Handwriting posture and manual motor asymmetry in sinistrals

DOES DYSLEXIA DEVELOP FROM LEFT-EYE DOMINANCE?

The purpose of this theoretical analysis and synthesis is to indicate how left-eye sighting dominance may lead to reading failure through dysfunctional right hemisphere letter encoding. Differing compensatory strategies are postulated to lead to outcomes that include the development of the phonologically impaired and phonologically proficient subtypes of dyslexia as well as specific spelling disability. Evidence is presented indicating that these disorders might be prevented by delaying the introduction of letter writing until the age of 8 years. Early childhood speech categorization in children genetically at-risk of developing dyslexia is also considered from this perspective. Convergent support for this premature writing hypothesis is provided by a comparison with the development of the left-hand inverted writing posture.

Concurrent Counting and Typing: Lateralized Interference Depends on a Difference Between the Hands in Motor Skill

In previous demonstrations of differences between left- and right-handers in dual-task performance, participants' hand preference has been confounded with asymmetry of manual skill. The present study was designed to disentangle those two factors as sources of lateralized interference in the concurrent-task paradigm. Forty-eight normal adults (24 females and 24 males) counted backward by ones or by twos while typing an easy or difficult sequence of letters with either hand. When participants were grouped according to self-reported hand preference, both groups showed bilaterally symmetric slowing, relative to single-task conditions. However, when the same participants were grouped according to manual asymmetry in the baseline condition, the cognitive task interfered significantly more with the faster hand than with the slower hand. Baseline typing rate, averaged across hands, did not influence dual-task interference. Both self-reported left-hand preference and left-hand superiority in baseline typing were associated with reduced interference on the cognitive task, and the reduced interference in those groups seemed to reflect relatively loose coupling between manual and cognitive tasks. The results support and extend Caroselli et al.'s (1997) findings regarding the effect of baseline manual asymmetry on the pattern of dual-task interference. Irrespective of the participant's hand preference, the presence or absence of baseline asymmetry may be sufficient to determine whether dual-task interference is lateralized.

Right-handers and left-handers show differences and important similarities in task integration when performing manual and vocal tasks concurrently

For a sample of 73 consistent left-handers, 46 inconsistent left-handers and 65 right-handers, reciprocal interactions between unimanual or bimanual tasks and concurrent reading tasks were shown to be a complex function of task characteristics and sample composition. The suggestion that subgrouping of left-handers [Hellige and Key, Cerebral Control of Speech and Limb Movements, North Holland, Amsterdam, 1990] influences task interactions in left-handers was borne out. Both manual and vocal performance were measured. The effects of the vocal task on the manual task were variable for groups and conditions, but an important common trend emerged: all handedness groups showed the same robust effect: vocal task performance was better when the right hand performed the manual task. Finally, speaking rate increased and tapping rate decreased during concurrent single hand tapping [cf. Hiscock, Brain Cognit. 1, 119-131, 1982]; and there was an opposite effect with concurrent bimanual tapping. The results favour a task integration model rather than an interference in cerebral functions space model.

Writing posture in right hemiplegic children

The handwriting posture of 29 children with congenital right hemiparesis was classified according to standard criteria. All 29 children, who ranged in age from 4 to 14 years, were classified unambiguously as noninverted writers. The prevalence of noninverted writing in this sample differs significantly from that reported for the general population of children. The data thus support the claim that neural factors contribute to handwriting posture.

Inverted handwriting posture in left handers is related to familial sinistrality incidence

McKeever (1979) reported that left handers who wrote with an inverted handwriting posture (IHP) had a higher incidence of left handedness in their families than did left handers who wrote with a non-inverted handwriting posture (NHP). Parlow and Kinsbourne (1981), however, failed to find a relationship between handwriting posture (HWP) and positive familial sinistrality (FS+). McKeever had classified his left handed subjects according to whether they had several, one, or no left handers among their first degree relatives and biologically-related second degree relatives. Parlow and Kinsbourne classified subjects as FS- or FS+ on the basis of first degree relatives and did not inquire about the handedness of second degree relatives. In the present study, handedness for writing of all first and second degree relatives was recorded for 216 NIP and 255 IHP left handers. Results showed a significant association of HWP and FS factors when second degree relatives were considered, but not when second degree relative handedness was ignored. The relationship was strongest within the maternal family line, where the proportion of left handers was more than twice as high for the IHP as for the NHP group. These findings suggest that HWP may well be of some importance for a complete understanding of the cerebral organization of left handers.

Asymmetrical transfer of training between hands: implications for interhemispheric communication in normal brain

Two experiments are presented which investigated claims of asymmetrical transfer of training between the hands/hemispheres. In Experiment 1, 96 right- and left-handed male undergraduates practiced an inverted-reversed printing task with either the right or the left hand. Transfer to the opposite hand was then compared to same-hand transfer, in a between-subject design. In Experiment 2, 176 right-handed boys and girls were tested at ages 7, 9, and 11 years. For right-handed subjects in both experiments, the left hand benefited more from opposite-hand training than did the right. The reverse was true for left-handers in Experiment 1, although one group (who wrote with the "inverted" position) showed little transfer in either direction. Two current models of interhemispheric interaction do not satisfactorily explain these findings. A third model, based on cross-activation, may provide a more effective alternative.

Writing postures in left-handers: inverters are hand-crossers

A behavioural approach shows that the inverted hand-writing posture typical of many left-handers is one element of a well-patterned bimanual strategy including (i) counterclockwise rotation of the page; (ii) horizontal orientation of the active forearm on the page; and (iii) positioning of the non-writing hand just below the beginning of the line of writing. The strategy as a whole is liable to represent an adaptation to the necessity of writing with an adduction movement, rather than the symptom of any neurological particularity.

Handwriting position and hemispheric asymmetry in right-handers

Right-handers who write with an inverted writing posture (N = 8) were compared to right-handers who write in the normal fashion (N = 16) on tests of handedness and hemispheric asymmetry. Inverted writers showed the same laterality effects as normal writers for dot location and the recognition of visually-presented nonsense syllables. In dichotic listening, inverted writers were much more likely to show a left-ear superiority than were normal writers. They also were more strongly right-handed on a speeded performance test, but not in hand preference. This pattern is quite different from that obtained with left-handers, and suggests that the inverting handwriting posture has a different basis in right-handers than in left-handers.