The development of spatial relation representations: evidence from studies of cerebral lateralization

Twenty-four-5-year-olds and 24 7-year-olds completed two divided-visual-field tasks; one task required subjects to categorize a dot as above or below a line, whereas the other required subjects to determine whether the dot was within 3 mm of the line. There was a relative left-hemisphere advantage for the above/below task and a relative right-hemisphere advantage for the distance task. The results indicate that distinct processing subsystems compute different kinds of visuo-spatial relations as early as 5 years of age.     

The corpus callosum and cerebral speech lateralization

In order to specify the callosal involvement in the establishment of cerebral lateralization, a dichotic listening task was administered to six subjects with congenital absence of the corpus callosum, two callosotomized patients, and two hemispherectomized patients. The acallosal subjects were also compared to six subjects matched for age, sex, and hand dominance as well as to six subjects also matched for IQ. Our findings indicate that language functions, as assessed by dichotic listening performance, are more strongly lateralized in callosal agenesis subjects than in IQ-matched normal controls. Our results also reveal that hemispherectomized patients typically show a strong ear-advantage favoring the ear contralateral to their remaining hemisphere. Callosotomized patients, on the other hand, show a more variable pattern of results that seems to be related to the postsurgical time interval. Finally, our findings suggest that there might be a relationship between IQ and lateralization in subjects with borderline or mild deficiency.     

Functional cerebral lateralization in subtypes of disabled readers

This study investigated the functional cortical organization of reading-disabled boys and age-matched normal readers. Subjects were initially classified according to E. Boder's (1971a. In B. Bateman (Ed.), Learning disorders. Seattle: Special Child Publications. Vol. 4.) distinction between dysphonetic children (who make nonphonetic, bizarre spelling errors), dyseidetic children (who make phonetically acceptable errors, but do not respond to some words as “wholes”), and children who display normal error patterns (who make phonetically acceptable errors and respond to words as “wholes”). It was hypothesized that different types of reading disability would be associated with different patterns of lateralized brain function. This proposal was examined by testing subjects on three experimental neuropsychological measures—hemispheric time-sharing, conjugate lateral eye movements, and tactile directional perception. Conjugate lateral eye movements were disregarded as the control subjects failed to show predicted asymmetries on this measure. The two other measures were considered valid and demonstrated atypical lateralization in the reading-disabled groups. Lateralization of verbal and/or spatial functiosn differed among the reading-disabled groups and an attempt was made to relate these atypical patterns to the type of reading difficulties presented.     

Intrahemispheric visual-motor information processing and cerebral functional lateralization.

A series of stimuli, words and faces, were presented tachistoscopically to 24 dextrals and 12 sinistrals. The stimuli were presented to one eye at a time and the subjects were instructed to respond to specific words or stimuli with a specific hand. The results indicate that (1) cerebral functional asymmetry is related to handedness; in the dextrals, the left hemisphere is more specialized in verbal recognition, while in the sinistrals, the right hemisphere is more specialized in recognizing non-verbal material. (2) An ipsilateral hand-and-eye combination is a valid method of measuring intrahemispheric information processing, provided that the tachistoscopically presented visual stimuli are capable of inciting specialized hemispheric function. The dominant relationship among the crossed and non-crossed visual pathways is discussed.     

Motor control by vision and the evolution of cerebral lateralization

Chicks (4 or 5 days old), which are able to use either eye freely, use the right eye (RE) preferentially in approach to a food dish when a lid, which has to be removed, is visible during approach. They use the left eye (LE) instead when no manipulation is required, but the same dish is similarly visible. The RE is also used preferentially in selecting food grains scattered over the floor; RE use in these two contexts is thus associated with visual control which brings the bill in planned contact with a visible target rather than with approach to a site where it is anticipated that feeding will occur. Zebrafish also use the RE preferentially when preparing to bite a target; during purely visual examination of the same target, this preference disappears. This evidence is used together with evolutionary evidence to support a new hypothesis for the origin of cerebral lateralization: paired anterior eyes evolved in filter-feeding ancestors of the vertebrates as part of the acquisition of prey catching. A key use for early vision was to predict likely contact with prey so as to inhibit reflexes of rejection and avoidance normally elicitated by tactile input to the mouth and so to allow ingestion. Innervation of mouth structures by the left side of the CNS caused control of mouth reflexes to become predominantly a left CNS affair. As visual abilities developed this starting condition meant that control of manipulation (which is by the mouth for most vertebrates) remained predominantly with the left side of the CNS.     

