When more is less: associations between corpus callosum size and handedness lateralization.
ABSTRACT Although not consistently replicated, a substantial number of studies suggest that left-handers have larger callosal regions than right-handers. We challenge this notion and propose that callosal size is not linked to left-handedness or right-handedness per se but to the degree of handedness lateralization. To test this hypothesis, we investigated the thickness of the corpus callosum in a large data set (n=361). We analyzed the correlations between callosal thickness and the degree of handedness lateralization in 324 right-handers and 37 left-handers at 100 equidistant points across the corpus callosum. We revealed significant negative correlations within the anterior and posterior midbody suggesting that larger callosal dimensions in these regions are associated with a weaker handedness lateralization. Significant positive correlations were completely absent. In addition, we compared callosal thickness between moderately lateralized left-handers (n=37) and three equally sized groups (n=37) of right-handers (strongly, moderately, and weakly lateralized). The outcomes of these group analyses confirmed the negative association between callosal size and handedness lateralization, although callosal differences between right- and left-handers did not reach statistical significance. This suggests that callosal differences are rather small, if examined as a dichotomy between two handedness groups. Future studies will expand this line of research by increasing the number of left-handers to boost statistical power and by combining macro- and microstructural, as well as functional and behavioral measurements to identify the biological mechanisms linking callosal morphology and handedness lateralization.
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ABSTRACT: The distinction between implicit and explicit forms of memory retrieval is long-standing, and important to the extent it reveals how different neural architecture supports different aspects of memory function. Similarly, distinctions have been made between kinds of repetition priming, a form of implicit memory retrieval. This study focuses on the production-identification (ID) priming distinction, which delineates priming tasks involving verification of stimulus features as compared to priming tasks that require use of a cue to guide response retrieval. Studies investigating this dissociation in dementia or similar patient populations indicate that these forms of priming may differ in their neural bases. The current study looks at degree of handedness as a way of investigating inferred neural architecture supporting these two forms of priming. A growing body of research indicates that degree of handedness (consistent, or CH, versus inconsistent, or ICH) is associated with greater interhemispheric interaction and functional access to right hemisphere processing in ICH, with superior performance seen in ICH on memory tasks reliant on this processing. Arguments about the theoretical mechanisms underlying ID and production forms of perceptual priming tasks suggest that performance on these tasks will differ as a function of degree of handedness. We tested this question in a group of CH and ICH young adults, who were asked to study lists of words prior to performing a production priming task (word stem completion, WSC), a perceptual word ID task, and a word stem cued recall task. While both handedness groups exhibited reliable priming across tasks, WSC priming was greater in ICH than CH participants, with ID priming not differing between groups. This dissociation supports the argument that production and ID forms of priming have different underlying neural bases.Frontiers in Psychology 02/2015; 6. DOI:10.3389/fpsyg.2015.00151 · 2.80 Impact Factor
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ABSTRACT: A robust inconsistent handed advantage exists for episodic retrospective memory processes. The current study was undertaken to test whether this handedness difference extends to the domain of prospective memory (PM). Two studies, one based on a self-report measure (the Prospective and Retrospective Memory Questionnaire) and the second involving a performance-based test (Memory for Intentions Screening Test), were carried out. Handedness effects were absent for both measures of PM. The absence of a handedness effect strongly suggests that PM processes are primarily characterized by executing intentions that depend on semantic networks for retrieval and do not necessarily rely on recalling spatio-temporal context, as is the case with episodic retrospective memory.Brain and Cognition 11/2014; 92. DOI:10.1016/j.bandc.2014.10.010 · 2.82 Impact Factor
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ABSTRACT: Lateralisatie in het brein (cerebrale lateralisatie) verwijst naar de taakverdeling binnen de hersenen. De hersenen bestaan uit twee hemisferen, waartussen het corpus callosum de voornaamste verbinding vormt. Bij de meeste mensen is het zo dat de mate waarin ieder van de hemisferen betrokken is bij een taak afhankelijk is van de aard van de taak. Zo is de linkerhemisfeer vaak veel meer betrokken bij talige taken (zoals het bedenken van woorden die beginnen met een bepaalde letter) dan de rechterhemisfeer. Fijne motoriek is ook vaak gelateraliseerd naar de linkerhemisfeer, wat zich op gedragsniveau uit als rechtshandigheid. Aan de andere kant is de rechterhemisfeer vaak juist sterker betrokken bij het uitvoeren van visuospatiële taken (zoals het vinden van de weg) dan de linkerhemisfeer. Deze specifieke verdeling van functies wordt een typisch lateralisatiepatroon genoemd. Atypische lateralisatiepatronen, zoals lateralisatie van taal naar de rechterhemisfeer en linkshandigheid, worden gerelateerd aan allerlei stoornissen, zoals dyslexie. De interesse in lateralisatie als onderwerp van onderzoek bestaat mede daardoor al sinds lange tijd. Vroeger dacht men dat cerebrale lateralisatie specifiek was voor de mens. Hoewel lateralisatie waarschijnlijk wel het sterkst is bij mensen, is lateralisatie inmiddels aangetoond bij alle gewervelde soorten en zelfs bij enkele ongewervelde soorten. Dit suggereert dat lateralisatie een evolutionaire oorsprong heeft. Het is echter nog altijd niet precies bekend hoe het komt dat we een gelateraliseerd brein hebben. Abstract Background Brain lateralization refers to the division of labour between the two hemispheres and is remarkably well developed in humans. However, its developmental process and plasticity remain elusive. It has been postulated that prenatal testosterone (pT) has an effect on human lateralization development. In the present study we examined the relationship between pT and two measures of lateralization: handedness and language lateralization. Method PT was assessed from amniotic fluid of healthy pregnant women. Strength and direction of handedness of the children (n=65 [31 girls, 34 boys], mean age [years]: 6.43, range: 5.97-7.53) was assessed based on hand choice during performance of age appropriate tasks. Language lateralization was measured using a dichotic listening task. The use of focused attention conditions in this task, enabled us to differentiate between potential effects of pT on the left hemisphere and effects on interhemispheric connectivity. Results We demonstrate a clear relationship between pT levels and language lateralization. The results suggest that in girls higher prenatal testosterone exposure facilitates left hemisphere language processing, whereas in boys it reduces the information transfer via the corpus callosum. With regard to handedness we showed that higher pT exposure was related to a decrease in strength of handedness independent of sex. Conclusion Together, our findings suggest that pT has a differential effect on callosal or hemispheric areas. This could be due to differences in developmental timing between these areas or to a difference in the distribution of androgen receptors across different areas of the corpus callosum or corresponding hemispheric areas.12/2011; 15(6):172-177. DOI:10.1007/s12474-011-0031-8