Previous visual search studies have revealed that target identification can be facilitated by foreknowledge of a salient distractor’s feature(s). The term ‘templates-for-rejection’ (coined by Woodman & Luck, 2007) has been used to refer to this effect and mechanisms of active suppression are proposed to be involved (e.g.,Gaspelin & Luck, 2019; Sawaki & Luck, 2010). To date, there has been an absence of research into the possible underlying neural mechanisms of templates-for-rejection for stimuli possessing socio-motivational relevance. The first three experiments of this thesis therefore used facial expression stimuli to investigate this issue. Experiment one (Chapter 2) presented face pairs bilaterally and participants reported the sex of the non-neutral face (neutral template-for-rejection block), or the non-angry face (angry template-for-rejection block). For comparison, participants also reported the sex of the neutral, or angry face (neutral vs. angry template-for-selection blocks). The template-matching face appeared beside either a happy, sad, surprised, or disgusted face. Electroencephalography (EEG) was recorded with event-related-potentials (ERPs) time-locked to the onset of the faces. Accuracy and response times were also measured. An ERP component previously considered to reflect active suppression was observed in response to neutral, but not angry, templates-for-rejection. However, what appeared to be active suppression of neutral templates could have been attentional capture by the more emotionally salient face on the opposite side. Experiments two (neutral templates) and three (angry templates) (Chapters 3 and 4) used the same faces, but with one face appearing on the midline and the other to the left or right of fixation, in order to isolate the ERP response to the lateral stimulus. For the neutral templates-for-rejection only, active suppression was revealed, consistent with previous studies using non-motivationally relevant stimuli. This effect appeared to arise at a relatively late stage of processing, suggesting that distractor representations may have been suppressed within visual working memory (VWM) to prevent ongoing engagement with task-irrelevant stimuli. In line with the findings from Experiment one, there was no evidence of active suppression of angry templates. This may indicate the resistance of angry facial expressions to suppression, but it is also the case that angry faces may have been harder than neutral faces to perceptually discriminate from paired emotional expressions; thus, an interpretation in terms of task difficulty for the absence of active suppression of angry face templatesfor-rejection cannot be ruled out. Experiment four (Chapter 5) was therefore carried out in part to explore the role of task difficulty on ERP indices of active suppression using non-motivationally relevant colour singletons. Attentional control was better under easy (crowded array), compared with hard (sparse array) task conditions. For crowded displays only, active suppression of non-targets occurred during late attention processing which was earlier than in experiment two. It is possible therefore that an apparent lack of active suppression for angry templates-for-rejection in experiments one and three may have been due to task difficulty as well as the aversive motivational significance of the stimuli. In contrast to experiment two (neutral template findings), there was no evidence that continued engagement with the non-motivationally relevant stimuli was actively terminated within the late VWM processing stage. Notably, the ERP profile of response in Experiment four corresponded to a template for rejection impairment with respect to speed and accuracy of target identification. Experiment five (Chapter 6) sought to explore the role of increased perceptual difficulty under equivalent high cognitive demands as experiment four, in order to investigate the conditions necessary for a behavioural templates-for-rejection benefit to emerge. To conclude, this thesis demonstrates that active suppression of template distractors may be influenced by the motivational relevance and threat value of stimuli, and also cognitive and perceptual task demands. The absence of a clear behavioural template-for-rejection benefit across experiments is explained in terms of delayed active suppression due to high cognitive demands associated with the requirements to switch between template features.
|Date of Award||14 May 2020|
|Sponsors||University of Roehampton|
|Supervisor||Amanda Holmes (Director of Studies) & Paul Bretherton (Co-Supervisor)|