Brain lateralization of complex movement: Neuropsychological evidence from unilateral stroke

Complex movement (CM) refers to the representation of a goal-oriented action and is classified as either transitive (use of tools) or intransitive (communication gestures). Both types of CM have three specific components: temporal, spatial, and content, which are subdivided into specific error types (SET). Since there is debate regarding the contribution of each brain hemisphere for the types of CM, our objective was to describe the brain lateralization of components and SET of transitive and intransitive CM. We studied 14 patients with a left hemisphere stroke (LH), 12 patients with a right hemisphere stroke (RH), and 16 control subjects. The Florida Apraxia Screening Test-Revised (FAST-R, Rothi et al., 1988) was used for the assessment of CM. Both clinical groups showed a worse performance than the control group on the total FAST-R and transitive movement scores (p < 0.001). Failures in Spatial and Temporal components were found in both clinical groups, but only LH patients showed significantly more Content errors (p < 0.01) than the control group. Also, only the LH group showed a higher number of errors for intransitive movements score (p = 0.017), due to lower scores in the content component, compared to the control group (p = 0.04). Transitive and intransitive CMs differ in their neurocognitive representation; transitive CM shows a bilateral distribution of its components when compared to intransitive CM, which shows a preferential left hemisphere representation. This could result from higher neurocognitive demands for movements that require use of tools, compared with more automatic communication gestures.     

Visual allesthesia in manual pointing: Some evidence for a sensorimotor cerebral organization

The authors of this paper first summarize some of the theoretical frames put forward to account for allesthesia. Then, pointing performances in a case of visual allesthesia is reported; unexpectedly, pointing behavior for identical targets was different according to hand. The most salient feature was that a same visual target elicited relatively short RTs with adequate pointing using the left hand versus much longer reaction times with allesthesic pointing using the right hand. Discussion is focused on the presented theoretical frames. In view of the results, authors are led to hypothesize that, in sensorimotor cerebral organization, some aspects of signal processing are dependent upon some premotor aspects of response elaboration.     

Seasonal variation in human functional cerebral lateralization and free testosterone concentrations

Men have previously been reported to exhibit seasonal fluctuations on specific types of cognitive performance. It has been speculated that this performance variability is a result of changes in cerebral asymmetry due to lowered testosterone concentrations in the spring relative to the fall. In the present study, functional cerebral lateralization was measured in a group of men and women in the spring and fall. Free testosterone concentrations were assessed for participants to determine what associations might exist between seasonal variability in lateralization and seasonal fluctuations in testosterone exposure. Men and women tested in the spring exhibited exaggerated patterns of asymmetry compared to participants tested in the fall. Lower testosterone concentrations were observed in the spring compared to the fall in women, but not men. No direct associations between testosterone and lateralization were detected for either sex at either season. These results illustrate that seasonal fluctuations in testosterone exposure do not directly influence seasonal changes in functional lateralization patterns.     

Socioeconomic status, a forgotten variable in lateralization development

Socioeconomic status (SES), a variable combining income, education, and occupation, is correlated with a variety of social health outcomes including school dropout rates, early parenthood, delinquency, and mental illness. Several studies conducted in the 1970s and 1980s largely failed to report a relationship between SES and hemispheric asymmetry as measured by lateral differences in dichotic listening, tactile dot enumeration, and visual emotion and word recognition. However, none of the studies used asymmetry measures correcting for both ceiling and floor effects in accuracy, raising the question of whether lower and higher SES groups were comparable. Here the published data are reanalyzed using a laterality coefficient that corrects for such effects. The results are consistent across studies in revealing reduced lateralization in lower SES groups. Developmentally, this finding is consistent with either maturation delay or reduced functional specialization, or both. Suggestions are made for further research that include the use of behavioral asymmetry measures to screen tasks for structural and functional brain imaging.     

Geschwind's theory of cerebral lateralization: developing a formal, causal model

Geschwind and Galaburda (1987) have proposed a complex and influential model of cerebral lateralization that is based on the argument that increased fetal testosterone levels modify neural development, immune development, and neural crest development. The theory can explain many aspects of cerebral lateralization and its relation to learning disorders, giftedness, and immune deficits. This article clarifies the structure of the theory by presenting it as a causal-path model. The internal coherence of the model is then evaluated by assessing the central concept of anomalous dominance, the role of timing in the articulation of the model, and the invocation of nonlinear processes. Finally, the article considers the problems implicit in testing a "grand" theoretical model and derives some principles for assessing the testability of various predictions, given the practical constraints of sample size and the problems of measurement error.     

