Hemispheric specialization and attention: effects of complete and partial callosal section and hemispherectomy on dichotic monitoring

Dichotic listening and interhemispheric integration after callosotomy: A systematic review

The right-ear advantage (REA) for recalling dichotically presented auditory-verbal stimuli has been traditionally linked to the dominance of the left cerebral hemisphere for speech processing. Early studies on patients with callosotomy additionally found that the removal of the corpus callosum leads to a complete extinction of the left ear, and consequently the today widely used models to explain the REA assume a central role of callosal axons for recalling the left-ear stimulus in dichotic listening. However, later dichotic-listening studies on callosotomy patients challenge this interpretation, as many patients appear to be able to recall left-ear stimuli well above chance level, albeit with reduced accuracy. The aim of the present systematic review was to identify possible experimental and patient variables that explain the inconsistences found regarding the effect of split-brain surgery on dichotic listening. For this purpose, a systematic literature search was conducted (databases: Pubmed, Web of Knowledge, EBSChost, and Ovid) to identify all empirical studies on patients with surgical section of the corpus callosum (complete or partial) that used a verbal dichotic-listening paradigm. This search yielded ks = 32 publications reporting patient data either on case or group level, and the data was analysed by comparing the case-level incidence of left-ear suppression, left-ear extinction, and right-ear enhancement narratively or statistically considering possible moderator variables (i.a., extent of the callosal surgery, stimulus material, response format, selective attention). The main finding was an increased incidence of left-ear suppression (odds ratio = 7.47, CI95%: [1.21; 83.49], exact p = .02) and right-ear enhancement (odds ratio = 21.61, CI95%: [4.40; 154.11], p < .01) when rhyming as compared with non-rhyming stimuli were used. Also, an increase in left-ear reports was apparent when a response by the right hemisphere was allowed (i.e., response with the left hand). While the present review is limited by the overall small number of cases and a lack of an appropriate control sample in most of the original studies, the findings nevertheless suggest an adjustment of the classical dichotic-listening models incorporating right-hemispheric processing abilities as well as the perceptual competition of the left- and right-ear stimuli for attention.

Dichotic-listening performance after complete callosotomy: No relief from left-ear extinction by selective attention

The surgical section of the corpus callosum (callosotomy) has been frequently demonstrated to result in a left-ear extinction in dichotic listening. That is, callosotomy patients report the left-ear stimulus below chance level, resulting in substantially enhanced right-ear advantage (REA) compared with controls. A small number of previous studies also suggest that callosotomy patients can overcome left-ear extinction when the instruction encourages to attend selectively to the left-ear stimulus. In the present case study, we re-examine the role of selective attention in dichotic listening in two patients with complete callosotomy and 40 age- and sex-matched controls. We used the standardised Bergen dichotic-listening paradigm which uses stop-consonant-vowel syllables as stimulus material and includes both a free-report and selective-attention condition. As was predicted, both patients showed a clear left-ear extinction. However, contrasting the earlier reports, we did not find any evidence for a relief from this extinction by selectively attending to the left-ear stimulus. We conclude that previous demonstrations of an attention-improved left-ear recall in callosotomy patients may be attributed to the use of suboptimal dichotic paradigms or residual callosal connectivity, rather than representing a genuine effect of attention.

Dominance specific visual extinction associated with callosal disconnection

Callosal disconnection signs are closely related to asymmetric hemispheric specialization of cognitive functions. Although extinction is more commonly associated with the right parietotemporal lesion, it may occur following lesions of the corpus callosum. After an infarction involving the left splenium, a 58-year-old right-handed man had no disconnection symptoms that had been reported earlier, but showed visual extinction with left or right visual hemifield dominant stimuli. Our results suggest that dominance specific visual extinction might be another disconnection sign associated with splenial lesion.

Dichotic listening in patients with splenial and nonsplenial callosal lesions

The authors found splenial lesions to be associated with left ear suppression in dichotic listening of consonant-vowel syllables. This was found in both a rapid presentation dichotic monitoring task and a standard dichotic listening task, ruling out attentional limitations in the processing of high stimulus loads as a confounding factor. Moreover, directed attention to the left ear did not improve left ear target detection in the patients, independent of callosal lesion location. The authors' data may indicate that auditory callosal fibers pass through the splenium more posterior than previously thought. However, further studies should investigate whether callosal fibers between primary and secondary auditory cortices, or between higher level multimodal cortices, are vital for the detection of left ear targets in dichotic listening.

