Concurrent verbal and emotion laterality effects exhibited by normally achieving and learning disabled children.

This study assessed both left- and right-hemisphere functions simultaneously when two-syllable words differing only in the initial stop consonant and spoken in different emotional tones were paired dichotically. Seventy-two right-handed normally achieving children, 12 boys and 12 girls at each of grades 1, 3, and 5, were instructed to detect either the presence of a specific word or of a specific emotion. In addition, 30 right-handed learning disabled (LD) children (age-matched to the normal controls) were assessed to determine whether LD children distribute verbal and nonverbal functions to different hemispheres. Results indicated that although both control and LD children demonstrated an overall REA for word stimuli and an LEA for emotional stimuli, and that emotional stimuli were easier to process than word stimuli, LD children were less accurate in processing both types of stimuli than their control counterparts. 'Complementary specialization,' as assessed through distribution of laterality effects, was found to be greater for control children than for LD children. However, the lack of consistency in complementary specialization found among the three developmental grade levels may be indicative that independent brain mechanisms underlying verbal and emotional processing have yet to be fully established in children. Further, in contrast to adult findings, a larger LEA was obtained for the emotion 'happy' than for the emotion 'sad.' It was concluded that whereas independent hemisphere processing for words and emotions is somewhat prevalent for control children, LD children might not be as strongly lateralized for opposite hemisphere processing of these functions.

Eye-dominance, writing hand, and throwing hand.

Handedness and eye-dominance are undoubtedly associated statistically, although a previous meta-analysis has found that the precise relationship is difficult to explain, with about 35% of right-handers and 57% of left-handers being left eye dominant. Of particular difficulty to genetic or other models is that the proportions are distributed asymmetrically around 50%. The present study asked whether this complicated pattern of association occurred because, following Peters, it is necessary to divide right-and left-handers into consistent handers (who write and throw with the same hand) and inconsistent handers (who write and throw with opposite hands). In an analysis of 10,635 subjects from questionnaire studies, 28.8% of left-handers and 1.6% of right-handers by writing were inconsistent for throwing. Our results also showed that writing hand and throwing hand both relate independently to eyedness, that throwing hand is somewhat more strongly associated with eyedness, and that the awkward asymmetry around 50% is now removed, 24.2% of consistent right-handers being left eye dominant compared with 72.3% of consistent left-handers, and 55.4% of inconsistent right-handers compared with 47.0% of inconsistent left-handers. We conclude that eyedness is phenotypically secondary to writing and throwing handedness. In our discussion we note that eyedness runs in families, we present new data suggesting that writing hand and throwing hand are co-inherited, and we argue that further data are now required to model properly the associations of writing hand, throwing hand, and eyedness, as well as probably also footedness and language dominance.

First-language acquisition in adolescence: evidence for a critical period for verbal language development.

It has been hypothesized that there is a critical period for first-language acquisition that extends into late childhood and possibly until puberty. The hypothesis is difficult to test directly because cases of linguistic deprivation during childhood are fortunately rare. We present here the case of E.M., a young man who has been profoundly deaf since birth and grew up in a rural area where he received no formal education and had no contact with the deaf community. At the age of 15, E.M. was fitted with hearing aids that corrected his hearing loss to 35 dB, and he began to learn verbal Spanish. We describe his language development over the 4-year period since his acquisition of hearing aids and conclude that the demonstrates severe deficits in verbal comprehension and production that support the critical period hypothesis.

Auditory attentional shifts in reading-disabled students: quantification of attentional effectiveness by the Attentional Shift Index.

A controversy has existed for some years regarding auditory attentional skills in reading-disabled children. Data have suggested highly developed attentional skills in groups of reading-disabled students, but reduced attentional shifts have also been documented in equivalent groups. Attentional shifts in dichotic listening with forced or directed attention are usually inferred from a significant interaction between attentional task and ear. However, this procedure cannot be used to evaluate individual test performance, and the interaction does not give a useful measure of attentional shifts in dichotic listening meaningful for comparison with other tests of attention. In this paper attentional shifts in dichotic listening are quantified with the Attentional Shift Index (ASI), a measure for evaluating the degree of attentional shift in individual subjects. The ASI is based on the log-odds ratio of hits and intrusion errors when the subject has been tested under conditions of directed or forced attention. When 58.3% of the normative sample showed significant attentional shifts, none of the reading-disabled sample did so. This finding is discussed in relation to different types of deficits that can account for for the lack of auditory attentional shifts.

Footedness is a better predictor than is handedness of emotional lateralization.

