The nature and contribution of space- and object-based attentional biases to free-viewing perceptual asymmetries.

Two experiments investigated the contribution of space- and object-based coordinates to previously reported leftward perceptual biases (pseudoneglect) at various locations across visual space. Neurologically intact participants (n = 34 and 27) made luminance discriminations between two left/right mirror-reversed luminance gradients (greyscales task), which were variously displaced around the midline in the participants' left and right hemispaces. The orientations of the stimuli were manipulated so that object- and space-based coordinates were congruent or incongruent. Experiment 1 confirmed the presence of a leftward object-based perceptual bias. The bias was moderated, however, by overattention to the more central stimulus. This central spatial effect could have resulted from the use of task-specific strategies, which were controlled by presenting the stimuli sequentially in Experiment 2. The findings of Experiment 1, a leftward object-based bias and a central spatial bias, were replicated. Overall, the results indicate a leftward object-based bias and a central spatial bias, both of which are relevant for the allocation of attention. The results are discussed with reference to a variety of models of the distribution of attention across space.