Interhemispheric cooperation of left- and right-handers in mental calculation tasks

Relations between handedness and interhemispheric processing in cognitive tasks were examined. Thirty six right-handers and thirty left-handers (familial and nonfamilial left-handers) were asked to add two numbers which were presented tachistoscopically. Two numbers were displayed either to one visual field, or one to the left and one to the right visual field simultaneously. In Experiment 1, the numbers were displayed in Arabic numerals, and in Experiment 2 one of the numbers was displayed in Kanji and one in Arabic numerals. The results of Experiment 1 showed a bilateral advantage, however no subject group difference was shown in the performance of the three (left unilateral, right unilateral, and bilateral) presentation conditions. Though familial left-handers showed a weaker tendency to different patterns than right and non-familial left-handers, the results of Experiment 2 were largely similar to those of Experiment 1. These findings suggest that interhemispheric processing may not differ between left- and right-handers.     

Bilateral field interactions and hemispheric asymmetry in number comparison

We investigated bilateral field interactions and hemispheric asymmetry in number comparison. A numerical comparison task with three different stimulus notations (Arabic numerals, word numerals and bar graphs) was used. In all conditions, a target was displayed in one visual field, simultaneously with a distractor of the same format in the other visual field. Participants had to indicate manually whether the magnitude of the target was small or large, ignoring the distractor stimulus. Only in the condition with Arabic numerals did we obtain some evidence for a LVF advantage, which argues against a strong laterality of number magnitude representations. Significant interactions between target and distractor values were observed, indicating rich interhemispheric interactions. The interactions were mainly situated at the response stage, but the presence of a bilateral identity effect also points to interactions at the input level.     

Temporal integration of form as a function of subject handedness and retinal locus of presentation

Two experiments are reported examining differences in perceptual processing as a function of subject handedness. Experiment 1 compared performance in the left vs right visual fields of right- vs left-handed subjects. Only right-handed subjects exhibited an interaction of visual field with spatial frequency (as mediated by interflash interval), suggesting that left-handers are not differentially lateralized for high vs low spatial frequency processing. Experiment 2 examined foveal performance in left-handers vs right-handers with or without familial sinistrality. Subjects with personal or familial sinistrality exhibited superior performance when the processing of low frequencies was required, suggesting that sinistrality may confer an advantage in the processing of lower spatial frequencies.     

Bilateral processing of redundant information: the influence of stimulus notation and processing speed in number comparison

Improved performance is generally observed when identical copies of a stimulus are presented bilaterally to both visual half fields (VHFs), relative to unilateral single presentations. We investigated the influence of stimulus notation and processing speed on this bilateral redundancy gain (B-RG) in number comparison. Two experiments were performed with consistent (i.e., two identical copies of Arabic digits or word numerals) and inconsistent (i.e., an Arabic digit in one VHF with a word numeral of the same value in the other VHF) displays (see also Marks and Hellige, 2003 for a similar design in number naming). In Experiment 1, the processing speeds of the Arabic digits were faster than the ones of the word numerals while in Experiment 2, the processing speeds of the two stimulus notations were kept equal. A significant difference between consistent and inconsistent conditions was restricted to Experiment 1 in which the processing speeds of the two stimulus notations diverged. These results are discussed in the framework of a simple race model in which two independent hemispheric processes compete for the control of response, and the faster process wins.     

Unconscious processing of Arabic numerals in unilateral neglect

This study explores the processing of Arabic numerals in three patients with dense left unilateral neglect. Three tasks have been used: a test of visual awareness (is the stimulus on the left, on the right or on both sides?), a number comparison task (is the number larger or smaller than 5?), a number parity judgment task (is the number odd or even?). The test of visual awareness indicated that all three patients were completely unaware of the stimuli presented in the left hemifield. Despite this, the number comparison and number parity judgment tasks clearly indicated that Arabic numerals were unconsciously processed at semantic level (i.e. quantity). These results show that patients with left unilateral neglect can still semantically process Arabic numerals unconsciously.     