A tremendous amount of experimental work has attempted to identify reliable behavioural predictors of cerebral lateralization. Preferred handedness has been the most popular predictor, but some recent reports suggest that preferred footedness may serve as a more accurate predictor of functional laterality, especially in the left-handed population. The present study sought to test this claim by selectively recruiting individuals with either 'crossed' lateral preferences (right-handed and left-footed or left-handed and right-footed) or 'uncrossed' lateral preferences (right-handed and right-footed or left-handed and left-footed). Lateralization of emotional perception was assessed with two blocks of the dichotic Emotional Words Test (EWT), and lateral preference for both handedness and footedness was assessed using self-report questionnaires. Ear advantage on the dichotic task varied significantly with preferred foot (P=0.003), but not with preferred hand. Cerebral lateralization may be more related to footedness than to other lateral preferences.

Footedness is a better predictor of language lateralisation than handedness.

A tremendous amount of experimental work has attempted to identify a reliable behavioural predictor of language lateralisation. Preferred handedness has probably received the most attention, but there are some recent reports that preferred footedness may serve as a more reliable predictor. The present study sought to test this claim by selectively recruiting 32 participants such that the factors of handedness, footedness, and gender were completely crossed. Language lateralisation was assessed with the Fused Dichotic Words Test (FDWT), and lateral preference for both handedness and footedness was assessed using self-report questionnaires. Ear advantage on the dichotic task varied significantly with preferred foot ( P <.001) but not with preferred hand ( P = .196). This result is problematic for evolutionary theories of cerebral lateralisation that claim left-hemispheric language is related to fine manual motor skill and sequencing. Language lateralisation may be more closely related to asymmetrical control of whole-body finely sequenced movements.

Handedness in mental retardation.

Mentally retarded and age-matched normal controls with no history of parental sinistrality were examined for their handedness with the Waterloo Handedness Questionnaire (Singh & Bryden, 1994). Normal controls showed a clear rightward bias, with the effect being significantly higher for skilled as compared to unskilled hand items. Mentally retarded subjects showed mixed handedness with a nonsignificant difference in performance between skilled and unskilled hand items.

Laterality effects in the processing of melody and timbre.

Laterality for the processing of melody and timbre was investigated in 64 right-handed non-musicians. In one block of dichotic-listening trials, participants listened for a prespecified target melody, and in a second block they listened for a prespecified target instrument. Females were more accurate on the left ear in the melody task (whereas males tended to show no ear advantage), but there were no significant ear differences in the timbre task for either sex. This supports the idea of a complementary sex-based pattern of lateralization, with males more strongly lateralized for verbal stimuli and females more strongly lateralized for non-verbal stimuli. No relation was observed between lambda measures for the two tasks, suggesting that laterality for melody processing is independent of laterality for timbre processing.

Mouse paw preference: effects of variations in testing protocol.

Two experiments using two inbred strains of mice were conducted to study mouse paw preference. In the first study, 250 (50 x 5) paw reaches from each of 12 male and 12 female C57BL/6J mice were observed over an 8-week period using the Collins paw preference apparatus to investigate questions relating to the independence of reaches, the size of the tube into which the animals reach for flakes of food, and practise effects. Animals appeared to be much more highly lateralized when two independent reaches per day for 25 days (I protocol) were observed than when 50 reaches were observed in a single session (S protocol). Paradoxically, however, we found no evidence for the lack of independence of reaches when we examined the sequences of reaches performed by animals under the S protocol conditions. With practise, animals became slightly more lateralized, but there were no effects of sex, and approximately equal numbers of mice were left-pawed as were right-pawed. The size of the tube had no effect. The second study used 30 BALB mice, each of which was tested with both the I and S protocols, with order of testing counterbalanced. There was a significant protocol x order interaction such that those doing S first were more highly lateralized on the I task, but those doing I first were highly lateralized on both tasks.

On the genetics and measurement of human handedness.

Lateral preferences in humans and other primates may be a key to many important issues in biology and psychology. There is strong evidence that the representation of language functions in the cerebral cortex is different in left-handed (LH) people than in right-handed (RH) people, and an understanding of handedness may lead to valuable clues as to how the brain becomes organised in the way that it does. Although there are clearly cultural effects that influence manual activities, there is nevertheless evidence that human handedness and other lateralities are at least in part genetically determined: compared with two RH parents, one RH and one LH parent are 2-3 times more likely to have an LH child, and two LH parents are 3-4 times more likely to have an LH child. Thus, one might wish to investigate the genetics of laterality with the goal of understanding the biological mechanisms that lead to the preferential use of one hand (or eye or foot). One may also see links between human handedness and footedness and the motor asymmetries found in many nonhuman primate species, and perhaps in lower mammals as well. From this perspective, one might see the study of human laterality as relevant to evolutionary biology. Investigators of both human and nonhuman asymmetries have to grapple with such difficult measurement issues as the relations between preference and performance, and the influences of postural and task demands, and have much to offer each other in the quest for the nature of laterality. Our recent work seems to indicate that the various sensory and motor lateralities may be related, in humans, but not in a simple way. In future work, the challenge will be to identify the relations between the various laterality ''profiles'' and patterns of functional cerebral organisation.

