Neural dynamics of shifting visual attention between objects and locations in early visual cortex

Effects of location- and object-based attention on sensory processing have been mostly studied in isolation leaving the relations between them less well understood. In an EEG experiment, temporal dynamics of location- and object-based attention were investigated with a probabilistic spatial cueing task to test temporal differences between sensory enhancement of two locations in one object. Stimuli consisted of two vertical rectangles/bars filled with a random noise pattern. Each rectangle was superimposed by two flickering clouds of dots to elicit steady-state visual evoked potentials (SSVEPs) that tagged the two ends of each object. A central cue pointed either to one (single cue) or to two (double cue) task-relevant locations on the bar. Targets occurred with a much higher probability on the cued location, and with a lower probability on uncued locations. Uncued locations were equidistant to the cued location either on the same object (unattended same) or the different object (unattended different). We replicated the finding of longer reaction times for targets at the uncued object (uncued different) compared to the uncued location of the cued object (uncued same). SSVEP amplitudes were significantly modulated by spatial and object-based integration relative to the pre-cue baseline. Interestingly, results indicated (a) an earlier enhancement of SSVEP amplitudes and (b) a sustained higher SSVEP amplitude for the cued compared to the uncued location of the same object. These results are in line with previous studies suggesting that object integration involves sequential activation and sustained enhanced activation of the to-be-attended attribute after the integration process.