The role of computational modeling in understanding hemispheric interactions and specialization

We describe results from three models of paired left and right cerebral regions communicating via a simulated corpus callosum. Conditions are identified under which lateralization emerges during learning, and under which an intact hemispheric region contributes to recovery when the contralateral one is damaged. It proved easy to demonstrate hemispheric specialization in the context of a variety of underlying cortical asymmetries, consistent with past arguments that lateralization of cognitive functions is a multi-factorial process. However, no single assumption about transcallosal influences could account for all existing experimental data. Possible solutions to this ‘callosal dilemma’ are suggested.     

Interhemispheric effects of simulated lesions in a neural model of letter identification

Experimental studies have produced conflicting results about the extent to which the intact, nonlesioned cerebral hemisphere is responsible for recovery from cognitive deficits following focal brain damage such as a stroke. To obtain a better theoretical understanding of interhemispheric interactions during recovery, we examined the effects of simulated lesions to a bihemispheric neural model of letter identification under various assumptions about hemispheric asymmetries, corpus callosum influence, and lesion size. Among other results, the model demonstrates that the intact hemispheric region's participation in the recovery process is a function of preexisting lateralization and lesion size, indicating that interpretation of experimental work should take these factors into account.     

The Role of the Corpus Callosum in Interhemispheric Transfer of Information: Excitation or Inhibition?

The corpus callosum is the major neural pathway that connects homologous cortical areas of the two cerebral hemispheres. The nature of how that interhemispheric connection is manifested is the topic of this review; specifically, does the corpus callosum serve to communicate an inhibitory or excitatory influence on the contralateral hemisphere? Several studies take the position that the corpus callosum provides the pathway through which a hemisphere or cortical area can inhibit the other hemisphere or homologous cortical area in order to facilitate optimal functional capacity. Other studies suggest that the corpus callosum integrates information across cerebral hemispheres and thus serves an excitatory function in interhemispheric communication. This review examines these two contrasting theories of interhemispheric communication. Studies of callosotomies, callosal agenesis, language disorders, theories of lateralization and hemispheric asymmetry, and comparative research are critically considered. The available research, no matter how limited, primarily supports the notion that the corpus callosum serves a predominantly excitatory function. There is evidence, however, to support both theories and the possibility remains that the corpus callosum can serve both an inhibitory and excitatory influence on the contralateral hemisphere.     

Lateralized lexical decision in schizophrenia: hemispheric specialization and interhemispheric lexicality priming

Reports of left-hemisphere dysfunction and abnormal interhemispheric transfer in schizophrenia are mixed. The authors used a unified paradigm, the lateralized lexical decision task, to assess hemispheric specialization in word recognition, hemispheric error monitoring, and interhemispheric transfer in male, right-handed participants with schizophrenia (n=34) compared with controls (n=20). Overall, performance and error monitoring were worse in patients. However, patients like controls showed left-hemisphere superiority for lexical processing and right-hemisphere superiority for error monitoring. Only patients showed selective-interhemispheric lexicality priming for accuracy, in which performance improved when the lexical status of target and distractor stimuli presented to each hemifield was congruent. Results suggest that schizophrenia is associated with impaired monitoring and with increased interhemispheric automatic information transfer rather than with changed hemispheric specialization for language or error monitoring.     

The right hemispheric dominance for face perception in preschool children depends on the visual discrimination level

The developmental origin of human adults’ right hemispheric dominance in response to face stimuli remains unclear, in particular because young infants’ right hemispheric advantage in face‐selective response is no longer present in preschool children, before written language acquisition. Here we used fast periodic visual stimulation (FPVS) with scalp electroencephalography (EEG) to test 52 preschool children (5.5 years old) at two different levels of face discrimination: discrimination of faces against objects, measuring face‐selectivity, or discrimination between individual faces. While the contrast between faces and nonface objects elicits strictly bilateral occipital responses in children, strengthening previous observations, discrimination of individual faces in the same children reveals a strong right hemispheric lateralization over the occipitotemporal cortex. Picture‐plane inversion of the face stimuli significantly decreases the individual discrimination response, although to a much smaller extent than in older children and adults tested with the same paradigm. However, there is only a nonsignificant trend for a decrease in right hemispheric lateralization with inversion. There is no relationship between the right hemispheric lateralization in individual face discrimination and preschool levels of readings abilities. The observed difference in the right hemispheric lateralization obtained in the same population of children with two different paradigms measuring neural responses to faces indicates that the level of visual discrimination is a key factor to consider when making inferences about the development of hemispheric lateralization of face perception in the human brain.     

