Language laterality in left-handers assessed by unilateral E.C.T.

A method for assessing language laterality (namely testing for dysphasia after unilateral E.C.T. to each side of the head) that has been shown to be reliable in right-handed patients has been used in a series of 24 left-handers free from cerebral disease. Results in these 24 patients, combined with those in 13 patients either tested by a previous method or suffering from organic cerebral disease, indicate that language is predominantly represented in the left hemisphere in about 70 per cent of left-handers. Neither degree of sinistrality as measured by the Oldfield score, nor a positive family history, reliably predicted language laterality.     

Reliability and validity of the dichotic monitoring test for language laterality

The dichotic monitoring test, whereby subjects detect a target word presented to either ear, was used to assess laterality for language function. Discriminant Function Analysis (DFA) using target hit rates and mean reaction times for left and right ears and handedness correctly classified 95% of a criterion sample of 37 patients who were assessed for language laterality using the unilateral intracarotid amobarbital test or a test of dysphasia following unilateral ECT. Test-retest reliability of the DFA classifications was measured for a separate group of 86 subjects. Of these, 80% were reliably classified, while 16% were classified bilateral on one test: only three cases switched from left to right or vice-versa. We conclude that DFA applied to measures of dichotic monitoring performance gives a valid non-invasive classification of hemisphere specialization for language functions and that the classifications are reliable on retest.     

Determining language laterality by fMRI and dichotic listening

For imaging studies on hemispheric specialization of the human brain, data about known functional asymmetries other than handedness would be valuable for a reliable interpretation of lateralized activation in individuals or groups of subjects. As certain aspects of language processing are observed to be a function of primarily the left, it can be used as a reference for other asymmetric processes such as sensory or cognitive skills. For analyzing language laterality, there are a variety of methods, but these differ in application or accuracy. In this study, we tested the reliability of two widely used methods - dichotic listening and fMRI - to determine language dominance in 30 individual subjects. The German adaptation of a dichotic listening test (H?ttig, H., Beier, M., 2000. FRWT: a dichotic listening test for clinical and scientific contexts, Zeitschr f Neuropsychologie 11. 233-245.) classified 54% of the 26 right-handed subjects as left hemispheric dominant. The results of the fMRI paradigm (Fernández, G., de Greiff, A., von Oertzen, J., et al., 2001. Language mapping in less than 15 min: real-time functional MRI during routine clinical investigation. Neuroimage 14, 585-594.) tested on the same subjects, however, classified 92% of the right-handed subjects as left dominant. The main reason for this discrepancy was that the ear dominance score of many subjects in the dichotic listening test was too low to determine a reliable ear advantage. As a consequence, this specific dichotic listening test cannot be used to determine language laterality in individual subjects. On the other hand, the fMRI results are consistent with numerous studies showing left dominant language processing in more than 90% of right-handers. In some subjects, however, language laterality critically depends on the areas used to determine the laterality index.     

Ultradian rhythms in cerebral laterality tested by a verbal dichotic listening paradigm

Dichotic listening (DL) performance of normal (healthy) adults was tested every 15 minutes continuously during 8-12 hours of daytime. The dichotic material consisted of 8 blocks, each containing 40 pairs of monosyllabic, concrete nouns in German. The subjects heard one block per each 15-min trial. DL performances determined by the number of correct free recalls gave no evidence for periodic variations, neither of right nor of left-ear performance. However, when determined by the number of first, but correctly recalled nouns DL performance showed distinct periodicities of about 3-4 hours at least in some subjects. Such periodicities were obvious in both right and left-ear DL performance and oscillated in counter-phase. This suggests that competitive processing of dichotic verbal stimuli mediated by the cerebral hemispheres could be modulated by an endogenous oscillatory system. Some subjects slept in the laboratory after they had performed in a long-term dichotic listening task. Since sleep stage was scored by means of electroencephalographic recordings phase relationships between ultradian cycles of DL performance and the sleep cycle could be analyzed. These findings suggest that sleep cycle of about 1 1/2-2 hours and the 3-4 hour periodicities of DL performance are organized in an approximately 1:2 period ratio of an intrinsic multi-oscillatory system.     

A dichotic index of laterality that scores linguistic errors

The measure of cerebral laterality most commonly used in dichotic listening to detect ear advantages does not take errors into consideration. Experimental data were used to compare the laterality coefficient measure commonly used with a new method which scores error responses using linguistic rules. This new method produced significant results when applied to data for which the traditional measure found no significant effects. This suggests that on dichotic tasks where only one response is required, the traditional method of computing ear asymmetry by excluding errors may overlook a significant amount of information in these responses, information which is recoverable with the linguistically based scoring system.     

