibition, conflict monitoring/resolution, general task-switching ability, phonological control, and semantic control. We also looked for dissociations between inner speech ability and executive function ability. Our results show tentative relationships between inner speech (across multiple measurement approaches) and all aspects of executive function except for response inhibition. We also found evidence for a double dissociation: many participants show intact executive function despite poor inner speech, and vice versa, so neither process is strictly reliant on the other. We suggest that this work provides preliminary evidence of a bidirectional relationship between inner speech and executive function: inner speech supports some aspects of executive function via internal self-cueing and certain aspects of executive function support performance on complex inner speech tasks.

Interdisciplinary approaches to understanding the inner speech, with emphasis on the role of incorporating clinical data

Neuroscience has largely conceptualized inner speech, sometimes called covert speech, as being a part of the language system, namely, a precursor to overt speech and/or speech without the motor component (impoverished motor speech). Yet interdisciplinary work has strongly suggested that inner speech is multidimensional and situated within the language system as well as in more domain general systems. By leveraging evidence from philosophy, linguistics, neuroscience and cognitive science, we argue that neuroscience can gain a more comprehensive understanding of inner speech processes. We will summarize the existing knowledge on the traditional approach to understanding the neuroscience of inner speech, which is squarely through the language system, before discussing interdisciplinary approaches to understanding the cognitive, linguistic and neural substrates/mechanisms that may be involved in inner speech. Given our own interests in inner speech after brain injury, we finish by discussing the theoretical and clinical benefits of researching inner speech in aphasia through an interdisciplinary lens.

Revealing the spatiotemporal brain dynamics of covert speech compared with overt speech: A simultaneous EEG-fMRI study

Covert speech (CS) refers to speaking internally to oneself without producing any sound or movement. CS is involved in multiple cognitive functions and disorders. Reconstructing CS content by brain-computer interface (BCI) is also an emerging technique. However, it is still controversial whether CS is a truncated neural process of overt speech (OS) or involves independent patterns. Here, we performed a word-speaking experiment with simultaneous EEG-fMRI. It involved 32 participants, who generated words both overtly and covertly. By integrating spatial constraints from fMRI into EEG source localization, we precisely estimated the spatiotemporal dynamics of neural activity. During CS, EEG source activity was localized in three regions: the left precentral gyrus, the left supplementary motor area, and the left putamen. Although OS involved more brain regions with stronger activations, CS was characterized by an earlier event-locked activation in the left putamen (peak at 262 ms versus 1170 ms). The left putamen was also identified as the only hub node within the functional connectivity (FC) networks of both OS and CS, while showing weaker FC strength towards speech-related regions in the dominant hemisphere during CS. Path analysis revealed significant multivariate associations, indicating an indirect association between the earlier activation in the left putamen and CS, which was mediated by reduced FC towards speech-related regions. These findings revealed the specific spatiotemporal dynamics of CS, offering insights into CS mechanisms that are potentially relevant for future treatment of self-regulation deficits, speech disorders, and development of BCI speech applications.

Inner speech in the daily lives of people with aphasia

Introduction

This exploratory, preliminary, feasibility study evaluated the extent to which adults with chronic aphasia (N = 23) report experiencing inner speech in their daily lives by leveraging experience sampling and survey methodology.

Methods

The presence of inner speech was assessed at 30 time-points and themes of inner speech at three time-points, over the course of three weeks. The relationship of inner speech to aphasia severity, demographic information (age, sex, years post-stroke), and insight into language impairment was evaluated.

Results

There was low attrition (<8%) and high compliance (>94%) for the study procedures, and inner speech was experienced in most sampled instances (>78%). The most common themes of inner speech experience across the weeks were 'when remembering', 'to plan', and 'to motivate oneself'. There was no significant relationship identified between inner speech and aphasia severity, insight into language impairment, or demographic information. In conclusion, adults with aphasia tend to report experiencing inner speech often, with some shared themes (e.g., remembering, planning), and use inner speech to explore themes that are uncommon in young adults in other studies (e.g., to talk to themselves about health).