LATERAL DIFFERENCES IN NORMAL MAN AND LATERALIZATION OF BRAIN FUNCTION*

The historical development of work on lateral differences (LDs) is described and an evaluation is attempted of the present state of the study and of the conceptual and methodological problems which it encounters. Contemporary work is mainly motivated by the notion that LDs reflect hemispheric specialization and thus provide a means of studying hemisphere function in easily available normal subjects. Work on LDs in visual recognition has first been conducted, however, within a completely different conceptual framework. Right visual field advantage (RVFA) for words and letters was first thought of as reflecting the left-to-right direction of latin writing which produces opportunity for differential perceptual learning in the two hemifields or, as was later considered, creates left-to-right scanning habits. On the contrary, right ear advantage (REA) in dichotic listening was, from its discovery by Kimura, shown to be linked to lateralization of speech control in the left hemisphere. The possibility that visual field effects could also be related to hemispheric specialization was then examined by looking for effects of handedness and for correlations between LDs and pathological data. After considerable initial resistance, the notion has become widely accepted. While the early work tended to deal with broad categories of tasks or of stimulating materials, more analytical approaches have been developed and investigators have tried to specify the component operations which are responsible for observed LDs. It has been shown that a same task can give rise to different patterns of asymmetry according to the particular operating mode which is adopted. Chronometric methods have been used to isolate processing stages with different lateralizations, and specific hypotheses have been advanced concerning levels at which lateralization originates. Regarding the mechanism of LDs, the most often considered interpretation is based on the notion of direct access, i.e. of an advantage associated with primary projection in the competent hemisphere. According to whether localization of the critical operations is strict or relative, one of two alternative versions of direct access, called respectively the callosal relay and efficiency models, apply. Other mechanisms based on hemispheric activation or priming and on interference between operations competing for hemispheric processing capacity have also been considered and presumably contribute, beside direct access, to observed LDs. There are also other sources for LDs than brain asymmetry, a fact which tends to be overlooked with the current focusing of interest on hemispheric interpretations. Even in cases where the role of hemispheric specialization has been established, other determinants can play amplifying or masking roles, as has been shown for auditory laterality effects. Current attempts at providing simple dichotomous general characterizations of hemisphere function, such as the one in terms of holistic vs. analytic processing, are described. The opinion is offered that such attempts are premature and that they reflect an underestimation of the complexity of the problem. It is argued (1) that the present catalogue of registered LDs is both too narrow and too subject to selective biases to provide a basis for ambitious generalizations; (2) that a convincing account of lateralization would require an understanding of its evolutionary origin; (3) that it would also require a more advanced understanding of the various mechanisms, including interhemispheric interactions, intervening between hemisphere competence and performance.     

Disentangling the relationship between hemispheric asymmetry and cognitive performance

It is widely believed that advantages of hemispheric asymmetries originated in better cognitive processing, hence it is often implied that the relationship between hemispheric asymmetry and cognitive performance is linearly positive: the higher the degree of lateralization in a specific cognitive domain, the better the performance in a corresponding task. Yet, the empirical evidence for this notion is mixed and the statistical methods to analyze this relationship have been criticized. The present study therefore investigated the relationship between hemispheric asymmetries and cognitive performance in two behavioral tasks (a left-lateralized word-matching task and a right-lateralized face-decision task) in 230 participants (140 women, 90 men) by using two different approaches. Both methods correspondingly revealed that a relationship between hemispheric asymmetries and cognitive performance does exist. Contrary to a positive (linear) relationship however, the data could be best described by an inverted U-shaped curve. Although the optimal degree of lateralization seemed to be task-specific, a slight or moderate degree of hemispheric asymmetry achieved best cognitive performance in all tasks. Moreover, performances deteriorated towards extreme ends of lateralization (i.e., participants with either extreme left or right hemispheric biases). Taken together, the present study provides evidence against the notion that higher lateralization is related to enhanced cognitive performance.     

Repetition priming within and between the two cerebral hemispheres

Two experiments explored repetition priming benefits in the left and right cerebral hemispheres. In both experiments, a lateralized lexical decision task was employed using repeated target stimuli. In the first experiment, all targets were repeated in the same visual field, and in the second experiment the visual field of presentation was switched following repetition. Both experiments demonstrated hemispheric specialization for the task (a RVF advantage for word identification) and hemispheric interaction for word processing (lexicality priming from contralateral distracters). In the first experiment, words were identified more quickly and accurately following repetition, with repetition facilitating faster but fewer correct responses for non-words. Complex interactions between visual field of first and second presentation in the second experiment indicate asymmetric interhemispheric repetition priming effects. These results provide a broad picture of hemispheric asymmetries in word processing and of complex interaction between the hemispheres during word recognition.     