Associations among familial sinistrality, allergies, and developmental language disorders.

Here we report the results of a study that tested what has become known as the Geschwind hypothesis (Geschwind & Galaburda, 1987). This study involved 526 people who were tested by an allergist for IgE-mediated allergies and who filled out a 12-item handedness questionnaire that also included questions about eyedness, prevalence of left-handedness in the immediate family, and familial developmental language problems. A series of log-linear analyses revealed that if the definition of "anomalous" included left-handedness and having a first-order left-handed relative, one was indeed more likely to be allergic than were those not so classified. When the definition of anomalous simply included those classified as left-handed, the results were not significant. Parallel findings resulted when the criterion variable was the presence of developmental language disorders. No evidence was found for relations between allergies and developmental language problems.

Biased attention and the fused dichotic words test.

This study examines the effect of biased attention on the fused dichotic words test (FDWT) and the CV syllables dichotic listening test (CVT). Eight males and eight females were given both tests with two different instructions: to direct attention to the left ear (DL), or to the right ear (DR). These instructions led to highly significant differences in response on the CVT, but only a marginal shift in performance on the FDWT. While the FDWT is not completely unaffected by attentional manipulations, it is far less influenced by such effects than the CVT. This indicates that subject-initiated shifts of attention are much less likely to affect performance on the FDWT than on other dichotic tests and makes it a more valuable task to assess cerebral speech lateralization.

Handedness and eye-dominance: a meta-analysis of their relationship.

About one in ten people is left-handed and one in three is left-eyed. The extent of the association of handedness and eyedness is unclear, as some eyedness measures are potentially contaminated by measures of handedness. A meta-analysis of hand-eye concordance in 54,087 subjects from 54 populations, found that concordance was 2.69 x greater in questionnaire rather than performance studies, 1.95 x greater in studies using unimanual monocular performance measures, and 6.29 x greater in studies using non-sighting measures of eye-dominance. In the remaining studies, which seemed to show no evidence of bias, the odds-ratio for hand-eye concordance was 2.53 x; in a population with 9.25% left-handedness and 36.53% left-eyedness, 34.43% of right-handers and 57.14% of left-handers are left-eyed. This pattern of hand-eye association poses problems for genetic models of cerebral lateralisation, which must invoke pleiotropic alleles at a single locus or epistatic interactions between multiple loci. There was no evidence that the incidence of eyedness, or the association between eyedness and handedness, differed between the sexes.

Inspecting asymmetric presentations of words differing in informational and morphemic structure.

Are laterality effects in visual word recognition a product of functional asymmetry of the cerebral hemispheres or a result of stimulus inspection factors such as asymmetric eye fixations before and during visual presentation, different movement time to asymmetric targets, an interaction of the informative components, or morphemic structure of a target with a drop in acuity with distance? Two experiments were conducted to address these questions. Experiment One recorded the inspection of prefixed and matched control words presented unilaterally for 200 msec. Experiment Two displayed suffixed words and their appropriate controls. The words also varied in their information distribution. Reader's eye movements were monitored while they inspected the target words in preparation for a synonym judgement task. Measures of the location of the initial fixation on the screen showed that subjects were not biased to the right visual field prior to stimulus onset. When only those occasions in which the subject had correctly fixated the central cursor were analyzed, a robust right visual field advantage was observed on many of the target inspection time measures, but no reliable difference was observed for faster movement time to the right visual field's target compared to that observed in the left visual field. Neither the distribution of information within the word nor its morphemic structure affected the pattern of any inspection time measures. Consequently, such factors can be ruled out as threats to the hypothesis concerned with the functional asymmetry of the cerebral hemispheres.

Magnitude of sex differences in spatial abilities: a meta-analysis and consideration of critical variables.

In recent years, the magnitude, consistency, and stability across time of cognitive sex differences have been questioned. The present study examined these issues in the context of spatial abilities. A meta-analysis of 286 effect sizes from a variety of spatial ability measures was conducted. Effect sizes were partitioned by the specific test used and by a number of variables related to the experimental procedure in order to achieve homogeneity. Results showed that sex differences are significant in several tests but that some intertest differences exist. Partial support was found for the notion that the magnitude of sex differences has decreased in recent years. Finally, it was found that the age of emergence of sex differences depends on the test used. Results are discussed with regard to their implications for the study of sex differences in spatial abilities.

Evaluating the empirical support for the Geschwind-Behan-Galaburda model of cerebral lateralization.

