Time course of visual attention across perceptual levels and objects.

Theories of shifts of visual attention based on attentional blink or dwell time do not directly address shifts of attention across different levels (global or local) involving multiple objects. Two experiments were conducted employing the attentional dwell time paradigm to investigate the shifts of visual attention between objects selected at same or different levels. Participants were instructed to identify two successive compound stimuli at a pre-specified level (global or local) presented at two different locations with variable SOA. The initial pair of locations in which the stimulus was presented was fixed in Experiment 1 but not in Experiment 2. Experiment 1 results showed very little impairment for second target identification when both the targets were at the global level. Attentional shift was better with both targets at the same level compared to different levels. Experiment 2 results showed that local followed by global target identification is difficult at short SOAs compared to other conditions. The results indicate that scope of attention affects the time course of visual attention. Global processing could be performed with very little capacity limitation simultaneously with distributed attention. The default mode of attention might be distributed and attention becomes focused for target identification. Different mechanisms may underlie shifts in focused attention between different locations and changes in attentional set required by changes in perceptual levels.