Event-related potential studies of prefrontal cortex contributions to episodic retrieval

Although the prefrontal cortex (PFC) is known to play roles in episodic memory retrieval, the specific processes it supports are not fully understood. The high temporal resolution of the event-related potential (ERP) technique provides one fruitful avenue for investigations of these processes. However, the PFC-supported retrieval operations that may be indexed by ERPs are currently under-specified. The work contained in this thesis is concerned with providing a more specific characterisation of PFC-supported processes that operate during episodic retrieval than is available currently. To this end, Experiments One to Three were designed in order to assess the likely functional significance of one known modulation which has been identified in ERP studies of episodic retrieval - the right-frontal ERP old/new effect. This effect is widely assumed to reflect activity generated within the PFC. Experiments Four and Five extended this work to related issues which arose as a result of the outcomes of the initial experiments. All five experiments reported in this thesis employed source memory tasks in which participants studied a list of words presented in one of two contexts (or sources). These words were presented visually in one of two colours in Experiments One, Two, Three, and Five (Visual condition), and were presented auditorily in a male/female voice in Experiments Four and Five (Auditory condition). At test, in all experiments, all studied words were presented visually in white letters intermixed with unstudied (new) words. For words judged to have been encountered previously (old words), a second judgment was required. In Experiments One and Two, this was a binary decision regarding the source in which the word had been previously presented (study colour). In Experiments Three, Four, and Five, a high/low source confidence rating was also made. For the right-frontal ERP old/new effect, strong evidence was provided in Experiments One and Two to rule out the potential contributions of three aspects of task design that may have contributed to disparate and seemingly contradictory findings in the published literature. These were: the presence/absence of copy cues at test, response requirements at test, and the difficulty of the retrieval task. Experiment Three was designed in order to test directly a "number of decisions account" for the right-frontal old/new effect (Dobbins & Han, 2006 Hayama, Johnson, & Rugg, 2008), and provided strong evidence inconsistent with such an account. A serendipitous finding in this study was evidence for a left-frontal ERP old/new effect that was functionally and electrophysiologically dissociable from the right-frontal old/new effect, and which differentiated between high and low confidence correct source judgments. There was no evidence for this effect in Experiment Four, however, when auditory (male/female voice) rather than visual (pink/yellow letters) source information was to be retrieved at test, suggesting the content-specificity of this frontally distributed ERP effect. This possibility was tested directly in Experiment Five in which versions of Experiments Three and Four were completed within-participants. Two separable frontal old/new effects were observed. These effects differed in their scalp distributions as a function of the forms of episodic content that were retrieved (i.e. visual vs. auditory source). The scalp distributions of the frontal old/new effects across Experiments Three to Five also varied according to whether the data from the first or the second halves of retrieval phases were analysed. These qualitative changes in neural activity according to time on task are interpreted in terms of processes involved in the resolution of interference, which presumably increases during the course of a retrieval task. The implications of this finding for conclusions made on the basis of averaged measures of neural activity across the entirety of a retrieval task are also discussed. In combination, the data reported in this thesis provide evidence that ERPs are sensitive to multiple neurally, functionally, and temporally distinct PFC-supported processes which operate during episodic retrieval, and offer insights into the roles played by PFC during episodic retrieval.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

The impact of prolonged disorders of consciousness on family caregivers’ quality of life – a scoping review

Providing long-term care for a family member diagnosed with a Prolonged Disorder of Consciousness (PDoC) can have a significant impact on the lives of family caregivers. This scoping review aimed to explore the current literature investigating the impact of caring for a person in a PDoC on family caregivers’ Quality of Life (QOL), as categorized using the WHOQOL-BREF model. We observed that articles employing quantitative methodologies mostly reported QOL outcomes relating to negative feelings, thinking, learning, memory and concentration, and personal relationships. Articles employing qualitative methodologies mostly reported QOL outcomes relating to negative feelings, personal relationships, positive feelings, and health and social care accessibility and quality. A descriptive content analysis of the QOL outcomes highlighted the limitations of the current literature base in representing the complexities of the experiences of family members providing care for a person in a PDoC. To provide valuable and personalized support to caregivers, without pathologizing or medicalizing their distress, it is vital to characterize more accurately the contextual subtleties of each person’s situation