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 differences between left and right number representations: effects of conscious and unconscious priming

The contribution of each hemisphere to the generation of number representations was investigated by two lateralized priming experiments in which participants had to compare Arabic digits to a fixed standard of four. In Experiment 1, unmasked primes (Arabic digits or word numerals) were used. In Experiment 2, masked primes were presented consciously or subconsciously. In both experiments similar priming effects were found in the left (LH) and the right hemisphere (RH) when the prime was presented consciously. However, asymmetries emerged when the primes were presented subconsciously: while the priming effects of digits and word numbers were equally large in the right visual half field (RVF-LH), the influence of the word prime on the semantic and the response stage of the left visual half field (LVF-RH) was absent, indicating that a word prime was no longer processed when it was presented subconsciously in LVF-RH. We believe that the origin of the latter effect can be attributed to a failure to transfer word number primes from the RH to the LH when attentional resources are restricted.     

Interhemispheric cooperation and activation in integration of verbal information

Subjects had to judge the semantic relation between two tachistoscopically presented German adjectives. In Exp. I, the two words were projected simultaneously, either both to the left visual field (i.e. to the right hemisphere, LVF-RH), or both to the right visual field (i.e. to the left hemisphere, RVF-LH), or one word to each visual field (i.e. to both hemispheres, EVF-BH). The results yielded a significantly higher performance in the condition EVF-BH than in the two unilateral conditions, without a significant difference between the latter two conditions. In Exp. II, the two words were shown successively. With bilateral presentation, a distinction was made as to whether the left visual field (EVF-BH-L) or the right visual field (EVF-BH-R) was stimulated first. The significantly highest performance was shown in conditions RVF-LH and EVF-BH-R compared with the condition EVF-BH-L, and the significantly poorest performance in the condition LVF-RH compared with the condition EVF-BH-L. The results provide evidence for (1) an intrahemispheric interference with simultaneous unilateral presentation, (2) a cooperative interhemispheric interaction with simultaneous bilateral presentation, and (3) an interhemispheric priming effect with successive bilateral presentation when the left hemisphere was stimulated first.     

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 findings 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 facilities processing of letter stimuli in the right hemisphere allowing for increased response speed and accuracy of bilateral visual field comparisons.     

Interhemispheric connectivity during lateralized lexical decision

The well‐established right visual field (RVF‐lh) advantage in word recognition is commonly attributed to the typical left hemisphere dominance in language; words presented to the LVF‐rh are processed less efficiently due to the need for transcallosal transfer from the right to left hemisphere. The exact stage for this hemispheric transfer is currently unsettled. Some studies suggest that transfer occurs at very early stages between primary visual regions, whereas other studies suggest that transfer occurs between the left visual word form area and its right hemisphere homolog. This study explores these conflicting accounts and finds evidence for both. Participants conducted a lateralized lexical decision task with both unilateral and bilateral display conditions. Connectivity analyses were conducted from magnetoencephalography signals that were localized to the left middle occipital gyrus (LMOG), right middle occipital gyrus (RMOG), left visual word form area (LVWFA), and right visual word form area (RVWA). Results from unilateral trials showed asymmetrical interhemispheric connectivity from the RMOG to LMOG and symmetrical interhemispheric connectivity between the LVWFA and RVWFA. Furthermore, bilateral presentations led to reduced interhemispheric connectivity between both homologous region of interest pairs. Together, these results suggest that lateralized word recognition involves multiple stages of interhemispheric interactions and that these interactions are reduced with bilateral displays.     