A general theory concerning the prenatal origins of cerebral lateralization in humans

The origins of cerebral lateralization in humans are traced to the asymmetric prenatal development of the ear and labyrinth. Aural lateralization is hypothesized to result from an asymmetry in craniofacial development, whereas vestibular dominance is traced to the position of the fetus during the final trimester. A right-ear sensitivity advantage may contribute to a left-hemispheric advantage in speech perception and language functions, whereas left-otolithic dominance may independently promote right-sided motoric dominance and a right-hemispheric superiority in most visuospatial functions. The emergence of handedness is linked to the assumption of an upright posture in the early hominids, whereas the failure to develop clear vestibular asymmetry may underlie the poor motoric lateralization found in several neurodevelopmental disorders.     

Survival with an asymmetrical brain: advantages and disadvantages of cerebral lateralization

Recent evidence in natural and semi-natural settings has revealed a variety of left-right perceptual asymmetries among vertebrates. These include preferential use of the left or right visual hemifield during activities such as searching for food, agonistic responses, or escape from predators in animals as different as fish, amphibians, reptiles, birds, and mammals. There are obvious disadvantages in showing such directional asymmetries because relevant stimuli may be located to the animal's left or right at random; there is no a priori association between the meaning of a stimulus (e.g., its being a predator or a food item) and its being located to the animal's left or right. Moreover, other organisms (e.g., predators) could exploit the predictability of behavior that arises from population-level lateral biases. It might be argued that lateralization of function enhances cognitive capacity and efficiency of the brain, thus counteracting the ecological disadvantages of lateral biases in behavior. However, such an increase in brain efficiency could be obtained by each individual being lateralized without any need to align the direction of the asymmetry in the majority of the individuals of the population. Here we argue that the alignment of the direction of behavioral asymmetries at the population level arises as an "evolutionarily stable strategy" under "social" pressures occurring when individually asymmetrical organisms must coordinate their behavior with the behavior of other asymmetrical organisms of the same or different species.     

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.     

Visual cerebral lateralization over phases of the menstrual cycle: a preliminary investigation

This study investigated whether visual cerebral asymmetries would change in phase with hormonal variations during the menstrual cycle. Lexical decision and line orientation tasks were administered during follicular, luteal, and menstrual phases of each woman's cycle. These tasks were also administered to a reference group of male subjects. Signal detection analyses indicated an unvarying RVF advantage in word/nonword discriminability (d') throughout the menstrual cycle, but a phase-dependent shift in left hemisphere response criterion (log beta). Gender differences were present for discriminability of line orientation, and female performance on this task varied over the cycle. The results imply that the neural systems subserving some cognitive functions are sensitive to fluctuations in gonadal steroids and suggest a hormonal basis for gender differences in some visual-spatial functions.     

Some cautions regarding statistical power in split-plot designs

We show that if overall sample size and effect size are held constant, the power of theF test for a one-way analysis of variance decreases dramatically as the number of groups increases. This reduction in power is even greater when the groups added to the design do not produce treatment effects. If a second independent variable is added to the design, either a split-plot or a completely randomized design may be employed. For the split-plot design, we show that the power of theF test on the betweengroups factor decreases as the correlation across the levels of the within-groups factor increases. The attenuation in between-groups power becomes more pronounced as the number of levels of the withingroups factor increases. Sample size and total cost calculations are required to determine whether the split-plot or completely randomized design is more efficient in a particular application. The outcome hinges on the cost of obtaining (or recruiting) a single subject relative to the cost of obtaining a single observation: We call this thesubject-to-observation cost (SOC) ratio. Split-plot designs are less costly than completely randomized designs only when the SOC ratio is high, the correlation across the levels of the within-groups factor is low, and the number of such levels is small.     

Evidence for stimulus-response compatibility effects in a divided visual field study of cerebral lateralization

The question addressed here was whether lateral asymmetry of processing might be influenced by response position, a factor which is usually considered irrelevant in divided visual field studies of cerebral lateralization. For this purpose a lexical decision task, which had previously been investigated with lateral unimanual two-finger choice reactions (Heister et al. 1983), was carried out under different manual and vocal response conditions so as to uncover possible S-R compatibility effects. In the first study, thirty-two subjects responded unimanually, with their responding hand held in a medial position. In the second study, twenty-four subjects responded vocally, i.e., in both cases the spatial (right/left) cues of the response position were eliminated. The reaction time advantage for compatible S-R pairings obtained with lateral hand position disappeared in the experiment with medial hand position, and the right-field superiority for vocal reactions was much smaller than the right-field superiority for right-hand reactions in the earlier lateral experiment. This indicates that an S-R compatibility effect contributed to the results of the earlier experiment. Thus, S-R compatibility can affect even unimanual reactions in lateralization studies.