Abstract
The human brain recurrently prioritizes task-relevant over task-irrelevant visual information. A central question is whether multiple objects can be prioritized simultaneously. To answer this, we let observers search for two colored targets among distractors. Crucially, we independently varied the number of target colors that observers anticipated, and the number of target colors actually used to distinguish the targets in the display. This enabled us to dissociate the preparation of selection mechanisms from the actual engagement of such mechanisms. Multivariate classification of electroencephalographic activity allowed us to track selection of each target separately across time. The results revealed only small neural and behavioral costs associated with preparing for selecting two objects, but substantial costs when engaging in selection. Further analyses suggest this cost is the consequence of neural competition resulting in limited parallel processing, rather than a serial bottleneck. The findings bridge diverging theoretical perspectives on capacity limitations of feature-based attention.
Highlights
Adaptive, goal-driven behavior demands the selection of relevant objects from the visual environment while irrelevant information is being ignored
To assess if prioritization of multiple targets is limited in terms of the number of attentional templates that can be simultaneously set up, limited in the number of templates that can be simultaneously engaged in the selection of target features in the display, or both, we independently manipulated 1) how many colors were task-relevant and 2) how many of these target colors appeared in the search display
This revealed a reliable onset difference between the 1TMP–1TGT (M = 216 ms) and 2TMP–1TGT (M = 237 ms) conditions (M = 21 ms, tc(23) = 2.21, p = 0.04; Figure 2C), indicating that attentional selection is delayed as a result of having to prepare for two different target colors compared to having to prepare for only a single target color
Summary
Goal-driven behavior demands the selection of relevant objects from the visual environment while irrelevant information is being ignored This requires the neural activation of task-relevant representations in memory – often referred to as attentional templates – which bias selection towards matching sensory input through top-down recurrent feedback loops (Duncan and Humphreys, 1989; Desimone and Duncan, 1995; Hamker, 2004; Eimer, 2014; Cohen and Tong, 2015). Because the N2pc can only distinguish between the left versus right hemifield, it is not able to simultaneously track the selection of multiple targets at different locations in more complex visual search displays To overcome this limitation, we used multivariate decoding, which has been proven to successfully track the spatiotemporal dynamics of feature-based selection processes at any location in a search display (Fahrenfort et al, 2017). This technique allowed us to independently track attentional selection over time for multiple concurrent targets at once, and to investigate the parallel versus serial nature of these selection processes
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