Discussion

High compliance and low attrition suggest design feasibility, and results emphasize the importance of collecting data in age-similar, non-brain-damaged peers as well as in adults with other neurogenic communication disorders to fully understand the experience and use of inner speech in daily life. Clinical implications and future directions are discussed.

Inner speech

Inner speech travels under many aliases: the inner voice, verbal thought, thinking in words, internal verbalization, "talking in your head," the "little voice in the head," and so on. It is both a familiar element of first-person experience and a psychological phenomenon whose complex cognitive components and distributed neural bases are increasingly well understood. There is evidence that inner speech plays a variety of cognitive roles, from enabling abstract thought, to supporting metacognition, memory, and executive function. One active area of controversy concerns the relation of inner speech to auditory verbal hallucinations (AVHs) in schizophrenia, with a common proposal being that sufferers of AVH misidentify their own inner speech as being generated by someone else. Recently, researchers have used artificial intelligence to translate the neural and neuromuscular signatures of inner speech into corresponding outer speech signals, laying the groundwork for a variety of new applications and interventions. This article is categorized under: Philosophy > Foundations of Cognitive Science Linguistics > Language in Mind and Brain Philosophy > Consciousness Philosophy > Psychological Capacities.

Inner Speech in Aphasia: Current Evidence, Clinical Implications, and Future Directions

Purpose Typical language users can engage in a lively internal monologue for introspection and task performance, but what is the nature of inner speech among individuals with aphasia? Studying the phenomenon of inner speech in this population has the potential to further our understanding of inner speech more generally, help clarify the subjective experience of those with aphasia, and inform clinical practice. In this scoping review, we describe and synthesize the existing literature on inner speech in aphasia. Method Studies examining inner speech in aphasia were located through electronic databases and citation searches. Across the various studies, methods include both subjective approaches (i.e., asking individuals with aphasia about the integrity of their inner speech) and objective approaches (i.e., administering objective language tests as proxy measures for inner speech ability). The findings of relevant studies are summarized. Results Although definitions of inner speech vary across research groups, studies using both subjective and objective methods have established findings showing that inner speech can be preserved relative to spoken language in individuals with aphasia, particularly among those with relatively intact word retrieval and difficulty primarily at the level of speech output processing. Approaches that combine self-report with objective measures have demonstrated that individuals with aphasia are, on the whole, reliably able to report the integrity of their inner speech. Conclusions The examination of inner speech in individuals with aphasia has potential implications for clinical practice, in that differences in the preservation of inner speech across individuals may help guide clinical decision making around aphasia treatment. Although there are many questions that remain open to further investigation, studying inner speech in this specific population has also contributed to a broader understanding of the mechanisms of inner speech more generally.

A Penny for Your Thoughts: Children's Inner Speech and Its Neuro-Development

Inner speech emerges in early childhood, in parallel with the maturation of the dorsal language stream. To date, the developmental relations between these two processes have not been examined. We review evidence that the dorsal language stream has a role in supporting the psychological phenomenon of inner speech, before considering pediatric studies of the dorsal stream's anatomical development and evidence for its emerging functional roles. We examine possible causal accounts of the relations between these two developmental processes and consider their implications for phylogenetic theories about the evolution of inner speech and the accounts of the ontogenetic relations between language and cognition.

Self-reported inner speech relates to phonological retrieval ability in people with aphasia

Many individuals with aphasia report the ability to say words in their heads despite spoken naming difficulty. Here, we examined individual differences in the experience of inner speech (IS) in participants with aphasia to test the hypotheses that self-reported IS reflects intact phonological retrieval and that articulatory output processing is not essential to IS. Participants (N = 53) reported their ability to name items correctly internally during a silent picture-naming task. We compared this measure of self-reported IS to spoken picture naming and a battery of tasks measuring the underlying processes required for naming (i.e., phonological retrieval and output processing). Results from three separate analyses of these measures indicate that self-reported IS relates to phonological retrieval and that speech output processes are not a necessary component of IS. We suggest that self-reported IS may be a clinically valuable measure that could assist in clinical decision-making regarding anomia diagnosis and treatment.

