Parasagittal asymmetries of the corpus callosum.

Significant relationships have been reported between midsagittal areas of the corpus callosum and the degree of interhemispheric transfer, functional lateralization and structural brain asymmetries. No study, however, has examined whether parasagittal callosal asymmetries (i.e. those close to the midline of the brain), which may be of specific functional consequence, are present in the human brain. Thus, we applied magnetic resonance imaging and novel computational surface-based methods to encode hemispheric differences in callosal thickness at a very high resolution. Discrete callosal areas were also compared between the hemispheres. Furthermore, acknowledging the frequently reported sex differences in callosal morphology, parasagittal callosal asymmetries were examined within each gender. Results showed significant rightward asymmetries of callosal thickness predominantly in the anterior body and anterior third of the callosum, suggesting a more diffuse functional organization of callosal projections in the right hemisphere. Asymmetries were increased in men, supporting the assumption of a sexually dimorphic organization of male and female brains that involves hemispheric relations and is reflected in the organization and distribution of callosal fibers.

Effect of neurofeedback on hemispheric word recognition.

We applied SMR/theta neurofeedback (NF) training at central sites of 20 Israeli children aged 10-12 years, half boys and half girls. Half of the subjects received C3 training and the other half C4 training, consisting of 20 half-hour sessions. We assessed the effects of training on lateralized lexical decision in Hebrew. The lateralized lexical decision test reveals an independent contribution of each hemisphere to word recognition (Barnea, Mooshagian, & Zaidel, 2003). Training increased accuracy and sensitivity. It increased left hemisphere (LH) specialization under some conditions but it did not affect interhemispheric transfer. Training did affect psycholinguistic processing in the two hemispheres, differentially at C3 and C4. Training also increased hemispheric independence. There were surprising sex differences in the effects of training. In boys, C4 training improved LH accuracy, whereas in girls C3 training improved LH accuracy. The results suggest that the lateralized NF protocol activates asymmetric hemispheric control circuits which modify distant hemispheric networks and are organized differently in boys and girls.

The alien hand syndrome: classification of forms reported and discussion of a new condition.

The term "alien hand" refers to a variety of clinical conditions whose common characteristic is the uncontrolled behavior or the feeling of strangeness of one extremity, most commonly the left hand. A common classification distinguishes between the posterior or sensory form of the alien hand, and the anterior or motor form of this condition. However, there are inconsistencies, such as the phenomenon of diagonistic dyspraxia, which is largely a motor syndrome despite being more frequently associated with posterior callosal lesions. We discuss critically the existing nomenclature and we also describe a case recently reported by us which does not fit any previously reported condition, termed agonistic dyspraxia. We propose that the cases of alien hand described in the literature can be classified into at least five broad categories: (i) diagonistic dyspraxia and related syndromes, (ii) alien hand, (iii) way-ward hand and related syndromes, (iv) supernumerary hands and (v) agonistic dyspraxia.

Effects of right and left hemisphere damage on performance of the "Right Hemisphere Communication Battery".

A Hebrew adaptation of Gardner and Brownell's (1986) "Right Hemisphere Communication Battery" (HRHCB) was administered to 27 right brain-damaged (RBD) patients, 31 left brain-damaged (LBD) patients, and 21 age-matched normal controls. Both patient groups showed deficits relative to controls and overall there was no difference between the two patient groups. A factor analysis of patients' scores on the HRHCB yielded two interpretable factors, a verbal and a nonverbal one. These factors were not lateralized. Performance of patients on the HRHCB correlated significantly and positively with performance on most tests of basic language functions, measured with a Hebrew adaptation of the "Western Aphasia Battery" (HWAB) and with other cognitive functions measured with standardized neuropsychological tests. There were stronger correlations of HRHCB with subtests of the HWAB in LBD patients and with nonlanguage cognitive tests in RBD patients. In the LBD group, HRHCB subtests' scores correlated negatively with lesion extent in frontal and temporal perisylvian regions. Such localization was not observed in RBD patients. The results argue against selective right hemisphere (RH) involvement in the RHCB, alleged to measure pragmatic aspects of language use, and show, instead, bilateral involvement. The results also argue against a modular organization of these functions of language use, especially in the RH.

Error monitoring in the hemispheres: the effect of lateralized feedback on lexical decision.