Event-related potential studies of prefrontal cortex contributions to episodic retrieval

Although the prefrontal cortex (PFC) is known to play roles in episodic memory retrieval, the specific processes it supports are not fully understood. The high temporal resolution of the event-related potential (ERP) technique provides one fruitful avenue for investigations of these processes. However, the PFC-supported retrieval operations that may be indexed by ERPs are currently under-specified. The work contained in this thesis is concerned with providing a more specific characterisation of PFC-supported processes that operate during episodic retrieval than is available currently. To this end, Experiments One to Three were designed in order to assess the likely functional significance of one known modulation which has been identified in ERP studies of episodic retrieval - the right-frontal ERP old/new effect. This effect is widely assumed to reflect activity generated within the PFC. Experiments Four and Five extended this work to related issues which arose as a result of the outcomes of the initial experiments. All five experiments reported in this thesis employed source memory tasks in which participants studied a list of words presented in one of two contexts (or sources). These words were presented visually in one of two colours in Experiments One, Two, Three, and Five (Visual condition), and were presented auditorily in a male/female voice in Experiments Four and Five (Auditory condition). At test, in all experiments, all studied words were presented visually in white letters intermixed with unstudied (new) words. For words judged to have been encountered previously (old words), a second judgment was required. In Experiments One and Two, this was a binary decision regarding the source in which the word had been previously presented (study colour). In Experiments Three, Four, and Five, a high/low source confidence rating was also made. For the right-frontal ERP old/new effect, strong evidence was provided in Experiments One and Two to rule out the potential contributions of three aspects of task design that may have contributed to disparate and seemingly contradictory findings in the published literature. These were: the presence/absence of copy cues at test, response requirements at test, and the difficulty of the retrieval task. Experiment Three was designed in order to test directly a "number of decisions account" for the right-frontal old/new effect (Dobbins & Han, 2006 Hayama, Johnson, & Rugg, 2008), and provided strong evidence inconsistent with such an account. A serendipitous finding in this study was evidence for a left-frontal ERP old/new effect that was functionally and electrophysiologically dissociable from the right-frontal old/new effect, and which differentiated between high and low confidence correct source judgments. There was no evidence for this effect in Experiment Four, however, when auditory (male/female voice) rather than visual (pink/yellow letters) source information was to be retrieved at test, suggesting the content-specificity of this frontally distributed ERP effect. This possibility was tested directly in Experiment Five in which versions of Experiments Three and Four were completed within-participants. Two separable frontal old/new effects were observed. These effects differed in their scalp distributions as a function of the forms of episodic content that were retrieved (i.e. visual vs. auditory source). The scalp distributions of the frontal old/new effects across Experiments Three to Five also varied according to whether the data from the first or the second halves of retrieval phases were analysed. These qualitative changes in neural activity according to time on task are interpreted in terms of processes involved in the resolution of interference, which presumably increases during the course of a retrieval task. The implications of this finding for conclusions made on the basis of averaged measures of neural activity across the entirety of a retrieval task are also discussed. In combination, the data reported in this thesis provide evidence that ERPs are sensitive to multiple neurally, functionally, and temporally distinct PFC-supported processes which operate during episodic retrieval, and offer insights into the roles played by PFC during episodic retrieval

Why use a mirror to assess visual pursuit in prolonged disorders of consciousness? Evidence from healthy control participants

