title: Can free-viewing perceptual asymmetries be explained by scanning, pre-motor or attentional biases?
abstract: Judgments of relative magnitude between the left and right sides of a stimulus are generally weighted toward the features contained on the left side. This leftward perceptual bias could be the result of, (a) left-to-right scanning biases, (b) pre-motor activation of the right hemisphere, or (c) a left hemispatial attentional bias. The relative merits of these explanations of perceptual asymmetry were investigated. In Experiment 1, English and Hebrew readers made luminance judgements for two left/right mirror-reversed luminance gradients (greyscales task). Despite different reading/scanning habits, both groups exhibited a leftward perceptual bias. English and Hebrew readers also performed a line bisection task. Scanning biases were controlled by asking participants to follow a marker as it moved left-to-right or right-to-left and then stop it as it reached the midpoint of the line. Despite controlling scanning, a leftward bias was observed in both groups. In Experiment 2, peripheral spatial cues were presented prior to the greyscales stimuli. English readers showed a reduction in the leftward bias for right-sided cues as compared to left-sided and neutral cues. Right-side cues presumably overcame a pre-existing leftward attentional bias. In both experiments, pre-motor activation was controlled using bimanual responses. Despite this control, a leftward bias was observed throughout the study. The data support the attentional bias account of leftward perceptual biases over the scanning and pre-motor activation accounts. Whether or not unilateral hemispheric activation provides an adequate account of this attentional bias is discussed.
authors.affiliation: Department of Psychology, University of Melbourne, Parkville, Australia. m.nicholls@psych.unimelb.edu.au
authors.forename: Michael E R
authors.initials: ME
authors.lastname: Nicholls
authors.forename: Georgina R
authors.initials: GR
authors.lastname: Roberts
mesh_terms: D000293:Adolescent; D000328:Adult; D001288:Attention; D001315:Australia; D015982:Bias; D001921:Brain / Q000502:physiology*; D005260:Female; D007839:Functional Laterality / Q000502:physiology*; D006801:Humans; D007557:Israel / Q000208:ethnology; D008297:Male; D011930:Reaction Time; D013647:Task Performance and Analysis; D014796:Visual Perception / Q000502:physiology*