Does each hemisphere have its own system for monitoring and responding to errors? Three experiments investigate the effect of presenting lateralized accuracy feedback in a bilateral lexical decision task. We presented feedback after each trial in either the left visual field (LVF) or right visual field (RVF). In Experiment 1 the feedback stimuli were faces smiling or frowning, in Experiment 2 we used colored squares, and Experiment 3 tested the effect of verbal feedback. Negative feedback presented in the LVF tended to improve performance on the following trial, while the same negative feedback in the RVF tended to disrupt performance on the following trial. This result was strongest with the faces as feedback, was less pronounced with colored squares, and disappeared with verbal feedback. The results are interpreted as suggesting a right hemisphere superiority for error monitoring that depends on the mode of feedback.

Hemispheric contributions to pragmatics.

Twenty-seven patients with right-hemisphere damage (RBD) and thirty-one patients with left-hemisphere damage (LBD) received a new pragmatics battery in Hebrew consisting of two parts: (1) comprehension and production of basic speech acts (BSAs), including tests of assertions, questions, requests, and commands, and (2) comprehension of implicatures, including implicatures of quantity, quality, relevance, and manner. Each test had a verbal and a nonverbal version. Patients also received Hebrew versions of the Western Aphasia Battery and of the Right Hemisphere Communication Battery. Both LBD and RBD patients were impaired relative to controls but did not differ from each other in their overall scores on BSAs and on Implicatures when scores were corrected by aphasia and neglect indices. There was a systematic localization of BSAs in the left hemisphere (LH) but not in the right hemisphere (RH). There was poor localization of Implicatures in either hemisphere. In LBD patients, BSAs were associated with language functions measured with the WAB, suggesting the radical possibility that the classic localization of language functions in aphasia is influenced by the localization of the BSAs required by aphasia language tests. Both BSAs and implicatures show greater functional independence from other pragmatic, language and cognitive functions in the RBD than in the LBD patients. Thus, the LH is more likely to contain an unmodular domain-nonspecific set of central cognitive mechanisms for applying means-ends rationality principles to intentional activity.

Parallel visuomotor processing in the split brain: cortico-subcortical interactions.

We tested nine patients with callosal pathology in a simple reaction time task with and without redundant targets in the same or opposite visual hemifield. Four patients showed large facilitation (redundancy gain) in the presence of a redundant target, exceeding probability summation models (neural summation). Five patients showed redundancy gain not exceeding probability models. Violation of probability models was not associated with a specific type of callosal lesion. Neural summation, which probably occurs at collicular level, may be modulated by cortical activity. To test this hypothesis, we used functional MRI. During detection of redundant simultaneous targets, activations in the extrastriate cortex were observed in a patient with callosal agenesis and redundancy gain violating probability models, but not in a patient with callosal agenesis and redundancy gain not exceeding probability models. We conclude that cortical activity in the extrastriate cortex may be a modulating factor in the magnitude of the redundancy gain during parallel visuomotor transforms.

Crossed-uncrossed difference in simple reaction times to lateralized flashes: between- and within-subjects variability.

In unimanual reaction times (RT) to lateralized flashes, contralateral responses tend to be slower than ipsilateral responses. This has been called Crossed-Uncrossed Difference (CUD). The CUD tends to show variability across subjects and across studies, but until now the stability of the CUD in an individual subject has not been investigated. To address the role of inter- and intra-subject variability in the CUD, three normal right handers were tested over 50 experimental sessions of 800 trials each, for a total of 40,000 trials of simple reaction times to lateralized flashes. In each subject, CUDs were computed for each session, over two, three, or more sessions, and over the entire dataset. These CUDs were then compared to the CUDs obtained in a group of 15 normal right handers, each tested once in a single session. Results show that: (i) CUD variability across several sessions in a single subject mimics the variability observed in a sample of subjects tested in a single session; (ii) this variability is considerably reduced when the CUD is computed over at least 2400 trials per subject; (iii) CUDs computed over 2400 and up to 12,000 of trials tend to be extremely similar ( approximately 2 ms) across the three subjects tested here; (iv) when reaction times are ordered from the fastest to the slowest and divided into bins, the CUD is remarkably stable over the entire reaction time distribution; and (v) in contrast to the variability of the CUD, the variability for crossed and uncrossed responses across several sessions in a single subject is small and does not mimic the variability observed in a sample of subjects tested in a single session. Taken together, these data suggest that the intersubject variability in the CUD observed in single experimental sessions does not represent a reliable intersubject difference and that the CUD computed over thousands of trials reflects hard-wired mechanisms of callosal transmission.

The crossed-uncrossed difference in simple reaction times to lateralized auditory stimuli is not a measure of interhemispheric transmission time: evidence from the split brain.