Interhemispheric cooperation for familiar but not unfamiliar face processing

Evidence for interhemispheric cooperation during language processing has been demonstrated for words, but not for meaningless pseudowords. Specifically, responses were found to be faster and more accurate when identical copies of a word were presented bilaterally to both hemispheres, relative to unilateral single presentations. This bilateral advantage for words seems to be a robust effect in normals. The present study addressed the question of whether the bilateral advantage is restricted to lexical material or whether it is a more global phenomenon occurring for meaningful material in general. Thirty healthy participants performed a familiarity decision in which one copy of familiar and unfamiliar faces was presented tachistoscopically to the right visual hemifield (RVF), the left visual hemifield (LVF) or simultaneously to both visual hemifields (bilateral condition, BVF). We obtained a highly significant familiarity by visual field interaction(P < 0.006) showing that only for familiar faces, a bilateral advantage was obtained. Unfamiliar face processing did not yield a bilateral advantage. We conclude that interhemispheric cooperation only occurs for meaningful material for which cortical representations can be assumed.     

Right hemispheric dominance and interhemispheric cooperation in gaze-triggered reflexive shift of attention

Aims: The neural substrate for the processing of gaze remains unknown. The aim of the present study was to clarify which hemisphere dominantly processes and whether bilateral hemispheres cooperate with each other in gaze‐triggered reflexive shift of attention. Methods: Twenty‐eight normal subjects were tested. The non‐predictive gaze cues were presented either in unilateral or bilateral visual fields. The subjects localized the target as soon as possible. Results: Reaction times (RT) were shorter when gaze‐cues were congruent toward than away from targets, whichever visual field they were presented in. RT were shorter in left than right visual field presentations. RT in mono‐directional bilateral presentations were shorter than both of those in left and right presentations. When bi‐directional bilateral cues were presented, RT were faster when valid cues were presented in the left than right visual fields. Conclusion: The right hemisphere appears to be dominant, and there is interhemispheric cooperation in gaze‐triggered reflexive shift of attention.     

Dyslexic adolescents: evidence of impaired visual and auditory language processing associated with normal lateralization and visual responsivity.

The questions of whether chronically dyslexic adolescent suffer any deficits of simple language stimulus processing or are less left hemisphere dominant than normal reading controls were addressed. The dyslexics were chosen for clarity of their specific reading problem and were older than dyslexics previously studied with lateralizing tests. Tasks administered in Experiment I were unilateral and bilateral tachistoscopic work recognitions and a tachistoscopic recognition report-time task for single lateralized letter stimuli. Experiment II, conducted a year later, readministered these tasks with modifications, and added dichotic digits and motor reaction time-stimulus detection tasks. It was concluded that right handed, chronic dyslexics: (1) possess left hemisphere language specialization; (2) show normal interhemispheric processing delays for single letter stimuli; (3) are, unlike nondyslexis but equally poor-reading Ss, clearly impaired in their efficiency of visual and auditory processing of simple language stimuli; (4) possess clear auditory memory deficits for verbal material; and (5) may possess an additional deficit of left hemisphere visual association area function.     

Neural dissociation of number from letter recognition and its relationship to parietal numerical processing

The visual recognition of letters dissociates from the recognition of numbers at both the behavioral and neural level. In this article, using fMRI, we investigate whether the visual recognition of numbers dissociates from letters, thereby establishing a double dissociation. In Experiment 1, participants viewed strings of consonants and Arabic numerals. We found that letters activated the left midfusiform and inferior temporal gyri more than numbers, replicating previous studies, whereas numbers activated a right lateral occipital area more than letters at the group level. Because the distinction between letters and numbers is culturally defined and relatively arbitrary, this double dissociation provides some of the strongest evidence to date that a neural dissociation can emerge as a result of experience. We then investigated a potential source of the observed neural dissociation. Specifically, we tested the hypothesis that lateralization of visual number recognition depends on lateralization of higher-order numerical processing. In Experiment 2, the same participants performed addition, subtraction, and counting on arrays of nonsymbolic stimuli varying in numerosity, which produced neural activity in and around the intraparietal sulcus, a region associated with higher-order numerical processing. We found that individual differences in the lateralization of number activity in visual cortex could be explained by individual differences in the lateralization of numerical processing in parietal cortex, suggesting a functional relationship between the two regions. Together, these results demonstrate a neural double dissociation between letter and number recognition and suggest that higher-level numerical processing in parietal cortex may influence the neural organization of number processing in visual cortex.     