The Geschwind-Behan-Galaburda (GBG) model of cerebral lateralization provides a complex but testable theory of the origins and associates of cerebral lateralization. An overall evaluation of the model suggest that it is not well supported by empirical evidence and that in the case of several key theoretical areas, the evidence that does exist is inconsistent with the theory. In particular: the concept of "anomalous dominance" is shown to be theoretically and methodologically flawed; a meta-analysis of the relationship between handedness and immune disorders finds a marginal overall association, and while three conditions (allergies, asthma, and ulcerative colitis) do show significant associations with left-handedness, two other conditions (myasthenia gravis and arthritis) show significant negative associations with left-handedness. Finally, a review of the origins of the neural crest, and its associations, suggests there is almost no empirical support for the GBG theoretical model in this area.

Laterality effects in normal subjects: evidence for interhemispheric interactions.

In this review we describe three methods of research that have been used to study relationships between the cerebral hemispheres in normal human subjects. We briefly mention reaction-time studies, which have been used to measure interhemispheric transmission time; more pertinent to laterality effects, however, are lateralized visual field studies, in which stimuli can be presented either unilaterally or to both visual fields simultaneously, and dichotic listening studies, wherein different stimuli are presented to the two ears simultaneously. Recent examples from the literature of experiments employing these techniques are discussed as they pertain to an understanding of interhemispheric interaction. Some suggestions are offered concerning appropriate procedures for studying interhemispheric interactions in normal human subjects.

A behavioral measure of hand preference as opposed to hand skill.

Twenty-five self-professed left-handers and 21 self-professed right-handers were given a variety of performance tests to assess handedness, along with a preference inventory and a dichotic listening test of language lateralization. The performance tests included the Annett pegboard task, that Tapley and Bryden dot-filling tasks, and two procedures, the long pegboard and long dots tasks, that were intended to assess the point in space at which a particular unimanual movement became sufficiently awkward for one to shift to the other hand. All four of these performance tests differentiated between left-handers and right-handers, although the differences between handedness groups were somewhat larger when handedness was defined in terms of the preference inventory rather than on the basis of self-report. When the difference between preferred and non-preferred hands was examined, the best predictor of hand preference was the long pegboard task. Such a finding is consistent with the view that the long pegboard provides a behavioral measure of hand preference, while the pegs and dots tasks are more closely linked to specific skills. In addition, the correlations between individual preference items and the dichotic right-ear advantage suggest that language lateralization is related to rather different handedness measures than those usually used to define handedness. This finding would suggest that handedness and language lateralization are determined by somewhat different mechanisms.

Simultaneous verbal and affective laterality effects.

By analyzing the error scores of normal participants asked to identify a specific word spoken in a specific tone of voice (for example, the word "tower" spoken in a happy tone of voice), we have been able to demonstrate concurrent verbal and affective cerebral laterality effects in a dichotic listening task. The targets comprised the 16 possible combinations of four two-syllable words spoken in four different tones of voice. There were 128 participants equally divided between left- and right-handers, with equal numbers of each sex within each handedness group. Each participant responded to 144 trials on the dichotic task, and filled in the 32-item Waterloo Handedness Questionnaire. Analysis of false positive responses on the dichotic task (responding "yes" when only the verbal or only the affective component of the target was present, or when both components were present but were at opposite ears) indicated that significantly more errors were made when the verbal aspect of the target appeared at the right ear (left hemisphere) and the emotional aspect was at the left ear (right hemisphere) than when the reverse was the case. A single task has generated both effects, so that differences in participants' strategies or the way in which attention is biased cannot account for the results. While the majority of participants showed a right-ear advantage for verbal material and a left-ear advantage for nonverbal material, these two effects were not correlated, suggesting that independent mechanisms probably underly the establishment of verbal and affective processing. We found no significant sex or handedness effects, though left-handers were much more variable than were right-handers. There were no significant correlations between degree of handedness as measured on the handedness questionnaire and extent of lateralization of verbal or affective processing on the dichotic task. We believe that this general technique may be able to provide information as to the nature and extent of interhemispheric integration of information, and is easily adaptable to other modalities, thus holds great promise for future research.

Laterality effects for tactile patterns.

A series of studies was carried out to investigate laterality effects in the discrimination of tactile patterns presented to the fingertips of right-handed subjects. When both hands were stimulated simultaneously, a small right-hand advantage (RHA) was observed for a sequential task and a small left-hand advantage (LHA) was found for a spatial task, but only in women and only when there was a delay or mask between presentation and response. This may indicate different attentional strategies for men and women, or better shielding against interhemispheric interference in women. When only one hand was stimulated at a time (unimanual tasks), no laterality effects were found with sequential tasks, but a substantial RHA for a spatial task was observed. Because the unimanual spatial task involves more possible alternatives than the unimanual sequential ones, this RHA may be a consequence of increased information load rather than a the spatial/sequential distinction. While laterality effects in tactile laterality studies are relatively inconsistent, there are clear differences between bimanual and unimanual presentation. Effects related to hemispheric specialization are much more evident with bilateral presentation.