Background: Evidence of reliable smooth visual pursuit is crucial for both diagnosis and prognosis in prolonged disorders of consciousness (PDOC). However, a mirror is more likely than an object to elicit evidence of smooth pursuit. Our objective was to identify the physiological and/or cognitive mechanism underlying the mirror benefit. Methods: We recorded eye-movements while healthy participants simultaneously completed a visual pursuit task and a cognitively demanding two-back task. We manipulated the stimulus to be pursued (two levels: mirror, ball) and the simultaneous cognitive load (pursuit only, pursuit plus two-back task) within subjects. Results: Pursuit of the reflected-own-face in the mirror was associated with briefer fixations that occurred less uniformly across the horizontal plane relative to object pursuit. Secondary task performance did not differ between pursuit stimuli. The secondary task also did not affect eye movement measures, nor did it interact with pursuit stimulus. Conclusions: Reflected-own-face pursuit is no less cognitively demanding than object pursuit, but it naturally elicits smoother eye movements (i.e. briefer pauses to fixate). A mirror therefore provides greater sensitivity to detect smooth visual pursuit in PDOC because the naturally smoother eye movements may be identified more confidently by the assessor

Strategic and Non-Strategic Semantic Expectations Hierarchically Modulate Neural Processing

Perception is facilitated by a hierarchy of expectations generated from context and prior knowledge. In auditory processing, violations of local (within-trial) expectations elicit a mismatch negativity (MMN), while violations of global (across-trial) expectations elicit a later positive component (P300). This result is taken as evidence of prediction errors ascending through the expectation hierarchy. However, in language comprehension, there is no evidence that violations of semantic expectations across local-global levels similarly elicit a sequence of hierarchical error signals, thus drawing into question the putative link between event-related potentials (ERPs) and prediction errors. We investigated the neural basis of such hierarchical expectations of semantics in a word-pair priming paradigm. By manipulating the overall proportion of related or unrelated word-pairs across the task, we created two global contexts that differentially encouraged strategic use of primes. Across two experiments, we replicated behavioral evidence of greater priming in the high validity context, reflecting strategic expectations of upcoming targets based on “global” context. In our preregistered EEG analyses, we observed a “local” prediction error ERP effect (i.e., semantic priming) ∼250 ms post-target, which, in exploratory analyses, was followed 100 ms later by a signal that interacted with the global context. However, the later effect behaved in an apredictive manner, i.e., was most extreme for fulfilled expectations, rather than violations. Our results are consistent with interpretations of early ERPs as reflections of prediction error and later ERPs as processes related to conscious access and in support of task demands

Relationship between aetiology and covert cognition in the minimally-conscious state

Objectives: Functional neuroimaging has shown that the absence of externally observable signs of consciousness and cognition in severely brain-injured patients does not necessarily indicate the true absence of such abilities. However, relative to traumatic brain injury, nontraumatic injury is known to be associated with a reduced likelihood of regaining overtly measurable levels of consciousness. We investigated the relationships between etiology and both overt and covert cognitive abilities in a group of patients in the minimally conscious state (MCS). Methods: Twenty-three MCS patients (15 traumatic and 8 nontraumatic) completed a motor imagery EEG task in which they were required to imagine movements of their right-hand and toes to command. When successfully performed, these imagined movements appear as distinct sensorimotor modulations, which can be used to determine the presence of reliable command-following. The utility of this task has been demonstrated previously in a group of vegetative state patients. Results: Consistent and robust responses to command were observed in the EEG of 22% of the MCS patients (5 of 23). Etiology had a significant impact on the ability to successfully complete this task, with 33% of traumatic patients (5 of 15) returning positive EEG outcomes compared with none of the nontraumatic patients (0 of 8). Conclusions: The overt behavioral signs of awareness (measured with the Coma Recovery Scale–Revised) exhibited by nontraumatic MCS patients appear to be an accurate reflection of their covert cognitive abilities. In contrast, one-third of a group of traumatically injured patients in the MCS possess a range of high-level cognitive faculties that are not evident from their overt behavior