Paul Satz

Pathological Left-Handedness: An Explanaory Model

Summary A model was discussed which attempts to account for the raised incidence of manifest left-handedness in brain-injured populations, particularly mentally retarded and epileptic (approximately 17 per cent). The model demonstrates, using hypothetical data, that the frequency of manifest left-handedness will increase because of the additive effects of natural right-handers, who, because of early left brain injury, become pathological left-handers. The model explains why the number of pathological left-handers will always be disproportionately higher than pathological right-handers, regardless of the probability of manual switch after unilateral brain injury. This probability value was estimated algebraically in order to account for the twofold increase in manifest left-handedness in epileptic groups. The model also generated a number of testable hypotheses, some of which were logically derived from the model. The hypotheses are assumed to have both theoretical and diagnostic utility and were empirically tested and supported in the present paper. Implications of the model were discussed as they related to the problem of left-handedness, and more generally, to the relationship between handedness and other clinical phenomena (e.g., dyslexia, cerebral speech dominance, etc.).

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

Abstract 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.

Left-handedness and early brain insult: an explanation.

Abstract A relationship between birth order and handedness was recently reported [1]. It was shown that the incidence of left-handedness was significantly higher (18 per cent) in first- or later-born than in second- or third-born Ss. It was suggested [1] that the former group runs a higher risk of birth complications due to neurological insult which accounts for the observed relationship. This hypothesized relationship, however, offers no explanation (other than circular) as to why the incidence of left-handedness should be altered, if at all, in Ss with early brain insult. A Model is presented [2] which could account for the present relationship [1] and other reports of a raised incidence of left-handedness in brain-injured populations [3].

Developmental parameters of the ear asymmetry: a multivariate approach.

The present paper has two objectives: (1) a critical review of dichotic listening studies in children, and (2) an investigation of developmental parameters of the ear asymmetry, based on factors revealed in the review. A dichotic listening task was administered to approximately 20 boys and 20 girls at each of five ages (5,6,7,9 and 11). The results were as follows: (1) significant ear asymmetry was not found in children younger than nine years of age; (2) the magnitude of the differences between ears, while not significant until age nine, continued to increase with age until eleven at which time the slope functions for each ear plateaued; and (3) ear asymmetry was independent of sex. An attempt is made to explain the present results, and those of previous research, within a developmental framework.

Cross-modal asymmetries in perception related to asymmetry in cerebral function

Abstract Cross-modal correlations between left-right asymmetry on the Dichotic Listening (D-L) test and a visual half-field ((VHF) test were obtained for 30 right-handed subjects with no family history of left-handedness, 30 right handed subjects with a positive family history of left-handedness and 30 left-handed subjects. Both groups of right-handed subjects demonstrated significant cross-modal correlations between the D-L (auditory) asymmetry and the VHF (visual) asymmetry. The left-handed subjects displayed no correlation between D-L and VHF asymmetries. This suggests a dissociation between auditory and visual laterality in left-handers. Differences between this finding and two previous studies which found no cross-modal asymmetry for right-or-left-handers are discussed. The visual half-field test utilized in this experiment controlled fixation by presenting digits sequentially at fixation, while simultaneously presenting other digits to either the right or left VHF. This procedure minimizes the effect of directional scanning on VHF asymmetry. The reliability of the D-L and VHF asymmetries was also computed. The D-L asymmetry proved to be highly reliable ( r = 0.86), while the VHF asymmetry was somewhat less reliable ( r = 0.46). The asymmetry on both tests was attributed to asymmetry of cerebral function.

Order of Report, Ear Asymmetry and Handedness in Dichotic Listening

When different numbers were presented simultaneously to each ear, Ss tended to report all the numbers from one channel before reporting any from the other (Broadbent effect). The results contradicted Kimuras findings on the low incidence of the ear order effect, but showed that when verbal input is increased, the frequency of the effect and over-all accuracy decrease. Kimuras ear asymmetry hypothesis, however, was shown to be true for varying amounts of verbal input. The fact that the ear asymmetry occurred for recall on the delayed “storage” half-spans suggested a rapproachement between both Broadbent and Kimura. The striking relationship between handedness and ear asymmetry was discussed in relation to speech-brainedness.

Superiority of right visual half-fields in right-handers for recall of digits presented at varying rates ☆

Abstract Right-handers exhibit a consistent superiority of the right visual half-field for recall of digits; this superiority may reflect the more direct connections between the right visual half field and the speech areas of the left hemisphere. Seven digits were presented in each trial, with four digits at the point of fixation and three in either the left or right visual half-field. The first six digits were presented in pairs, with one digit at fixation and one in the half-field, followed by one additional digit at fixation. The greatest asymmetry occured at the faster presentation rates and for right-handers without family history of sinistrality.

Some Predictive Antecedents of Specific Reading Disability A Preliminary Two-Year Follow-Up

This paper reports one phase of an ongoing longitudinal project designed to identify the precursors of developmental dyslexia. The present study is based on the second year follow-up of an original population of 497 white male kindergarten children administered a developmental and neuropsychological test battery in 1970, and an evaluation of the predictive accuracy of these tests to the criterion of reading level at the end of grade 1 in 1972. The results of the multivariate analyses revealed that over 90% of both the High Risk and Low Risk children were correctly classified. The results were shown to lend preliminary support for the theory and for the predictive utility of these preschool tests.

Pathological left-handedness: Evaluation of a model

Abstract This study investigated the relationship between manifest handedness and the localization of EEG abnormalities in a large group of mentally retarded persons. Persons who had abnormal EEG recordings, regardless of whether the dysfunction was bilateral or asymmetric, exhibited approximately twice the incidence of left-handedness than persons who had normal EEGs. Data for persons having asymmetric damage support a model of pathological left-handedness, with left-handers showing a substantially higher probability of having left brain dysfunction than right-handers.

Within-modal and cross-modal reliabilities of two laterality tests.

Abstract Sixteen right handers were given four tests of a dichotic listening recall task (concrete words) and a visual half-field recall task (letters). There was a significant right ear and right visual half-field superiority for the four test sessions, with improvement in recall scores over sessions. Pearson correlations for each ear on the four dichotic tests ranged from .74 to .90. Correlations on the visual half-field scores, however, were significant only after the first session. Analysis of stability in lateral preferences across modalities revealed stable and reliable performance, particularly in the last two testings, where 81.3% of the subjects had a right-sided asymmetry. Cross-modal correlations, however, were not significant. Implications regarding predictions of speech laterality are discussed in the light of these findings.