Interaction between visual attention and the processing of visual emotional stimuli in humans : eye-tracking, behavioural and event-related potential experiments
Acunzo, David Jean Pascal
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Past research has shown that the processing of emotional visual stimuli and visual attention are tightly linked together. In particular, emotional stimuli processing can modulate attention, and, reciprocally, the processing of emotional stimuli can be facilitated or inhibited by attentional processes. However, our understanding of these interactions is still limited, with much work remaining to be done to understand the characteristics of this reciprocal interaction and the different mechanisms that are at play. This thesis presents a series of experiments which use eye-tracking, behavioural and event-related potential (ERP) methods in order to better understand these interactions from a cognitive and neuroscientific point of view. First, the influence of emotional stimuli on eye movements, reflecting overt attention, was investigated. While it is known that the emotional gist of images attracts the eye (Calvo and Lang, 2004), little is known about the influence of emotional content on eye movements in more complex visual environments. Using eye-tracking methods, and by adapting a paradigm originally used to study the influence of semantic inconsistencies in scenes (Loftus and Mackworth, 1978), we found that participants spend more time fixating emotional than neutral targets embedded in visual scenes, but do not fixate them earlier. Emotional targets in scenes were therefore found to hold, but not to attract, the eye. This suggests that due to the complexity of the scenes and the limited processing resources available, the emotional information projected extra-foveally is not processed in such a way that it drives eye movements. Next, in order to better characterise the exogenous deployment of covert attention toward emotional stimuli, a sample of sub-clinically anxious individuals was studied. Anxiety is characterised by a reflexive attentional bias toward threatening stimuli. A dot-probe task (MacLeod et al., 1986) was designed to replicate and extend past findings of this attentional bias. In particular, the experiment was designed to test whether the bias was caused by faster reaction times to fear-congruent probes or slower reaction times to neutral-congruent probes. No attentional bias could be measured. A further analysis of the literature suggests that subliminal cue stimulus presentation, as used in our case, may not generate reliable attentional biases, unlike longer cue presentations. This would suggest that while emotional stimuli can be processed without awareness, further processing may be necessary to trigger reflexive attentional shifts in anxiety. Then the time-course of emotional stimulus processes and its modulation by attention was investigated. Modulations of the very early visual ERP C1 component by emotional stimuli (e.g. Pourtois et al., 2004; Stolarova et al., 2006), but also by visual attention (Kelly et al., 2008), were reported in the literature. A series of three experiments were performed, investigating the interactions between endogenous covert spatial attention and object-based attention with emotional stimuli processing in the C1 time window (50–100 ms). It was found that emotional stimuli modulated the C1 only when they were spatially attended and task-irrelevant. This suggests that whilst spatial attention gates emotional facial processing from the earliest stages, only incidental processing triggers a specific response before 100 ms. Additionally, the results suggest a very early modulation by feature-based attention which is independent from spatial attention. Finally, simulated and actual electroencephalographic data were used to show that modulations of early ERP and event-related field (ERF) components are highly dependent on the high-pass filter used in the pre-processing stage. A survey of the literature found that a large part of ERP/ERF reports (about 40%) use high-pass filters that may bias the results. More particularly, a large proportion of papers reporting very early modulations also use such filters. Consequently, a large part of the literature may need to be re-assessed. The work described in this thesis contributes to a better understanding of the links between emotional stimulus processing and attention at different levels. Using various experimental paradigms, this work confirms that emotional stimuli processing is not ‘automated’, but highly dependent on the focus of attention, even at the earlier stages of visual processing. Furthermore, the uncovered potential bias generated by filtering will help to improve the reliability and precision of research in the ERP/ERF field, and more particularly in studies looking at early effects.