The complex interaction between different attentional control mechanisms during visual search

The brain selectively processes incoming sensory information and plans adequate motor output aimed at behaviourally relevant objects in the environment, based on different attentional control (AC) mechanisms. The contribution of single AC mechanisms to visual attention has been extensively investigated; still, it remains unclear how those different biasing signals interact with one another in order to reach the final choice of which spatial location (or stimulus) is worth of attention. The main goal of my PhD project was to investigate whether different AC mechanisms jointly act to shape priority of given stimuli and locations or whether, at any given moment, one mechanism prevails over the others, gaining precedence onto the neural representation of the visual space, known as spatial priority map. By using variants of the same visual search task, we implemented a series of behavioural and EEG experiments to test the unique and combined effect of two AC mechanisms: top-down control, modulated via endogenous cueing (valid vs. neutral cues), and experience-dependent control, implemented through a statistical learning (SL) protocol (high vs. low target frequency locations). Our results revealed that both cue validity and SL enhanced performance respectively for targets predicted by valid (vs. neutral) cue and for targets at high (vs. low) frequency locations. The benefit of top-down control was also confirmed by larger CNV and P1, i.e. EEG markers of general preparation and early categorization for target selection, respectively. Most importantly, when activated together, top-down control and SL display an interesting interaction, with the behavioural effect of the latter being overridden by the presence of the former. However, in terms of N2pc, an EEG index of selection, the cueing effect selectively emerged for targets in the low- (vs. high) frequency location, suggesting that, even if not behaviourally evident, the SL effect was not totally blocked by top-down guidance; rather, it could affect attentional deployment, at least at some point of the target selection process. Finally, in our tasks, we could also indirectly assess the impact of a salient distractor on individuals’ performance; this irrelevant bottom-up AC signal indeed diverted attention from the target and interfered with the task, regardless of the presence or absence of the other AC mechanisms.