Electrophysiological measures of residual language comprehension to improve diagnostic accuracy in prolonged disorders of consciousness

Vidal Gran, Consuelo (2021). Electrophysiological measures of residual language comprehension to improve diagnostic accuracy in prolonged disorders of consciousness. University of Birmingham. Ph.D.

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Abstract

Behavioural scales are the clinical standard to assess consciousness in patients diagnosed with disorders of consciousness (DOC), although patients remain unresponsive to environmental stimuli beyond reflexes. Electrophysiological studies have identified modulation of brain activity in some patients when following commands; however, command-following requires complex brain processing that most patients lack due to the severity of their brain injury. Passive EEG paradigms differentiating automatic from strategic processing have detected residual cognitive processing in DOC patients. The local-global paradigm measures trial by trial violations of local expectations (short time-scales), which elicit a mismatch negativity (MMN) response that reflects automatic processing; whereas violations of global expectations (longer time-scales), that are generated using contextual information, show a positive ERP component (P3b) that accounts for controlled processing. Following the same rationale, this thesis includes four experiments that investigate the influence of semantic local and global expectations on word processing, aiming to assess residual language comprehension in DOC patients. By employing a relatedness proportion paradigm in a semantic priming task, we provide a ‘local’ context within trials (i.e., related/unrelated word-pairs), as we simultaneously manipulate global expectations by providing a context across the task (i.e., prime validity: cueing participants about the probability of a related target following the prime). We first report three behavioural visual studies in healthy participants, which suggest that individuals show greater priming effects in high validity contexts relative to low validity contexts (i.e., relatedness proportion effects), as individuals use the global context strategically to generate expectations about the target. Strategic involvement is supported by self-report measures, as only individuals that applied conscious strategies while performing the task showed these behavioural global effects. In a subsequent study we investigate the neural correlates of the generation of local and global expectations in this paradigm. The results show an earlier ‘local’ prediction error ERP effect around 250ms; followed by a later ‘global’ effect approximately at 350ms that interacted with the global context. We then adapt the task for DOC patients to identify auditory markers of strategic expectancy generation in this paradigm. The results in the healthy group suggest an early ‘local’ ERP effect (around 350ms), showing a prediction error ERP signal; and a later effect (around 550ms) reflecting violations of the ‘global’ semantic context i.e., larger error signal for unexpected targets in high validity relative to low validity context. Moreover, we present two DOC patient cases, where we detect a ‘local’ ERP effect in a patient (MCS diagnosis), whereas no ERP effect is observed in the other case (VS/UWS diagnosis). In conclusion, we propose this auditory ERP paradigm as a tool to detect residual language comprehension in DOC patients, as it detects hierarchical differentiated effects for strategic and non-strategic semantic expectations, similar to the local-global paradigm but in the clinically relevant domain of language processing.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):