Cerebral lateralization in individuals diagnosed as autistic in early childhood

Early infantile autism was found to be associated with an atypical pattern of cerebral lateralization. Based on EEG measures of hemispheric activation during cognitive processing, it was found that 7 of the 10 autistic individuals tested showed a pattern of hemispheric specialization rarely seen in the normal population; namely, a “reversal” in lateralization reflective of a lack of left-hemisphere specialization for linguistic functions. Furthermore, the autistic individuals' pattern of cognitive strengths and weakness is suggestive of a selective impairment of the left cerebral hemisphere.     

Speech lateralization in deaf populations: evidence for a developmental critical period

Cerebral lateralization for speech in right-handed normal hearing and deaf adolescents was assessed using the dual-task paradigm. Subjects with normal hearing and deafness acquired after 3 years of age displayed left hemispheric dominance for speech production, whereas both congenitally deaf and those with an early acquired deafness (onset 6-36 months) showed atypical, anomalous cerebral representation. These results suggest the presence of a developmental critical period for cerebral lateralization during which exposure to adequate environmental stimulation may be needed to activate left hemispheric dominance for speech.     

Sex differences in the lateralization of spatial abilities: a spatial component analysis of extreme group scores

Sex differences in the cerebral lateralization of two discrete components of spatial processing were investigated in high and low ability males and females using the dual-task paradigm. In the first phase of the experiment, the results indicated a pattern of right hemispheric control for a spatial visualization component, regardless of sex and ability level. In the processing of the spatial orientation component of spatial ability, high ability males and females showed left hemispheric lateralization, whereas low ability males and females displayed right hemispheric control. In the second phase of this study, it was observed that high ability females and low ability males may use a verbal mediation strategy in processing spatial visualization tasks. No verbal mediation effects were found for the spatial orientation component.     

Atypical speech lateralization in adults with developmental coordination disorder demonstrated using functional transcranial Doppler ultrasound

Research using clinical populations to explore the relationship between hemispheric speech lateralization and handedness has focused on individuals with speech and language disorders, such as dyslexia or specific language impairment (SLI). Such work reveals atypical patterns of cerebral lateralization and handedness in these groups compared to controls. There are few studies that examine this relationship in people with motor coordination impairments but without speech or reading deficits, which is a surprising omission given the prevalence of theories suggesting a common neural network underlying both functions. We use an emerging imaging technique in cognitive neuroscience; functional transcranial Doppler (fTCD) ultrasound, to assess whether individuals with developmental coordination disorder (DCD) display reduced left‐hemisphere lateralization for speech production compared to control participants. Twelve adult control participants and 12 adults with DCD, but no other developmental/cognitive impairments, performed a word‐generation task whilst undergoing fTCD imaging to establish a hemispheric lateralization index for speech production. All participants also completed an electronic peg‐moving task to determine hand skill. As predicted, the DCD group showed a significantly reduced left lateralization pattern for the speech production task compared to controls. Performance on the motor skill task showed a clear preference for the dominant hand across both groups; however, the DCD group mean movement times were significantly higher for the non‐dominant hand. This is the first study of its kind to assess hand skill and speech lateralization in DCD. The results reveal a reduced leftwards asymmetry for speech and a slower motor performance. This fits alongside previous work showing atypical cerebral lateralization in DCD for other cognitive processes (e.g., executive function and short‐term memory) and thus speaks to debates on theories of the links between motor control and language production.     

Effects of irrelevant object structure on early attention deployment

Event-related potential (ERP) studies have suggested that Gestalt principles or grouping factors in a visual scene modulate attention deployment in early visual processing. The present study examined the effects of an extrinsic grouping factor, region commonality, on early ERP spatial attention. Effects of sex were also explored, since the processing of task-irrelvant objects may differ between the sexes. Twenty-four participants were required to discriminate one side of rapidly-presented bilateral letters, and attention effects were indexed by hemispheric lateralization accroding to attended visual fields. In results, an early P1 attention effect (70–110 ms) increased when the letters were surrounded by an object, compared to the control stimulus without a complete object, and this result was more prominent in women than in men. The present study demonstrates that visual object and sex differences play a novel role at very early cortical stages of processing in attention deployment to a task-irrelevant visual structure.     

Comparison of hemispheric activation during mental word and rhyme generation using transcranial Doppler sonography

The Wada test is at present the method of choice for preoperative assessment of patients who require surgery close to cortical language areas. It is, however, an invasive test with an attached morbidity risk. By now, an alternative to the Wada test is to combine a lexical word generation paradigm with non-invasive imaging techniques. However, results of this approach are still not in complete agreement with the findings of the Wada test (r = .92; Knecht, Deppe, Ebner et al., 1998). We attempted to obtain a more distinct language lateralization with the development of a phonological rhyme generation paradigm based on pseudoword stimuli. To examine the predictive value of both paradigms we performed simultaneous bilateral functional transcranial Doppler ultrasonography on 19 right-handed male native German subjects. The rhyme generation condition produced a significantly stronger lateralization to the supposed hemisphere of language dominance than the classical word generation condition. The observed differences suggest that the rhyme generation paradigm is more robust in detecting hemispheric language dominance than other neuropsychological paradigms and might be most valuable for preoperative assessment of cortical language areas.     

