Hemispheric specialization: What, how and why

Lateralized cognitive processes and lateralized task control in the human brain

The principles underlying human hemispheric specialization are poorly understood. We used functional magnetic resonance imaging of letter and visuospatial decision tasks with identical word stimuli to address two unresolved problems. First, hemispheric specialization depended on the nature of the task rather than on the nature of the stimulus. Second, analysis of frontal candidate regions for cognitive control showed increased coupling between left anterior cingulate cortex (ACC) and left inferior frontal gyrus during letter decisions, whereas right ACC showed enhanced coupling with right parietal areas during visuospatial decisions. Cognitive control is thus localized in the same hemisphere as task execution.

Separable Mechanisms in Face Processing: Evidence from Hemispheric Specialization

This article addresses three issues in face processing: First, is face processing primarily accomplished by the right hemisphere, or do both left- and right-hemisphere mechanisms play important roles? Second, are the mechanisms the same as those involved in general visual processing, or are they dedicated to face processing? Third, how can the mechanisms be characterized more precisely in terms of processes such as visual parsing? We explored these issues using the divided visual field methodology in four experiments. Experiments 1 and 2 provided evidence that both left- and right-hemisphere mechanisms are involved in face processing. In Experiment 1, a right-hemisphere advantage was found for both Same and Different trials when Same faces were identical and Different faces differed on all three internal facial features. Experiment 2 replicated the right-hemisphere advantage for Same trials but showed a left-hemisphere advantage for Different trials when one of three facial features differed between the target and the probe faces. Experiment 3 showed that the right-hemisphere advantage obtained with upright faces in Experiment 2 disappeared when the faces were inverted. This result suggests that there are right-hemisphere mechanisms specialized for processing upright faces, although it could not be determined whether these mechanisms are completely face-specific. Experiment 3 also provided evidence that the left-hemisphere mechanisms utilized in face processing tasks are general-purpose visual mechanisms not restricted to particular classes of visual stimuli. In Experiment 4, a left-hemisphere advantage was obtained when the task was to find one facial feature that was the same between the target and the probe faces. We suggest that left-hemisphere advantages shown in face processing are due to the parsing and analysis of the local elements of a face.

Gestalt laws of perceptual organization in an embedded figures task: evidence for hemispheric specialization

An embedded figures task was used to investigate hemispheric specialization for visual parsing. Error data from 40 normal right-handed males revealed a right-hemisphere advantage for parsing governed by the Gestalt laws of organization. In contrast, for parsing that violated those laws, subjects who could perform such parsing at better-than-chance levels showed a left-hemisphere advantage. A simple left hemisphere/analytic, right hemisphere/holistic dichotomy is too ambiguous to account for the results, because it fails to specify the level of visual structure (e.g. whole figures, intermediate components, or elementary segments) that is to be taken as a reference point for defining particular tasks as analytic or holistic; what is analytic from one point of view is holistic from another. We focus instead on principles of perceptual organization that characterize the results of perceptual processing, regardless of whether those results are seen as the outcome of analytic or holistic processing.

Shifting ear differences in melody recognition through strategy inducement

In a previous study, the pattern of ear superiority displayed by nonmusicians in a task where dichotically presented target melodies have to be recognized among four probes was found to be correlated with subjects' descriptions of their operating modes: those who reported focusing on critical local differences in pitch tended to show right ear advantage (REA) and the others LEA. The purpose of the present study was to examine if laterality patterns can be affected by treatments designed to induce particular strategies. Treatments supposed to orient toward analytic processing, advising attention to critical notes, with (Experiment 2) or without (Experiment 1) the additional task of reporting their location, produced the expected shift toward REA. To the contrary, treatments supposed to orient toward holistic processing, advising attention to overall contour or requesting aesthetic judgments (Experiment 1), failed to produce the expected shift toward LEA. The result can be attributed either to a basic difference in susceptibility to strategic control between the different operating modes or, more simply, to ineffectiveness of the present holistically oriented treatments.