In a complete commissurotomy patient, the difference in simple (detection) reaction times between responses to contralateral and ipsilateral auditory stimuli was found to be small (less than 5 ms) and not reliable, whereas the difference between contralateral and ipsilateral responses to lateralized visual stimuli was found to be large (ranging from 25 ms to 45 ms in different previous studies) and always reliable. This suggests that the reaction times difference in detecting lateralized auditory stimuli is not a valid estimate of interhemispheric transmission time.

Effects of right- and left-hemisphere damage on understanding conversational implicatures.

Processing of implicatures was examined in 27 right-brain-damaged (RBD) and 31 left-brain-damaged (LBD) stroke patients with focal lesions using a new implicatures battery (IB) as part of an exploration of the neural basis and modularity of natural language pragmatics. Following Grice, we sampled implicatures of Quantity, Quality, Relation, and Manner. Verbal implicatures consisted of two-sentence conversational vignettes which are literally problematic. Nonverbal implicatures consisted mostly of famous paintings that are literally problematic (e.g., Magritte's "Le Domain d'Arnheim"). The patient has to identify and solve the problem. To compare with performance on the IB, patients also received a Hebrew adaptation of Gardner and Brownell's Right Hemisphere Communication Battery, a new test of basic speech acts (verbal and nonverbal assertions, questions, requests, and commands), a Hebrew version of the Western Aphasia Battery, and standardized neuropsychological tests. Both LBD and RBD patients were significantly impaired in implicature processing relative to age-matched normal controls. In general, both patient groups showed weak correlations of implicatures with extents of lesions in left perisylvian language area or its right-hemisphere (RH) homolog. However, performance of LBD and RBD patients on the IB revealed different patterns of correlations with other pragmatic, language, and nonlanguage tests. In LBD patients, there was a greater association between performance on verbal and nonverbal implicatures and between performance on implicatures and basic speech acts than in RBD patients. Given the different modes in which right-and left-hemisphere (LH) damage affect the processing of conversational implicatures, it remains to be discovered how the two hemispheres interact to process natural language pragmatics in the normal brain in real time.

Redundancy gains for visual search after complete commissurotomy.

Redundant-targets effects (RTE) for visual search were investigated in 2 commissurotomy patients (L.B., N.G.). L.B., who showed no evidence of visual interhemispheric transfer, exhibited a paradoxical enhancement of the redundancy gain in the bilateral compared with the within-hemifield redundant-targets conditions, whereas N.G., who showed evidence of interhemispheric transfer of visual information, exhibited no enhancement of the bilateral redundancy gain. When only uncrossed responses were considered, both bilateral and within-field RTE were evident only when attentional demands were high. Bilateral redundant targets led to stronger gains, some indicative of coactivation, in the slower response hand. The authors suggest that the enhancement of the bilateral RTE comes about by neural coactivation, which is especially pronounced when the slower hemisphere elicits the response.

Individual differences in the hemispheric specialization of dual route variables.

The dual route model suggests that reading of letter strings can occur through both a lexical and a nonlexical route. Hemispheric specialization of these routes has also been posited, suggesting that the left hemisphere has both lexical and nonlexical routes while the right hemisphere has only a lexical route. However, some recent data conflict with this hemispheric dual route model, suggesting that both hemispheres may have access to both routes. The purpose of the present study was to investigate individual differences in the hemispheric specialization of these routes and to determine whether these group differences in their specialization might explain conflicts in the literature. The effect of four individual difference factors was explored: handedness, biological sex, menstrual stage (i.e., fluctuations in estrogen), and self-rated degree of masculinity (i.e., sexual attribution). We looked at the interaction of these individual differences with the following dual route variables: (i) string length, (ii) word frequency, (iii) regularity of grapheme-phoneme correspondences of words, and (iv) the interaction of frequency and regularity using a bilateral lexical decision task. We observed that sex, menstrual stage, and masculinity each affected hemispheric specialization of the dual route variables, but did so in different ways. We posit that both hemispheres have orthographical (lexical) access as well as phonological (nonlexical) access to words. Further, we suggest that the presence of phonological processing in the right hemisphere depends on available resources and the strategies used, which are subject to individual differences.

Stereognosis in the chronic split brain: hemispheric differences, ipsilateral control and sensory integration across the midline.

