title: A stimulus-dependent dissociation between the cerebral hemispheres under free-viewing conditions.
abstract: 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.
authors.affiliation: Department of Psychology, University of Melbourne, Parkville, VIC 3010, Australia. mokubo@unimelb.edu.au
authors.forename: Matia
authors.initials: M
authors.lastname: Okubo
authors.forename: Michael E R
authors.initials: ME
authors.lastname: Nicholls
mesh_terms: D000293:Adolescent; D000328:Adult; D001288:Attention; D015982:Bias; D005260:Female; D005425:Flicker Fusion / Q000502:physiology; D007839:Functional Laterality / Q000502:physiology*; D006801:Humans; D008297:Male; D010468:Perceptual Disorders / Q000503:physiopathology*; D010775:Photic Stimulation / Q000379:methods; D011597:Psychomotor Performance / Q000502:physiology; D011930:Reaction Time / Q000502:physiology; D013028:Space Perception / Q000502:physiology*; D013223:Statistics as Topic; D013997:Time Factors; D014794:Visual Fields / Q000502:physiology*