Callosal disconnection syndrome in a left-handed patient due to infarction of the total length of the corpus callosum

We report on a left-handed patient with an ischemic infarction affecting exclusively the total length of the corpus callosum. This lesion clinically correlated with an almost complete callosal disconnection syndrome as described in callosotomy subjects, including unilateral verbal anosmia, hemialexia, unilateral ideomotor apraxia, unilateral agraphia, unilateral tactile anomia, unilateral constructional apraxia, lack of somesthetic transfer and dissociative phenomena. Despite the patient's left-handedness, his pattern of deficits was similar to the disconnection syndrome found in right-handers. Our report focusses on motor dominance and praxis. We followed-up the improvement in left apraxia and investigated the ability to initiate and learn a new visuo-motor skill. The results permit two tentative assumptions: (1) that the improvement in left apraxia was due to a compensatory increase in ipsilateral proximal muscle control, and (2) that motor dominance, i.e. the competence to initiate and learn a new movement pattern, was hemispherically dissociable from manual dominance in the sense of praxis control.

Blood glucose levels selectively influence memory for word lists dichotically presented to the right ear

The relationship between performance on a dichotic listening task and blood glucose levels was examined. It was predicted that, if the ability of blood glucose to supply the brain with its basic fuel limits performance under conditions of cognitive demand, in a dichotic listening task information directed to the left hemisphere would be particularly susceptible to the level of blood glucose. Via headphones subjects heard lists of words directed to both ears, although randomly they were asked to attend to those directed to one ear. Those receiving a glucose drink recalled more of a list of words directed to the right ear and hence the left hemisphere. Subjects with low baseline blood glucose levels recalled more from the attended ear, and those with high baseline blood glucose more from the unattended ear. In those who received a glucose drink, a fail in blood glucose during the dichotic task was associated with a right ear superiority. The pattern of findings can be explained by the assumptions that under conditions of cognitive demand the supply of glucose to the brain limits performance and that there are individual differences in the ability to efficiently take glucose from the blood stream into the brain.

The effect of illness duration on perceptual asymmetry in schizophrenia

Perceptual asymmetry in a group of schizophrenic subjects who were clinically stable and living in the community was compared with a group of normal control subjects matched for age and sex using a dichotic monitoring task of language processing. Schizophrenic subjects showed reduced target detection and slower reaction times for both left and right ear inputs. In relation to performance asymmetry, the schizophrenic subjects had a reduced speed of responding to left ear items compared to normal controls. Within the schizophrenic group differences in performance emerged according to duration of illness. Shorter duration of illness was associated with poorer target detection overall and comparatively greater magnitude of the normal right ear advantage. The latter was accounted for by a relative augmentation of the normal left ear performance decrement. These results were partly reflected in the reaction time measures. The findings suggest that as illness duration increases there may be a tendency for certain aspects of the information processing abnormality in schizophrenia to normalise, in spite of continued deficits as reflected in prolonged reaction times.

Tactile extinction on complex stimulation in normal children

The Schwartz Quality Extinction Test was administered to 221 normal right-handed children aged 4 to 7 years to find out at what age their ability to perceive complex bimanual simultaneous stimuli stabilizes. Tactile extinction was present in some 4- and 5-yr.-old children and tended to disappear later. We found a significant difference in responses between 5- and 6-yr.-old children but no difference regarding side and sex at different ages. Among the principal theories regarding the pathogenesis of extinction, incomplete maturation of attentional mechanisms seems the one which better explains extinction in younger children.

Differences in dichotic listening asymmetries in depression according to symptomatology

Subjects suffering from major depressive disorder were compared to normal controls on two verbal dichotic listening tasks. Although there were no significant differences between the groups on a task primarily of language perception, significant differences were obtained on a task with an attentional component. Overall performance was lower for the depressed group and ear asymmetry was reduced. Within the depressed group ear asymmetry varied according to symptomatology; withdrawal-retardation was associated with a lack of asymmetry and anxiety with a normal right ear advantage. The results were interpreted in terms of an interaction between affect and attention, and possible underlying mechanisms of cerebral hemisphere function were discussed.

Dichotic listening and manual performance in relation to magnetic resonance imaging after closed head injury

In order to investigate post-traumatic hemispheric disconnection effects, dichotic listening and intermanual tasks were administered to 69 patients who had sustained a closed head injury of varying severity. The manual tasks consisted of naming objects palpated in either hand, transfer of postures from one hand to the other and writing. Consistent with predictions, the degree of ear asymmetry in dichotic listening performance was directly related to the severity of the head injury as reflected by the degree of impaired consciousness. Depth and localisation of parenchymal lesion characterised by magnetic resonance imaging were also related to the degree of ear asymmetry. Parenchymal lesions situated in sites which could potentially interfere with callosal auditory or geniculocortical pathways produced a greater disparity in response to left versus right ear inputs as compared with parenchymal lesions in areas such as the frontal lobes which are purportedly unrelated to asymmetries in dichotic listening performance. The results provide further evidence for the effects of multifocal brain lesions involving the white matter on tasks which require intra and/or interhemispheric integration.

