Interhemispheric collaboration during digit and dot number-matching in younger and older adults.

Digit and dot number-matching stimuli were used to replicate findings reported for younger adults by Patel and Hellige (2007) and to explore whether performance would differ for younger versus older participants. Participants were to make numerical matches of digits only, dots only, and digits and dots mixed conditions to determine whether reaction time (RT), percentage error, and efficiency scores that combine latency and accuracy for match trials were better on within- versus across-hemisphere trials. Sixty-six younger and 42 older participants were screened with the Mini-Mental State Examination (MMSE) and the Geriatric Depression Scale. They performed the three experimental conditions and were assessed with Digit Span Forward and Backward subscales from the Wechsler Adult Intelligence Scale-III. Results for younger adults demonstrated a within-hemisphere advantage for the Digits and Mixed conditions and an across-hemisphere advantage for the Dots condition, consistent with previous literature. Older participants showed a stronger within-hemisphere advantage for the Digits condition compared with younger participants and no advantage for within- or across-hemisphere processing for the Mixed condition when RT was considered, but they performed similarly to younger adults when efficiency scores were used and showed a relative across-hemisphere advantage for the Dots condition. Although RT suggests age-related differences in how information is distributed across the hemispheres of the brain, more comprehensive efficiency scores indicate that younger and older adults appear to use similar strategies in the coordination of interhemispheric transfer of information. MMSE scores regardless of age were related to type of task but not to across- versus within-hemisphere performance.

Exploring interhemispheric collaboration in older compared to younger adults.

Physical and Name Identity letter-matching tasks were used to explore differences in interhemispheric collaboration in younger and older adults. To determine whether other factors might also be related to across/within-hemisphere processing or visual field asymmetries, neuropsychological tests measuring frontal/executive functioning were administered, and comparisons were made for participants split into low and high efficiency groups based on performance on the letter-matching tasks. A Task by Across/Within interaction was found for both groups, but with a stronger within-hemisphere advantage for the Physical Task and a weaker across-hemisphere advantage for the Name Task for older participants. More efficient groups and better performers on several neuropsychological tasks showed a reduced across-hemisphere advantage for the Name Identity task. Findings suggest that computational complexity, specific task demands, and perhaps trade-offs between age-related changes in gray and white matter all contribute to whether processing loads are distributed across or within hemispheres as we age.

Hemispheric differences in processing handwritten cursive.

Hemispheric asymmetry was examined for native English speakers identifying consonant-vowel-consonant (CVC) non-words presented in standard printed form, in standard handwritten cursive form or in handwritten cursive with the letters separated by small gaps. For all three conditions, fewer errors occurred when stimuli were presented to the right visual field/left hemisphere (RVF/LH) than to the left visual field/right hemisphere (LVF/RH) and qualitative error patterns indicated that the last letter was missed more often than the first letter on LVF/RH trials but not on RVF/LH trials. Despite this overall similarity, the RVF/LH advantage was smaller for both types of cursive stimuli than for printed stimuli. In addition, the difference between first-letter and last-letter errors was smaller for handwritten cursive than for printed text, especially on LVF/RH trials. These results suggest a greater contribution of the right hemisphere to the identification of handwritten cursive, which is likely related visual complexity and to qualitative differences in the processing of cursive versus print.

Benefits of interhemispheric collaboration can be eliminated by mixing stimulus formats that involve different cortical access routes.

Previous studies indicate that the benefits of dividing an information processing load across both cerebral hemispheres outweigh the costs of interhemispheric transfer as tasks become more difficult or cognitively complex. This is demonstrated as better performance when two stimuli to be compared are presented one to each visual field and hemisphere than when both stimuli are presented to the same single hemisphere (an across-hemisphere advantage). Two experiments indicate that this finding does not generalize to complex tasks that require matching numeric quantities represented by two very different visual formats whose processing involves somewhat different cortical areas: digits and dice-like dot patterns. In fact, mixing these stimulus formats consistently produces a within-hemisphere advantage. We propose that, when two simultaneously presented stimuli are presented in sufficiently different visual formats, identification of the two stimuli may take place in parallel, via different cortical access routes and with little or no interference, even when they are presented to the same cerebral hemisphere.