EEG immediately after unilateral ECT

EEG was continuously recorded in 15 patients for a period extending from just before to 1/2 hour after unilateral ECT. Fourier analysis was performed on the EEG following 15 right-sided treatments and five left-sided treatments. During the induced seizure, epileptic slow-wave activity had significantly greater power on the treated side. Immediately after the seizure, there was significantly more delta activity and less alpha and beta activity on the treated side. This asymmetry, though becoming less marked, was usually still present at the end of the recording period. Analysis of other variables associated with the treatment showed that there was a significant correlation between the time to eye-opening after ECT and both the duration of the seizure and the amount of anaesthetic administered. The similarity between these induced unilateral seizures and unilateral seizures occurring spontaneously in some epileptics is discussed.     

Reading comprehension performance and laterality: evidence for concurrent validity of dichotic, dichhaptic and EEG laterality measures

EEGs were recorded while 55 boys read and answered questions about passages. Concurrent validity between parietal alpha ratios obtained from these recordings and dichotic/dichhaptic scores supports their utility as measures of lateral processing. The evidence was particularly strong between right hand advantage for dichhaptically presented sequential patterns and left hemispheric activity while answering. Results also indicate that coefficients for dichotically presented digits and alpha ratios during silent reading are indices of receptive laterality and measure the deployment of lateral structures rather than degree of structural laterality. This supports a dynamic rather than static model of lateral processing during receptive language.     

Correlations between assessed manual laterality and predicted speech laterality in a normal population

The correlation of handedness and lateral speech representation was investigated in an adult normal population of left- and right-handers. Handedness was assessed by multivariate analysis of manual test scores. Assessment of speech laterality was based on scores derived from the Dichotic Listening Test.

Results showed that: (1) self-reports of “left-handedness” were unreliable estimates of manual performance; (2) self-classified left-handers, in contrast to right-handers, varied along levels of both manual and speech laterality; (3) the association between speech and manual laterality was greater when handedness was based on multiple performance scores; (4) familial sinistrality was twice as frequent in those test-classified left- and right-handers who showed speech representation on the same side as the dominant hand.     

Language laterality determined by MEG mapping with MR-FOCUSS

Magnetoencephalography recordings were made on 27 patients with localization related epilepsy during two different language tasks involving semantic and phonological processing (verb generation and picture naming). These patients underwent the semi-invasive intracarotid amobarbital procedure (IAP), also referred to as the Wada test, to determine the language-dominant hemisphere. Magnetoencephalography (MEG) data were analyzed by MR-FOCUSS, a current density imaging technique. A laterality index (LI) was calculated from this solution to determine which hemisphere had more neural activation during these language tasks. The LIs for three separate latencies, within each language task, were calculated to determine the latency that correlated best with each patient's IAP result. The LI for all language processing was calculated for the interval 150-550 ms, the second LI was calculated for the interval 230-290 ms (Wernicke's activation), and the third LI was calculated for the interval 396-460 ms (Broca's activation). In 23 of 24 epilepsy patients with a successful IAP, the LIs for Broca's activation, during the picture naming task, were in agreement with the results of the IAP (96% agreement). One of three patients who had an undetermined or bilateral IAP had an LI calculated for Broca's activation (396-460 ms) that agreed with intracranial mapping and clinical testing. These results indicate an 89% agreement rate (24 of 27) for magnetoencephalographic LI determination of the hemisphere of language dominance.     

Schizophrenia subgroups differing in dichotic listening laterality also differ in neurometabolism and symptomatology

Schizophrenia patients vary in right ear advantage (REA) on dichotic listening tests for assessing left hemispheric dominance for language processing. The authors examined if patients with low REA differed from other patients in symptoms and in resting brain metabolism. SPECT was conducted during visual fixation for 9 healthy control subjects and 16 schizophrenia patients: 8 with normal and 8 with diminished REA. REA-diminished patients had greater positive symptoms and lower mental status scores (all P<0.05) and had right middle temporal gyrus hypermetabolism. Both schizophrenia groups had decreased right frontal and increased medial temporal lobe metabolism vs. control subjects. REA-diminished patients had right temporal lobe hypermetabolism under a resting condition (eyes open, visual fixation). Results suggest reduced right ear (left hemisphere) advantage for dichotic word perception in schizophrenia is related to a predisposition to overactivate right temporal lobe regions and to positive symptoms. In contrast, the prefrontal-medial temporal imbalance present in both patient groups may typify the schizophrenia syndrome.     

Speech lateralisation in callosal agenesis assessed by the dichotic fused words test

Child patients with left-hemisphere damage (n = 2), with total callosal agenesis (n = 2), and with partial callosal agenesis or hypotrophy (n = 3) were submitted to dichotic verbal stimulation by the Fused Words Test. The controls were nine normal children, right-, left-, and mixed-handed. As expected, the left-injured patients presented a massive advantage of the ear contralateral to the intact hemisphere. Among the controls the right-handers showed a significant right-ear prevalence, whereas left-and mixed-handers exhibited rather inconsistent earasymmetries. The major finding of this study is the striking difference in perceptual asymmetry between partial and total acallosals. Based on the notion that the dichotic verbal asymmetry is an effect of the underlying functional asymmetry for language, these findings are interpreted as an indication that development of the unilateral speech control is influenced by the presence, during ontogenesis, of callosal connections.