Spatial processing of images and hemisphere function

Origins of asymmetry in the CNS

Vertebrate ancestors had extreme asymmetry of the CNS, largely imposed by bodily asymmetry. In the zebrafish a key asymmetry is that of the habenulae. Their major outflow on the left is concerned with visual control of sustained response to targets, and on the right with response to potent releasers of innate responses. Mammals retain comparable outflows but without obvious asymmetry. Behavioural asymmetries associated with the processing of perceptual information are, if anything, exaggerated. Evidence from insects suggests that these latter asymmetries are of great value in any complex CNS. Bodily asymmetry may therefore not be essential for their evolution.

Prefrontal cortex hemispheric specialization for categorical and coordinate visual spatial memory

During visual spatial perception of multiple items, the left hemisphere has been shown to preferentially process categorical spatial relationships while the right hemisphere has been shown to preferentially process coordinate spatial relationships. We hypothesized that this hemispheric processing distinction would be reflected in the prefrontal cortex during categorical and coordinate visual spatial memory, and tested this hypothesis using functional magnetic resonance imaging (fMRI). During encoding, abstract shapes were presented in the left or right hemifield in addition to a dot at a variable distance from the shape (with some dots on the shape); participants were instructed to remember the position of each dot relative to the shape. During categorical memory retrieval, each shape was presented centrally and participants responded whether the previously corresponding dot was 'on' or 'off' of the shape. During coordinate memory retrieval, each shape was presented centrally and participants responded whether the previously corresponding dot was 'near' or 'far' from the shape (relative to a reference distance). Consistent with our hypothesis, a region in the left prefrontal cortex (BA10) was preferentially associated with categorical visual spatial memory and a region in the right prefrontal cortex (BA9/10) was preferentially associated with coordinate visual spatial memory. These results have direct implications for interpreting previous findings that the left prefrontal cortex is associated with source memory, as this cognitive process is categorical in nature, and the right prefrontal cortex is associated with item memory, as this process depends on the precise spatial relations among item features or components.

Lateralized spatial processes and their lexical implications

Numerous lateralized tasks have been identified as involving spatial processing, but the extent to which they use the same spatial process is largely unknown. The present research investigated relationships among significant asymmetries from seven visuo-spatial and two verbal tasks, combining samples from previous factor analytic experiments with those from three new studies (combined N=789). All of the spatial intercorrelations were negligible (r<0.20), unlike previous outcomes finding robust correlations within certain clusters of verbal asymmetries. However, lateral differences involving spatial quantitative processing on the one hand and figure-ground separation on the other, showed significant but independent correlations with one from visual lexical processing (word recognition). The results support three major conclusions. First, there are at least several and possibly many lateralized spatial processes. Second, hemispheric processes follow a modular architecture in preference to a diffuse or parallel distributed architecture. Finally, the dissociated correlations are consistent with a visual lexical processing model having both occipital and parietal components, potentially reconciling a current controversy over the cerebral localization of language.

Coordinate and categorical judgements in spatial imagery. An fMRI study

We aimed at verifying whether the hemispheric specialisation for categorical/coordinate spatial judgements also applies to the spatial imagery domain by the use of whole-brain fMRI. In a block-design experiment we used the "coordinate" mental clock test, contrasting it with a "categorical" task applied to the same clock stimuli; as a control task we used a syllable counting task requiring a verbal-phonological judgement on the same material of the two imagery tasks. Our results showed that categorical and coordinate spatial judgements on imagined stimuli rely on the activation of a set of cortical areas, centred upon the superior parietal lobule (SPL) bilaterally. These regions, together with other parietal and prefrontal areas, showed a pattern of relative lateralization, with the left hemisphere being mainly activated during the categorical task and the right in the coordinate task. These data confirm the strong involvement of the SPL in spatial processing. Moreover, our findings suggest that different interconnected neural networks are activated to comply with specific test requirements, giving rise to functional imaging patterns compatible with psychological theories on hemispheric specialization.

Hemispheric and facial asymmetry: faces of academe

Facial asymmetry (facedness) of selected academic faculty members was studied in relation to brain asymmetry and cognitive specialization. Comparisons of facedness were made among humanities faculty (H), faculty members of mathematics and physics (M-P), psychologists (P), and a group of randomly selected individuals (R). Facedness was defined in terms of the relative sizes (in square centimeters) of the two hemifaces. It was predicted that the four groups would show differences in facedness, namely, H, right face bias; M-P, left face bias; P, no bias; and R, no bias. The predictions were confirmed, and the results interpreted in terms of known differences in hemispheric specialization of cognitive functions as they relate to the dominant cognitive activity of each of the different groups. In view of the contralateral control of the two hemifaces (below the eyes) by the two hemispheres of the brain, the two sides of the face undergo differential muscular development, thus creating facial asymmetry. Other factors, such as gender, also may affect facial asymmetry. Suggestions for further research on facedness are discussed.

Transcranial Doppler ultrasonic assessment of middle cerebral artery blood flow velocity changes during verbal and visuospatial cognitive tasks

Hemisphere specific changes of blood flow velocity in the right and left middle cerebral artery (MCA) induced by cognitive demands of verbal and nonverbal tasks were examined by means of a newly developed technique of simultaneous bilateral transcranial ultrasonic Doppler sonography (TCD). Thirty-one right-handed healthy volunteers served as subjects. Identical stimulus and response procedures were used with all tasks to avoid possible differential effects of these conditions on blood flow velocity. Compared to the preceding resting phase, the increase in flow velocity induced by each of the verbal tasks (sentence completion, similar or contrasting word meaning, similarities) proved to be significantly higher in the left than in the right MCA. Among the non-verbal visuospatial tasks only the "identical pictures" (perceptual speed) task led to a complementary higher increase in right MCA blood flow velocity. No such asymmetry in blood flow acceleration was observed, however, with the tasks "figure assembly" and "cube comparison" which require visualization and mental rotation of figures. The findings underline the recently emerging uncertainty in neuropsychological research with regard to the functional specialization of the right hemisphere.