Visual field differences in semantic comparative judgments with digits and Kanji stimulus materials

Hemisphere specialization for two types of semantic comparative judgments were examined. In the first experiment a pair of Arabic numerals of different physical size were displayed and subjects were asked to judge if the physical size of the stimulus and the numerical magnitude were congruent or not by pressing the appropriate button. Reaction times were faster in the right visual field than in the left visual field. In the second experiment a pair of different physical sized Kanji representing concrete objects were displayed and subjects were required to judge if the physical size of the Kanji was congruent or not with the relative real life object size by pressing the appropriate button. Reaction times were faster in the left visual field. In the third experiment a pair of different physical sized Kanji representing numerical magnitude were displayed and subjects judged if the physical size of the Kanji and the numerical magnitude were congruent or not. Results showed a right visual field advantage and eliminated the possibility that Kanji complexity was the cause of the left visual field advantage in the second experiment. These results were interpreted as providing support for the proposal that there are two types of semantic comparative judgments, one relying on the verbal code and the other relying on the use of imagery, and suggesting that the former calls for stronger engagement of the left hemisphere while the latter calls for a stronger contribution from the right hemisphere.     

Interhemispheric cooperation during word processing: evidence for callosal transfer dysfunction in schizophrenic patients

Functional lateralization and interhemispheric interaction during word processing were investigated in schizophrenic patients (n=12) and matched healthy controls (n=18). Words and phonologically regular pseudowords were presented tachistoscopically either in the left or right visual field (unilateral conditions), or simultaneously in both visual hemifields (bilateral condition). Consistent with earlier findings, healthy controls showed a right visual field advantage (RVFA), indicating left-hemispheric dominance for language. The patients showed a RVFA similar to that of controls, consistent with normal left-hemispheric language dominance. Importantly, controls performed much better on words presented in the bilateral condition, when two copies of the same word appeared twice, compared to stimulation in only one of the visual hemifields. This bilateral advantage, which has been interpreted as evidence for cooperation between the hemispheres, was absent in schizophrenics. These data show that schizophrenic patients can exhibit similar lateralization patterns as healthy controls. Their specific functional deficit may be a lack of cooperation between the hemispheres.     

ERP correlates of the bilateral redundancy gain for words

Neurophysiological correlates of hemispheric asymmetry and interhemispheric interaction in lexical processing were investigated in a lexical decision task with tachistoscopic stimulus presentation either unilaterally, to the right or left visual field, or bilaterally, with identical stimulus copies to each visual hemi-field. Behavioral data confirmed both right visual field advantage and bilateral redundancy gain for words but not for pronounceable orthographically regular pseudowords. ERPs showed a significant amplitude increase 160-200 after stimulus presentation specifically for words after bilateral redundant stimulation, which was present in the recordings from both hemispheres. Localization of cortical sources using minimum norm estimation indicated stronger cortical activity for words in temporal regions of both hemispheres after bilateral presentation compared with each of the unilateral stimulation conditions individually. Pseudoword presentation did not lead to a general increase of cortical activation in the bilateral condition compared with unilateral presentation. The specific activation increase for words in the bilateral redundant condition relative to unilateral stimulation and the absence of this effect for pseudowords, which became manifest in a significant interaction of the factors lexicality and presentation mode, is best explained by summation of neuronal activation from both hemispheres within distributed lexical circuits. Source estimation indicates that temporal areas, particularly in the left hemisphere, are the primary cortical loci where such stimulus-specific activity increases occurred.     

The lateralization of hemispheric function in the recognition of Chinese characters

Two experiments were conducted to investigate the lateralization of hemispheric functions. In experiment 1 single Chinese characters, English words and random dot forms were used as stimuli presented to the left or right visual field of adults. In experiment 2 adults and school children were asked to recognize single characters presented in one field and Arabic numerals in the opposite field. The results indicated that adults and children all showed left hemisphere superiority in the recognition of characters. Some possible causes were given to explain why Chinese characters, being an ideographic language, showed a similar superiority as phonetic languages.