Benton's test of Stereognosis was administered separately to the left and right hands of six patients with complete cerebral commissurotomy and two patients with partial commissurotomy sparing the splenium. Experiment 1 displayed the multiple choice card in free vision for all patients whereas Experiment 2 displayed it for ocular exploration in one visual field at a time, either the same or opposite to the exploring hand, in two complete commissurotomy patients fitted with special contact lens systems. There were deficits in stereognosis without primary somatosensory impairment in both disconnected hemispheres and the deficit was more marked in the hemisphere with predominant extracallosal damage. The two disconnected hemispheres appeared to use different strategies for recognizing shapes. As can be expected, naming of stimuli explored with the left hand was worse than of stimuli explored by the right hand, but it was above chance and variable across patients. Left hand naming was not affected by restricting stimulus exploration to distal joints. Naming of stimuli explored in the left visual hemifield was worse than of stimuli explored with the left hand, suggesting that it reflects left rather than right hemisphere speech. There was surprisingly good performance in the cross-hemisphere condition where one hand explored the stimulus and the multiple choice card was explored in the opposite visual field. The left hand-right hemifield combination was more effective than the right hand-left field combination, suggesting better "ipsilateral" tactile/kinesthetic projections in the left than right hemisphere.

Different psychological status in the two hemispheres of two split-brain patients.

Questions of a psychological nature were presented to two split-brain patients from the California series encouraging each hemisphere to respond simultaneously and independently. The responses of both patients indicated that their hemispheres were responding independently. For the first patient, his right hemisphere appeared to be more disturbed than his left by childhood memories of being bullied. The right hemisphere of the second patient seemed to regard himself more positively, but it also seemed to feel more negative emotions such as loneliness and sadness. We discuss the possible significance of the findings.

The role of the corpus callosum in visual orienting: importance of interhemispheric visual transfer.

In a tachistoscopic visual search task, the effects of ipsi- and contralateral distractors on target search were investigated in two complete commissurotomy patients. Pop-out distractors slowed the search for contralateral targets in both patients, i.e. search was not independent in both hemifields. In normals, we previously observed an extinction-like asymmetry in that distractors in the right visual hemifield interfered with target search in the left visual hemifield, but not vice versa. This pattern was also found in one of our patients, N.G., whereas the other, L.B., showed a reversed laterality effect. While N.G. is able to transfer visual shape information between hemispheres, L.B. is not. The data suggest that the reversal of the contralateral distractor asymmetry in L.B. is due to the disruption of ipsilateral visual input to the right hemisphere.

Individual differences in lateralization: effects of gender and handedness.

Male and female left- and right-handers participated in 3 experiments designed to investigate 3 components of performance asymmetry in lateralized tasks. Experiment 1 used a consonant-vowel-consonant (CVC) identification task measuring quantitative differences in hemispheric abilities and hemispheric control and qualitative differences in hemispheric strategies. The quantitative data revealed that left-handers have a smaller performance asymmetry than do right-handers and that both groups have the same degree of increased accuracy when stimuli are presented bilaterally. Handedness affected the qualitative measures of men, not of women. Experiment 2 used nominal and physical letter-matching tasks with bilateral presentations and measured the flexibility of callosal function. The results suggest that left-handers have less flexible interhemispheric communication than do right-handers and show no effect of gender. Experiment 3 used a chair identification task indexing hemispheric arousal bias. Left-handers tended to have more aroused right than left hemispheres, whereas the distribution of right-handers was centered around 0 arousal bias. Intertask analyses revealed a relationship between arousal bias and metacontrol, where individuals with more aroused right hemispheres tended to use a right-hemisphere strategy in the bilateral condition of the CVC experiment. Intercorrelations between measures from the experiments revealed only a limited relationship between metacontrol patterns in the CVC task and a measure of callosal flexibility in the physical letter-matching task. The results are discussed in the context of the relationships between dimensions of hemispheric asymmetry.

Interhemispheric relations in hierarchical perception: a second look.

This study reevaluates the role of interhemispheric interactions in the consistency effect (global interference with local decisions) in hierarchical perception. In an earlier study, Robertson et al. [22] (Neuropsychology, Vol. 7, pp. 325-342, 1993) tested three split-brain patients on a hierarchical perception task in which stimuli, consisting of large (global) letters made up of smaller (local) letters, were unilaterally or bilaterally presented for identification. They found that, in general, the consistency effect did not occur in split-brain patients and argued that the effect is interhemispheric and normally mediated by the corpus callosum. We repeated the experiment with new stimuli in two of the same split-brain patients. We found that both patients demonstrated evidence for global interference, implying that the neocortical commissures are not necessary for eliciting the consistency effect in hierarchical perception.

Does the previous trial affect lateralized lexical decision?

