Can task specific perceptual bias be distinguished from unilateral neglect?

Can free-viewing perceptual asymmetries be explained by scanning, pre-motor or attentional biases?

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.

Attentional asymmetries following ECT in patients with major depression

Neuropsychological abnormalities of lateralization have been reported after right unilateral electroconvulsive therapy (ECT), that may reflect temporary disruption of the treated hemisphere. A visuospatial task sensitive to lateralization of spatial attention was administered in a test-retest design to patients with unipolar major depression and a group of age and gender matched controls. The patient group underwent right unilateral ECT between the two test sessions. The patient and control groups did not differ significantly at the initial baseline testing. After ECT, the patient group showed a significant shift of attentional bias toward the left, while the control group showed no significant shift in the second session relative to the first. The results suggest that approximately 1 h after termination of ictus there is a leftward attentional bias, possibly reflecting a change in right hemisphere cerebral activity.

Laterality of expression in portraiture: putting your best cheek forward

Portraits, both photographic and painted, are often produced with more of one side of the face showing than the other. Typically, the left side of the face is overrepresented, with the head turned slightly to the sitter's right. This leftward bias is weaker for painted male portraits and non-existent for portraits of scientists from the Royal Society. What mechanism might account for this bias? Examination of portraits painted by left- and right-handers and of self-portraits suggests that the bias is not determined by a mechanical preference of the artist or by the viewer's aesthetics. The leftward bias seems to be determined by the sitters and their desire to display the left side of their face, which is controlled by the emotive, right cerebral hemisphere. When we asked people to portray as much emotion as possible when posing for a family portrait, they tended to present the left side of their face. When asked to pose as scientists and avoid portraying emotion, participants tended to present their right side. The motivation to portray emotion, or conceal it, might explain why portraits of males show a reduced leftward bias, and also why portraits of scientists from the Royal Society show no leftward bias.

Language experience and right hemisphere tasks: the effects of scanning habits and multilingualism

This study explores the effects of multilingualism and reading scanning habits on right hemisphere (RH) abilities. Native Hebrew speakers and Arabic-Hebrew bilinguals performed three tasks. Experiment 1 employed an odd/even decision paradigm on lateralized displays of bar graphs. Both groups of subjects displayed the expected LVFA within the range previously reported for readers of English. Experiment 2 consisted of a chair identification task designed to tap asymmetry of hemispheric arousal and a chimeric face task designed to tap RH specialization for facial emotion. Neither scanning habits nor language experience affected performance on the chair task. Scanning habits seem to have affected performance on the chimeric faces task: there was no preference for the left smile in these right-to-left readers, as opposed to previous results in the literature using left-to-right readers. Correlations between measures from the three tasks and all the subject's scores on an English proficiency test and on a Hebrew test for the bilinguals reveal tentative relationships between proficiency in a second language and RH abilities. The results do not support the hypothesis that multilingualism can affect the manner in which these nonlanguage tasks are subserved by the RH. They do support the hypothesis that scanning habits particular to specific languages can affect performance asymmetries on some nonlanguage tasks that have been posited to reflect RH specialization.