Impaired hemispheric communication in schizophrenia: a study using the consonant-vowel-consonant task.

The ability of the left and right hemispheres to interact and cooperate in the processing of information is important for normal brain functioning. Some investigators have suggested that this cooperation in functioning occurs through a process called "metacontrol." Because studies have suggested that patients with schizophrenia may have problems with interhemispheric interaction, we considered that it could be informative to study such patients from the viewpoint of interhemispheric cooperation, and more specifically that of metacontrol. To do this we utilized an assessment procedure called the consonant-vowel-consonant (CVC) task, in which nonsense three-letter combinations (trigrams) are presented vertically to the left, right and both visual fields. The pattern of errors can be used to determine the type of processing the hemispheres use under the different conditions of presentation. Previous studies have shown that when trigram information is presented to both hemispheres, the left hemisphere appears to switch into a mode that more resembles right hemisphere processing, and we wished to determine if evidence for this switch in the mode of processing also occurred in a sample of patients with schizophrenia. To determine the specificity of any findings to schizophrenia, we also studied a group of patients with bipolar disorder. For the control and bipolar groups, our results showed the typical finding of superior processing in the right visual field (left hemisphere) over the left visual field, with presentation to both visual fields demonstrating a qualitative error pattern resembling that of the left visual field (right hemisphere). Patients with schizophrenia, however, did not demonstrate this switching into a right hemisphere mode upon bilateral presentation of the trigrams and instead seemed to show no switching, or actually switching into more of a left hemisphere mode. Our results suggest that patients with schizophrenia (but not bipolar disorder) may have abnormalities in the cooperative processing of information when it is presented to both hemispheres, and that this disturbance may reflect problems with metacontrol of hemispheric processing.

Hemispheric differences for identification of words and nonwords in Urdu-English bilinguals.

Hemispheric asymmetry was examined for Urdu-English bilinguals identifying printed Urdu words and nonwords, separated Urdu letter strings, digits, and English nonwords. In all cases, fewer errors occurred when stimuli were presented to the right visual field/left hemisphere (RVF/LH) than to the left visual field/right hemisphere (LVF/RH). Qualitative error patterns suggested that separated Urdu letter strings were processed more serially than Urdu letter strings joined to form words or pronounceable nonwords and more serially on RVF/LH than on LVF/RH trials. This qualitative laterality effect is similar to that found for Hebrew and Arabic but opposite that found for English and suggests that the qualitative manner of processing printed verbal material is influenced by language-specific factors such as scanning direction, orthographic-to-phonological mapping rules, and morphology.

Lateralized cognitive dysfunction and psychotic symptoms in schizophrenia.

Our understanding of hemispheric asymmetries in schizophrenia can be attributed to extensive neuropsychological and neuroimaging research on this topic; however, it has yet to be determined whether lateralized cognitive dysfunction represents a single core trait in schizophrenia or whether the lateralized impairments are domain specific. To test whether lateralized deficits are core features in schizophrenia we examined performance across a wide range of lateralized cognitive domains including attention, fluency, recognition memory, perception, and arousal. We also examined the relationship between lateralized impairments and psychotic and affective symptoms to determine whether abnormal hemispheric asymmetries were possibly state-related. The sample consisted of 43 subjects with schizophrenia and 66 normal healthy comparison subjects without psychiatric illness. Schizophrenia subjects exhibited abnormal right hemisphere performance on a test of recognition memory and abnormal left hemisphere performance on a measure of arousal. These findings suggest that lateralized cognitive disturbances in schizophrenia do not represent a single core lateralized deficit. Regarding the symptom analyses, severity of positive symptoms was related to right hemisphere cognitive impairment (including fluency and recognition memory), whereas severity of negative symptoms was related to left hemisphere cognitive impairment (including fluency). Overall, our findings suggest that lateralized dysfunction can occur in both hemispheres in schizophrenia, and that the positive psychotic symptoms may relate more to right hemisphere impairment, whereas negative psychotic symptoms may related more to left hemisphere impairment.