We investigated the effect of previous trial variables on performance in the current trial in a lexical decision task with unilateral presentation of one letter string or bilateral simultaneous presentation of two different letter strings, one cued to be processed (target) and the other uncued, to be ignored (distractor). The variables included correctness of the previous trial, visual hemifield and wordness of the previous trial, and presentation mode of the previous trial (unilateral or bilateral). An incorrect response on the previous trial enhanced the accuracy in the current trial only in the left visual field (LVF). A previous LVF target produced faster correct responses to LVF targets in the current trial and LVF word recognition was more accurate when the previous LVF target was a word rather than a nonword. Target processing in the current trial was not inhibited or facilitated if the target belonged to the same response category as the unattended stimulus in the previous trial (absence of "negative priming'). Taken together, our data suggest that previous trial effects in lateralized lexical decision are stronger for word decisions in the LVF, and may account for the inconsistency of the wordness effect in the LVF across different lateralized lexical decision experiments. Our data also suggest that behavioral laterality experiments are well advised to use random sequences that change across subjects in order to minimize previous trial effects.

Bifurcation patterns in the human sylvian fissure: hemispheric and sex differences.

The sylvian fissure bifurcates posteriorly into ascending and descending rami. The diversity in the specific arrangement of these rami and in the length of a more anterior segment of the fissure, between the bifurcation point and Heschl's gyrus (segment H-B), was analyzed qualitatively and quantitatively, in both left and right hemispheres and in both males and females. Qualitatively, four basic patterns of bifurcation appeared: (A) the ascending ramus is larger than the descending one (61.25% of the cases). In half of these cases segment H-B was very short (designated as 'short H-B'). The other patterns were: (B) the descending ramus is larger than the ascending one (7.5% of the total); (C) both rami are of approximately equal size (10% of the cases); and (D) both are of approximately equal size but the ascending ramus is oriented frontally instead of caudally as in the other cases (21.25% of the cases). Type D has not been reported before, and may be considered a new variant of bifurcating rami that in many cases corresponds to what other authors have referred to as the absence of an ascending ramus. We found a biased distribution of the fissure types according to hemispheres and also sex, with type A being more common in males and in the right hemisphere, and type D more common in females and in the left hemisphere. When the two hemispheres of each subject were matched, no correspondence was observed between the fissurization pattern of one hemisphere and that of the other, indicating that fissurization develops independently in each hemisphere. Quantitative analyses confirmed these findings and showed some new relations between components of the sylvian fissure. For example, when pooling together all fissure types a negative correlation between segment H-B and the ascending ramus was observed in males but not in females. On average, segment H-B was larger on the left side while the ascending ramus was larger on the right, confirming previous reports. Since earlier studies indicate that the planum temporale is larger on the left side, we suggest that the latter usually corresponds to segment H-B. However, the 'short H-B' cases described above have an unusually long and deep ascending branch, indicating that the planum temporale may run into the latter in these cases. The present classification of fissure types therefore describes a new variant of fissurization patterns in the sylvian fissure, which is asymmetrically distributed across the hemispheres and is perhaps sexually dimorphic. Furthermore, our analysis of fissure morphology and asymmetry is of direct relevance to the definition and location of the planum temporale in the sylvian fossa. Finally, our quantitative analyses are amenable to the use of morphometric techniques in the study of variability in fissurization patterns.

The effects of procedural variations on lateralized Stroop effects.

Several issues in the classic Stroop effect remain open, including (i) the stage of processing which gives rise to the effect, (ii) the effect of some procedural manipulations, (iii) the effect of hemispheric specialization and of interhemispheric interactions, and (iv) the existence of individual differences. In this paper, we investigate these issues using a series of experiments with central, lateral, and bilateral presentations of the Stroop stimuli. A total of 146 right-handed subjects took part in a multiexperiment study with relatively equal numbers of the two sexes participating in each experiment. We found that manual responses diluted but did not abolish the Stroop interference relative to vocal responses, arguing for a late processing stage account of the effect. However, separating the color patch from the color word either unilaterally or bilaterally did not significantly change the magnitude of the Stroop interference, arguing against a cost of interhemispheric transfer in the Stroop effect in normal subjects. We did, however, find evidence for hemispheric specialization in Stroop interference, greater in the left hemisphere than in the right hemisphere. Color words in the RVF produced the greatest Stroop effects regardless of the location of the color patch. In most cases, this laterality effect interacted with the sex of the subject, such that only males and a subgroup of females (i.e., those tested during a low estrogen phase in the menstrual cycle) showed evidence for greater Stroop effects when a color word projected to the left hemisphere than when it projected to the right hemisphere, regardless of the placement of the color patch.