A stimulus-dependent dissociation between the cerebral hemispheres under free-viewing conditions.

Under free-viewing conditions, the leftward stimulus features are frequently overestimated (pseudoneglect). This asymmetry could reflect the operation of: (a) spatial attention mechanisms in the right hemisphere (RH) or, (b) stimulus-specific activation. To test these propositions, we attempted to induce a stimulus-specific dissociation between the hemispheres under free-viewing conditions. In two experiments (n=23, n=22) dextrals carried out two tasks. The 'grayscales' task required luminance judgments between two mirror-reversed luminance gradients. The flicker task required temporal frequency judgments between two mirror-reversed temporal gradients. The grayscales and flicker tasks suited the processing styles of the right and left hemispheres, respectively. For both experiments, a strong leftward bias was observed for the grayscales task, which could be the result of both of the mechanisms outlined above. In Experiment 1, there was a rightward bias for the flicker task-but only for participants with longer reaction times. In Experiment 2, where all responses were delayed, a rightward bias was found for the flicker task for shorter stimuli. The data demonstrate that stimulus-specific dissociations can be induced under free-viewing conditions. However, the fact that the rightward bias was: (a) weaker than the leftward bias and, (b) reduced by increases in length, demonstrates that RH spatial attention mechanisms are also important.