Role of working memory in the inhibition of unwanted memories: An electrophysiological & behavioural exploration.

The experiments in this thesis were designed to contribute to an understanding of the links between working memory capacity (WMC) and cognitive control over retrieval from long-term memory. The specific focus was on control over recollection, as measured by event-related potentials (ERPs), and how this was moderated by individual differences in WMC. Two broad assumptions underlying the investigations were that: (i) cognitive control over recollection can be exerted only when there are sufficient cognitive resources to do so, and (ii) WMC indexes resource availability. Evidence that could be interpreted in line with this account was accrued over the course of three experiments. Two consistent findings emerged. First, that there was no relationship between how well people completed the tasks and the extent to which control over recollection was exerted. Second, that there was a consistent positive correspondence between resource availability (as measured by WMC) and ERP evidence for the degree to which control over recollection occurred. This correspondence took two forms. First, a correlation between WMC and ERP evidence for the degree of control over recollection (Experiment 2). Second, the absence of evidence for control over recollection when WMC was temporarily reduced via the requirement to complete a demanding cognitive task prior to the memory retrieval task (Experiment 3). In addition to these findings, the third experiment in this thesis permitted a direct investigation of whether control over recollection was accomplished via inhibition of task-irrelevant memory contents. The people for whom there was evidence of a high degree of control over what was recollected showed poorer memory subsequently for the memory contents that were subjected to control than did people who exerted less control. This finding is consistent with the view that cognitive control was exerted by inhibiting certain memory contents, thereby making them less accessible at a later point in time. Taken together, these data strongly suggest that inhibition of unwanted information occurs during memory retrieval when participants have the working memory resources available to support active inhibition processes

Using recognition testing to support semantic learning in developmental amnesia

Patients with developmental amnesia (DA) have suffered hippocampal damage in infancy and subsequently shown poor episodic memory, but good semantic memory. It is not clear how patients with DA learn semantic information in the presence of episodic amnesia. However, patients with DA show good recognition memory and it is possible that semantic learning may be supported by recognition. Building on previous work, we compared two methods for supporting semantic learning in DA; recognition-learning and recall-learning. In each condition, a patient with DA (aged 8 years) was presented with semantic information in animated videos. After each presentation of a video, learning was supported by an immediate memory test. Two videos were paired with a cued recall test. Another two videos were paired with a multiple-choice test to enable recognition-based learning. The outcome measure was semantic recall performance after a short delay of 30 min and a long delay of one week. Results showed a benefit of recognition-learning compared to recall-learning on cued recall in the patient with DA (76% vs. 35%). This finding indicates that young people with severe hippocampal damage can utilize recognition to support semantic learning. This has implications for the support of school-aged children with episodic memory difficulties

Working memory capacity is related to variations in the magnitude of an electrophysiological marker of recollection

The links between the resources available for cognitive control and the ability to recover and maintain episodic content were investigated by contrasting an ERP index of recollection (the left-parietal ERP old/new effect) with a measure of working memory capacity (WMC). Participants were given the O-Span measure of WMC and completed a retrieval task in which they had to make responses on one key to previously studied words (targets) and responses on a second key to words that were presented at retrieval on either one or two occasions (new words and non-targets, respectively). The size of the ERP index of recollection associated with correct responses to targets was correlated with WMC, a finding consistent with the view that this ERP effect is linked to operations associated with maintaining information on-line in service of task goals. In addition, the degree to which left-parietal ERP old/new effects for targets were larger than for non-targets increased as WMC increased. Larger left-parietal ERP old/new effects for targets than for non-targets have been interpreted as evidence of successful prioritisation of recollection of target information. The link with WMC reported here is consistent this view, in so far as WMC indexes the availability of resources that are necessary to exert cognitive control over memory retrieval

The role of working memory capacity in the control of recollection

The links between control over recollection and working memory capacity (WMC) were investigated using event-related potentials (ERPs) and behavioural assays. Electrophysiological evidence for a relationship between greater control over recollection and higher scores on a measure of WMC was obtained. In addition, people with high WMC who first completed a task requiring cognitive control showed no electrophysiological evidence for control over recollection on a subsequent task. This outcome suggests a causal link between control over recollection and the availability of WMC, in so far as the consequence of completing the first task was a reduction in WMC that impacted on completion of the subsequent task. All participants also completed a final recall task, on which they were asked to remember the stimuli they had encountered during the task in which ERPs were acquired. Only those participants who showed electrophysiological evidence for the exertion of control over recollection showed differences between the likelihoods of recalling stimuli over which control either had or had not been exerted. In combination, the findings provide insights into the conditions under which control over recollection occurs, and make a strong argument for including individual difference measures of resource availability when assessing how and when people exert control over what they remember

Neural correlates of individual differences in strategic retrieval processing

Processes engaged when information is encoded into memory are an important determinant of whether that information will be recovered subsequently. Also influential, however, are processes engaged at the time of retrieval, and these were investigated here by using event-related potentials (ERPs) to measure a specific class of retrieval operations. These operations were revealed by contrasts between ERPs elicited by new (unstudied) test items in distinct tasks, the assumption being that these contrasts index operations that are engaged in service of retrieval and that vary according to the demands of different retrieval tasks. Specific functional accounts of this class of retrieval processing operations assume that they influence the accuracy of memory judgments, and this experiment was designed to test for the first time whether this is in fact the case. Toward this end, participants completed 2 retrieval tasks while ERPs were acquired, and the extent to which processes were engaged differentially across tasks in service of retrieval was operationalized as the magnitude of the differences between the new-item ERPs that were elicited. This measure correlated positively with response accuracy on the tasks, which provides strong evidence that this class of retrieval processing operations benefits the accuracy of memory judgments

Reduced specificity of autobiographical memory and depression: The role of executive control

It has been widely established that depressed mood states and clinical depression, as well as a range of other psychiatric disorders, are associated with a relative difficulty in accessing specific autobiographical information in response to emotion-related cue words on an Autobiographical Memory Test (AMT; J. M. G. Williams and K. Broadbent, 1986). In 8 studies the authors examined the extent to which this relationship is a function of impaired executive control associated with these mood states and clinical disorders. Studies 1-4 demonstrated that performance on the AMT is associated with performance on measures of executive control, independent of depressed mood. Furthermore, Study 1 showed that executive control (as measured by verbal fluency) mediated the relationship between both depressed mood and a clinical diagnosis of eating disorder and AMT performance. Using a stratified sample in Study 5, the authors confirmed the positive association between depressed mood and impaired performance on the AMT. Studies 6-8 involved experimental manipulations of the parameters of the AMT designed to further indicate that reduced executive control is to a significant extent driving the relationship between depressed mood and AMT performance. The potential role of executive control in accounting for other aspects of the AMT literature is discussed