Focused and divided attention in each half of space with disconnected hemispheres

The effects of spatial location and strategy of attention on the processing of speech messages were investigated in ten right-handed subjects and four persons with complete forebrain commissurotomy and one case of right hemispherectomy. Lists of simultaneous but different words, with one of each pair spoken by a male and the other by a female, were monitored for target words. The lists were presented to the left ear, the right ear and to both ears (central) in separate conditions. Unimanual responses were made to targets in either voice (divided attention) or to only one of the voices (focused attention). The performance of the clinical subjects was generally less efficient than that of the controls. In contrast to intact subjects, they responded to more distractor and unattended items. They also showed a nonsignificant tendency for better processing of right rather than centrally-presented words. With right-sided presentation two clinical subjects, with extracallosal damage to the right cortex, were unable to divide attention and responded to only one of the voices. These results implicate an intact corpus callosum for efficient use of attention strategies as well as hemispheric differences in the control of attention.

The effects of antipsychotic drugs and symptomatology on perceptual asymmetry in schizophrenia

Perceptual asymmetry was measured in acutely psychotic schizophrenic patients and normal controls using a dichotic monitoring task. Although the schizophrenic group differed from the controls in terms of accuracy and speed of response, there were no significant group differences with respect to asymmetries in these variables. The pattern of responding in the schizophrenic group was also examined in relation to differences in symptomatology and antipsychotic drug administration. These variables, in isolation and in interaction with each other, were found to have a number of effects on both asymmetry and overall level of responding. The results were interpreted in the context of an information processing model of functional hemisphere asymmetry in schizophrenia, and the implications regarding the nature of schizophrenic symptomatology and the effects of antipsychotic drugs were discussed.

Dichotic listening asymmetries and psychotic symptoms in schizophrenia: a preliminary report

Schizophrenic subjects were compared with both psychiatric control and normal control groups on two verbal measures of functional hemispheric asymmetry on a dichotic listening task. One measure of language perception showed little variation in ear differences across groups. However, the task that contained both attentional and language-processing components showed variations in asymmetry according to symptomatology. Subjects with high scores on scales of hallucinations and delusions showed a pattern of ear differences that was not found in those with low scores on these symptom dimensions. The results may reflect cognitive factors in a dysfunctional information-processing system and, in particular, attentional disturbance rather than anomalies in the lateralization of language.

Dichotic listening in commissurotomized and hemispherectomized subjects

Three commissurotomized and two left-hemispherectomized subjects were tested on spoken report of sequences of three dichotic pairs of digits. With instruction to report only one digit from each pair, there was an overall advantage to the ear contralateral to the hemisphere mediating speech, but report of ipsilateral-ear digits ranged from 40 to 100%. In commissurotomized subjects, the more extreme ipsilateral suppression under instructions to report all digits may be due to failure to gain access to unattended information stored in the right hemisphere, rather than to suppression of the ipsilateral pathway. However one commissurotmized patient did appear to have access to right-hemisphere items, the result either of subcortical transfer or of external cross-cueing. The hemispherectomized subjects seemed able to store both attended and unattended information in the same hemisphere.

Tactual extinction without anomia: evidence of attentional factors in a patient with a partial callosal disconnection

Tactual extinction was examined in a 43-yr-old left-handed male patient (MM) with a hematoma in the trunk of the corpus callosum sparing the genu and the ventral part of the splenium as well as the cerebral hemispheres. Experiments I and II showed no deficit in tactual perception nor in the inter-hemispheric transfer of tactual information. In Experiment III, MM palpated stimuli differing in their association value, and responded vocally or manually. There was no difference in performance between the hands in unimanual control conditions, but in a dichhaptic condition an overall right-hand advantage was observed, depending on the kind of stimulus and on the mode of response: such results were not observed in control subjects. Results showed that MM's left-hand performance reached right-hand levels after specific modifications in the allocation of attention between the hands or the load of information in haptic memory (Experiment IV). Experiment V revealed a major deficit in the matching of left- and right-hand information only for the dichhaptic procedure. The results are discussed with a view of cerebral mechanisms as dynamic rather than structurally determined: it is suggested that extinction phenomena are sensitive to cognitive strategies and attentional factors and that the corpus callosum plays a critical role in the lateral control of attention to tactual as well as to auditory and visual information.