Tactile perception and the right hemisphere: A masculine superiority for imagery coding

Recent research regarding hemispheric lateralization has suggested complex interactions among observed asymmetries, subject coding strategies, and sex differences in the utilization of and access to information codes. Prior to tactile perception of ambiguous figures, subjects were instructed to use either a verbal (left hemisphere) or imagery (right hemisphere) strategy. A non-instruction control group was included. Although a significant overall relationship between instructed strategy and tactile asymmetries was observed, this largely was due to males' privileged access to right-hemisphere imagery codes.     

Evoked potential and visual half-field evidence for task-dependent cerebral asymmetries in congenitally deaf adults

Evoked potentials to laterally presented stimuli were collected from left and right tempero-parietal sites during performance of two visual half-field tasks, lexical decision, and line orientation discrimination. Reaction time and accuracy data were simultaneously collected. The behavioral data indicated the development of a right field advantage for the lexical decision task as a function of practice. A principal components analysis revealed three independent evoked potential components which displayed task-dependent hemispheric asymmetries. Multiple regression analyses revealed that visual half-field asymmetries in response accuracy were closely related to hemispheric asymmetries on several independent evoked response components. Subject's scores on independent tests of verbal reasoning and spatial relations were also found to be closely related to hemispheric asymmetry on several independent evoked response components. These data support a multidimensional concept of cerebral specialization. They also suggest that visual field asymmetries reflect the confluence of several underlying processes which have independent lateralization distributions across the population. In general, the results underscore the need for further research on the nature of the relationship between cerebral and perceptual asymmetries.     

Sex-related hemispheric lateralization of amygdala function in emotionally influenced memory: an FMRI investigation

The amygdala appears necessary for enhanced long-term memory associated with emotionally arousing events. Recent brain imaging investigations support this view and indicate a sex-related hemispheric lateralization exists in the amygdala relationship to memory for emotional material. This study confirms and further explores this finding. Healthy men and women underwent functional Magnetic Resonance Imaging (fMRI) while viewing a series of standardized slides that were rated by the subjects as ranging from emotionally neutral to highly arousing. Two weeks later, memory for the slides was assessed in an incidental recognition test. The results demonstrate a significantly stronger relationship in men than in women between activity of the right hemisphere amygdala and memory for those slides judged as arousing, and a significantly stronger relationship in women than in men between activity of the left hemisphere amygdala and memory for arousing slides. An ANOVA confirmed a significant interaction between sex and hemisphere regarding amygdala function in memory. These results provide the strongest evidence to date of a sex-related hemispheric lateralization of amygdala function in memory for emotional material. Furthermore, they underscore the view that investigations of neural mechanisms underlying emotionally influenced memory must anticipate, and begin to account for, the apparently substantial influence of sex.     

胼胝体调节大脑两半球相互作用的机制

胼胝体是哺乳动物脑内最大的白质结构,不仅可以在大脑两半球间传递信息,也能调节半球间的相互作用,“抑制模型”和“兴奋模型”是当前解释胼胝体调节机制的主要模型.前者假设,胼胝体对半球间的信息传递起抑制作用,表现为优势半球的功能增强,非优势半球的活动抑制,因而可以提高半球加工的独立性和功能不对称性;后者则认为,胼胝体对半球间信息传递起促进作用,导致两半球同时性活动,并增强其功能连接性,因而可以降低半球的功能不对称性,有利于半球间的信息共享与功能整合.近期的研究显示,胼胝体并不是一个结构与功能的单一体,而是包含了在空间与时间上既分离又互动的多通道信息加工复合体.抑制与兴奋信息可以通过胼胝体的不同空间通道以不同的速度在半球间进行传递,并受到任务计算类型与复杂性的调节,因此,胼胝体的抑制与兴奋的协同可以调节两半球的动态相互作用.     

Linguistic lateralization and asymmetries in interhemispheric transmission time

The Interhemispheric Conduction Delay (ICD) theory of cerebral lateralization claims that longer interhemispheric transfer times (IHTT's) should result in greater functional lateralization for time-critical tasks. IHTT was estimated in 40 normal participants using both a visual and an auditory version of the Poffenberger (1912) paradigm. There was a significant response-hand by side-of-presentation interaction, and the length of IHTT for auditory information being transferred from the right to left hemisphere was significantly related to linguistic lateralization as assessed with a dichotic-listening task. These results support the ICD theory of cerebral lateralization.