Aging and Individual Variation in Interhemispheric Collaboration and Hemispheric Asymmetry.

Thirty younger (Mean Age = 19.9 years) and 20 older adults (Mean Age = 74.7 years) performed Physical and Name Identity letter-matching tasks (matches were either within or between hemispheres) to study age-related changes in 1) the efficiency with which the two hemispheres interact with each other and 2) hemispheric asymmetry. In order to determine whether age-related effects were associated with differences in cognitive resources, the same individuals completed a set of memory span tasks. Performance on the letter-matching tasks indicated that the costs of interhemispheric collaboration were greater for older than for younger participants. However, within the older group, the advantage of spreading processing across both hemispheres increased as memory span decreased, suggesting that older individuals who are challenged by cognitive complexity are more likely to show increased benefits from between-hemisphere processing than individuals who are not so challenged. There was also an overall left visual field/right hemisphere advantage for the younger but not for the older group, suggesting greater age-related declines in right- than left-hemisphere function.

Interhemispheric interaction in bilateral redundancy gain: effects of stimulus format.

Previous visual laterality experiments have shown that identification is better when 2 copies of the same stimulus are presented--1 to each hemisphere--than when only a single copy is presented to 1 hemisphere. New experiments were conducted to vary whether the 2 stimuli on a bilaterally redundant trial were physically identical or identical in format in addition to leading to identical responses. Substantial bihemispheric gain was obtained even when the 2 stimuli were perceptually distinctive (e.g., letter trigrams differing in case and font or the same numeric quantity represented by digits and dots). Thus, much of the bihemispheric advantage involves relatively abstract aspects of information processing. However, when the formats were sufficiently distinctive, there were small effects on bihemispheric performance, suggesting some role for less abstract processes that are sensitive to physical characteristics.

Corpus callosum abnormalities in psychopathic antisocial individuals.

CONTEXT: Psychopathic antisocial individuals have previously been characterized by abnormal interhemispheric processing and callosal functioning, but there have been no studies on the structural characteristics of the corpus callosum in this group. OBJECTIVES: To assess whether (1) psychopathic individuals with antisocial personality disorder show structural and functional impairments in the corpus callosum, (2) group differences are mirrored by correlations between dimensional measures of callosal structure and psychopathy, (3) callosal abnormalities are associated with affective deficits, and (4) callosal abnormalities are independent of psychosocial deficits. DESIGN: Case-control study. SETTING: Community sample. PARTICIPANTS: Fifteen men with antisocial personality disorder and high psychopathy scores and 25 matched controls, all from a larger sample of 83 community volunteers. MAIN OUTCOME MEASURES: Structural magnetic resonance imaging measures of the corpus callosum (volume estimate of callosal white matter, thickness, length, and genu and splenium area), functional callosal measures (2 divided visual field tasks), electrodermal and cardiovascular activity during a social stressor, personality measures of affective and interpersonal deficits, and verbal and spatial ability. RESULTS: Psychopathic antisocial individuals compared with controls showed a 22.6% increase in estimated callosal white matter volume (P<.001), a 6.9% increase in callosal length (P =.002), a 15.3% reduction in callosal thickness (P =.04), and increased functional interhemispheric connectivity (P =.02). Correlational analyses in the larger unselected sample confirmed the association between antisocial personality and callosal structural abnormalities. Larger callosal volumes were associated with affective and interpersonal deficits, low autonomic stress reactivity, and low spatial ability. Callosal abnormalities were independent of psychosocial deficits. CONCLUSIONS: Corpus callosum abnormalities in psychopathic antisocial individuals may reflect atypical neurodevelopmental processes involving an arrest of early axonal pruning or increased white matter myelination. These findings may help explain affective deficits and previous findings of abnormal interhemispheric transfer in psychopathic individuals.