Interhemispheric integration of information in a surface estimation task

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.

Bilateral visual field processing and evoked potential interhemispheric transmission time

The relationship between the efficiency of interhemispheric interactions via the corpus callosum and the speed and accuracy in making comparisons of information simultaneously presented to the right and left visual fields was studied by comparing bilateral (vs unilateral) advantages in matching letters, with evoked potential measures of interhemispheric transmission time (EP-IHTT). The primary finding was a strong correlation suggesting that larger bilateral field advantages in reaction time are associated with faster EP-IHTT. However, the association between EP-IHTT and bilateral advantage was strong only for transmission speed from left hemisphere to right hemisphere, but not for speed of transmission in the opposite direction. The data are consistent with a hypothesis of asymmetric homologue enhancement, i.e. a directionally asymmetric callosal influence which facilitates processing of letter stimuli in the right hemisphere allowing for increased response speed and accuracy of bilateral visual field comparisons.

The bilateral field advantage on a letter-matching task

A series of studies have reported that responding is faster when letter pairs to be matched are projected to two hemispheres rather than one. Four experiments described here tested this bilateral field advantage and identified factors that influence its extent. Subjects were shown letter pairs drawn from the ensemble "AaBb", and classified the letter pairs as "Match" if the letters had the same name (regardless of case) and "No Match" if they did not. In the first two experiments the letter pairs were presented unilaterally (both letters in one visual field), bilaterally (one letter in each visual field), or centrally (both letters on the vertical midline, above and below fixation), in order to investigate how the bilateral field advantage is influenced by screen location. The third experiment added a bilateral-diagonal position (to check for artefacts related to horizontal scanning strategies), and the fourth experiment added distractor digits (to equate initial processing demands in the bilateral and unilateral conditions). Results indicate that the bilateral field advantage is a robust phenomenon, although several manipulations reduced its magnitude. Implications of these findings for models of hemispheric collaboration and interhemispheric processing are discussed.

Hemispheric interactions: the bilateral advantage and task difficulty

Twenty-five normal subjects made "same-different" responses to dot patterns presented in the LVF, RVF or bilaterally. Task difficulty was manipulated in each condition by varying the number of dots in the two patterns presented from two to four to six. The pairs of patterns always had the same number of dots on a given trial. Response latency and accuracy worsened as the number of dots increased for all three presentation conditions and for both "same" and "different" judgements. Overall, responding was faster and the number of errors lower on Bilateral presentations. For response latencies to identical patterns of dots, the size of the bilateral advantage increased relative to RVF responding as task difficulty increased but did not change significantly relative to LVF responding. When the two patterns were not identical the size of the advantage did not change as task difficulty increased. "Same" judgements were faster but less accurate than "different" judgements. A model of hemispheric interactions is proposed to account for the findings.

Bilateral processing of numbers: parallel processing versus functional unity

Thirty-six normal subjects had to add two numbers ranging from 3 to 9 which were presented tachistoscopically. The two numbers were displayed either unilaterally to the left or right visual field, or bilaterally with one number to the left and one to the right visual field. They were represented as two word names, two dot clusters, or two bargraphs, or the two numbers were represented inconsistently in two different of these representations. The results showed that bilateral processing of a word and a bargraph was performed without visible interactions, and the hemispheres performed independently. When the display included a dot cluster, however, its processing (sequential vs. holistic) appeared to depend on the type of stimulus processed by the contralateral hemisphere, indicating strong interhemispheric interactions.

Hemispheric specialization for tactile perception opposed by contralateral noise

The aim of this study was to investigate the contribution of each cerebral hemisphere in the categorization of tactile information with and without contralateral tactile noise. Two groups of subjects rated the dissimilarity of paired stimuli varying in shape and texture presented to the left or right hand, with contralateral noise in the other hand for one group only. Analysis of variance on derived structured indices showed a general left hand advantage in the treatment of this haptic information. Moreover, a group by hand interaction revealed that noise-exposed subjects showed better differentiation of tactile information for stimuli presented to the left hand. The data suggest a unidirectional interference by the left hemisphere in the processing of tactile information in noise-free conditions and that quality of interhemispheric transfer of information may modulate cerebral functional lateralization.

Effects of spatial arrangement of letter pairs in a name-matching task with unilateral and bilateral hemifield stimulation

In tachistoscopic studies requiring "same/different" judgements on pairs of stimuli, differences between unilateral and bilateral presentation have been interpreted in terms of higher level cognitive processing. However, unilateral presentation is usually confounded with vertical and bilateral presentation with horizontal arrangement of stimuli. The effect of various stimulus arrangements was examined in two experiments. The results gave no evidence for an independent effect of uni- vs bilateral presentation, but rather a strong effect of the spatial arrangement such that pairs arranged on a straight line through the fixation point yield faster reaction times than any other vertical or horizontal arrangement.