Reading upside down and mirror text in groups differing for right minus left hand skill

On the process of recognizing inverted words: does it rely only on orientation-invariant cues?

Following a demonstration by Parks (1983) of failure to notice the reflection of a letter of an inverted word, two experiments were conducted to test a hypothesis about the process of recognizing inverted words that is termed here invariant cues only (ICO)-a letter-by-letter identification process based only on orientation-invariant letter features. In Experiment 1, subjects were presented with whole strings-words and nonwords, either upright or inverted-in which either all the letters were normal or one of the letters was reflected, and they were asked to make lexical decisions. In Experiment 2, subjects made a reflection judgment about an upright or inverted letter within a string immediately after they had been presented with the other, nonreflected string letters, again either upright or inverted. The results do not support the ICO hypothesis: Lexical decisions were greatly affected by the reflection of a letter in upright and inverted stimuli alike. Reflection judgments were considerably facilitated by word context in the upright and the inverted modes alike. The results are accommodated better by the notion that recognition of disoriented words requires some correction used to restore orientation-sensitive features.

Handedness and lexical skills in undergraduates

The right shift (RS) theory of handedness and cerebral dominance (Annett, 1972, 1985) suggested that individual differences for patterns of cerebral dominance may be associated with different types of risk to cognitive functions. The higher prevalence of dextrality than sinistrality in humans depends on a single gene (RS+) which facilitates left hemisphere specialisation for speech but at the expense of some right hemisphere function (Annett, 1992b; Kilshaw and Annett, 1983). RS-- genotypes have random asymmetries for hand and brain while RS++ genotypes have strong right shift. Three samples of undergraduates were given tests of phonological processing and of real word homophone discrimination. As predicted, phonology was poor in comparison with homophone discrimination toward the left of the laterality continuum while the reverse was true to the right of the continuum. When groups with specific difficulty for each type of test in comparison with the other were distinguished, those with poor phonology were less dextral and those with poor homophone discrimination were more dextral for hand preference than the rest of the sample. Siblings differed for handedness also, consistent with the argument for a genetic influence.

Familial sinistrality: the utility of calculating exact genotype probabilities for individuals

Measures of familial sinistrality are frequently used in neuropsychological research. However they suffer from the problem that they are essentially a global measure of phenotypes in the family, whereas they are interpreted as an indicator of the genotype of the individual concerned. In this paper it is shown how to calculate a precise probabilistic estimate of a proband's genotype, given a particular genetic model of handedness, using all of the information available in a family tree. Example calculations are provided for a range of families with one or two sinistral relatives. Genotype probabilities are also calculated for a large population data set (the National Childhood Encephalopathy Study) and it is shown that familial sinistrality does not account for all of the variability present in the genotype probabilities, and that genotype probabilities provide an additional prediction of individual sinistrality after taking familial sinistrality into account, whereas all of the information in familial sinistrality is accounted for by the genotype probabilities. Finally it is shown how genotype probabilities can be used to assess whether there is heterozygote advantage for a characteristic, and using this method it is suggested that there is no support for heterozygotes having higher intellectual ability than homozygotes.

Parallels between asymmetries of Planum temporale and of hand skill

Relationships between the skill of each hand and differences between the hands are described for a sample of 14-15-year-olds representative of the general population. The description is modelled on the account of Galaburda et al. (Neuropsychologia 25, 853-868, 1987) of relationships between asymmetries of the Planum Temporale (PT) and measures of PT area on each side, in order to show parallels between findings for the two types of asymmetry. For both PT and hand skill, the extent of asymmetry appears to depend on the greater variability of the weaker side rather than of the larger or "better" side. The findings are also described as a replication of the study of hand skill in children by Annett and Manning (Br. J. Psychol. 80, 213-226, 1989). As no score transformations were needed in the present sample, the grounds for doubt raised by Bishop (Handedness and Developmental Disorder, Blackwell, Oxford, 1990) about the earlier sample are removed. Repetition of the analyses for right-handed and left-handed writers separately demonstrates very close similarities for findings in the two hand preference groups. Theories about the mechanisms responsible for asymmetries appear to require the presence of factors which reduce brain area and hand skill on one side. For most people the reduction is to the right PT and to the left hand.

Speech lateralisation and phonological skill

A phoneme segmentation task requiring Ss to produce deliberate spoonerisms was performed by 78 undergraduates, along with a dichotic monitoring task and tests of hand preference and hand skill. The hypothesis that those performing badly on the speech task would include a higher proportion of individuals with left ear advantage on the dichotic task was supported. The distribution of cases was consistent with other findings for relationships between cerebral speech lateralisation and side of ear advantage and also consistent with predictions of the right shift theory of handedness and cerebral speech (Annett, 1972, 1985).