A Test Battery for Inner Speech Functions

Objective

Inner speech, or the ability to talk to yourself in your head, is one of the most ubiquitous phenomena of everyday experience. Recent years have seen growing interest in the role and function of inner speech in various typical and cognitively impaired populations. Although people vary in their ability to produce inner speech, there is currently no test battery which can be used to evaluate people's inner speech ability. Here we developed a test battery which can be used to evaluate individual differences in the ability to access the auditory word form internally.

Methods

We developed and standardized five tests: rhyme judgment of pictures and written words, homophone judgment of written words and non-words, and judgment of lexical stress of written words. The tasks were administered to adult healthy native British English speakers (age range 20-72, n = 28-97, varies between tests).

Results

In all tests, some items were excluded based on low success rates among participants, or documented regional variability in accent. Level of education, but not age, correlated with task performance for some of the tasks, and there were no gender difference in performance.

Conclusion

A process of standardization resulted in a battery of tests which can be used to assess natural variability of inner speech abilities among English speaking adults.

The Subjective Experience of Inner Speech in Aphasia Is a Meaningful Reflection of Lexical Retrieval

Purpose Individuals with aphasia often report that they feel able to say words in their heads, regardless of speech output ability. Here, we examine whether these subjective reports of successful "inner speech" (IS) are meaningful and test the hypothesis that they reflect lexical retrieval. Method Participants were 53 individuals with chronic aphasia. During silent picture naming, participants reported whether or not they could say the name of each item inside their heads. Using the same items, they also completed 3 picture-based tasks that required phonological retrieval and 3 matched auditory tasks that did not. We compared participants' performance on these tasks for items they reported being able to say internally versus those they reported being unable to say internally. Then, we examined the relationship of psycholinguistic word features to self-reported IS and spoken naming accuracy. Results Twenty-six participants reported successful IS on nearly all items, so they could not be included in the item-level analyses. These individuals performed correspondingly better than the remaining participants on tasks requiring phonological retrieval, but not on most other language measures. In the remaining group ( n = 27), IS reports related item-wise to performance on tasks requiring phonological retrieval, but not to matched control tasks. Additionally, IS reports were related to 3 word characteristics associated with lexical retrieval, but not to articulatory complexity; spoken naming accuracy related to all 4 word characteristics. Six participants demonstrated evidence of unreliable IS reporting; compared with the group, they also detected fewer errors in their spoken responses and showed more severe language impairments overall. Conclusions Self-reported IS is meaningful in many individuals with aphasia and reflects lexical phonological retrieval. These findings have potential implications for treatment planning in aphasia and for our understanding of IS in the general population.

Neurolinguistics Research Advancing Development of a Direct-Speech Brain-Computer Interface

A direct-speech brain-computer interface (DS-BCI) acquires neural signals corresponding to imagined speech, then processes and decodes these signals to produce a linguistic output in the form of phonemes, words, or sentences. Recent research has shown the potential of neurolinguistics to enhance decoding approaches to imagined speech with the inclusion of semantics and phonology in experimental procedures. As neurolinguistics research findings are beginning to be incorporated within the scope of DS-BCI research, it is our view that a thorough understanding of imagined speech, and its relationship with overt speech, must be considered an integral feature of research in this field. With a focus on imagined speech, we provide a review of the most important neurolinguistics research informing the field of DS-BCI and suggest how this research may be utilized to improve current experimental protocols and decoding techniques. Our review of the literature supports a cross-disciplinary approach to DS-BCI research, in which neurolinguistics concepts and methods are utilized to aid development of a naturalistic mode of communication.