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.
"One explanation for the differences in episodic memory is related to the increased collaboration between the left and right hemisphere. In support of the findings of superior episodic memory performance by weaker DH participants, studies have shown that the size of the corpus callosum is larger in weaker versus stronger DH participants (Clarke & Zaidel, 1994; Cowell, Kertesz, & Denenberg, 1993; Luders et al., 2010). Here a region, the precuneus, that has been linked to episodic memory revealed activation that was modulated by DH (Aggleton & Pearce, 2001; Binder & Desai, 2011; Binder, Desai, Graves, & Conant, 2009; Epstein, Parker, & Feiler, 2007; Vincent et al., 2006). "
[Show abstract][Hide abstract] ABSTRACT: The impact of handedness on language processing has been studied extensively and the results indicate that there is a relationship between the two variables; however, the nature of the relationship is not at all clear. In the current study we explored degree of handedness (DH) opposed to direction in a group of right-handed individuals. fMRI was used to explore the impact of DH on the sentence comprehension network. The results revealed that during sentence comprehension activation in regions linked to semantic memory (e.g., anterior temporal cortex) were modulated by DH. Also, unexpectedly the precuneus/posterior cingulate gyrus which has been linked to episodic memory was also affected by DH. These results extend those reported previously by showing that the neural architecture that supports sentence comprehension is modulated by DH. More specifically, together the results presented here support the hypothesis proposed by Townsend, Carrithers, and Bever (2001) that DH interacts with the language system and impacts the strategy used during sentence comprehension.
Brain and Cognition 03/2014; 86C(1):98-103. DOI:10.1016/j.bandc.2014.02.002 · 2.48 Impact Factor
"For example, Potter and Graves (1988) argued that CR-handers had a poorer interhemispheric transfer performance during a line drawing task when compared with non-right-handers. Luders et al. (2010) showed a negative association between corpus callosum size and strength of handedness, regardless of direction of handedness. This largely supports the idea that strength of handedness may demonstrate something important about how the hemispheres interact. "
[Show abstract][Hide abstract] ABSTRACT: Hand preference is often viewed as a troublesome variable in psychological research, with left-handers routinely excluded from studies. Contrary to this, a body of evidence has shown hand preference to be a useful variable when examining human behaviour. A recent review argues that the most effective way of using handedness as a variable, is a comparison between individuals who use their dominant hand for virtually all manual activities (consistent handers) versus those who use their other hand for at least one activity (inconsistent handers). The authors contend that researchers should only focus on degree of handedness rather than direction of preference (left versus right). However, we argue that the field suffers from a number of methodological and empirical issues. These include a lack of consensus in choice of cut-off point to divide consistent and inconsistent categories and importantly a paucity of data from left-handers. Consequentially, researchers predominantly compare inconsistent versus consistent right-handers, largely linked to memory, cognition and language. Other research on response style and personality measures shows robust direction of handedness effects. The present study examines both strength and direction of handedness on self-reported Behavioural Inhibition System (BIS) and Activation System (BAS) scores, using evidence from a large (N=689) dataset including more than 200 left-handers. There were degree of handedness effects on BIS and BAS-Fun Seeking, but effects are largely driven by differences between consistent left-handers and other groups. Choice of cut-off point substantively influenced results, and suggests that unless a suitable sample of left-handers is included, researchers clarify that their degree of handedness effects are applicable only to right-handers. We concur that strength of hand preference is an important variable but caution that differences related to consistency may not be identical in right and left-handers.
Frontiers in Psychology 02/2014; 5:134. DOI:10.3389/fpsyg.2014.00134 · 2.80 Impact Factor
"Differences in consistency are apparent within the first two years of life (Nelson, Campbell, & Michel, 2013) and may stem from genetic variation (Arning et al., 2013). Multiple neuroanatomical studies have shown that some regions of the corpus callosum are larger in inconsistent-handers than consistenthanders (Cowell, Kertesz, & Denenberg, 1993; Habib et al., 1991; Luders et al., 2010; Witelson, 1985). Although some studies have not found these structural differences (Jäncke & Steinmetz, 2003; Welcome et al., 2009), behavioural studies have indicated that interhemispheric interaction is greater in inconsistent-handers than consistenthanders (Chase & Seidler, 2008; Lyle & Martin, 2010; Potter & Graves, 1988). "
[Show abstract][Hide abstract] ABSTRACT: Making repetitive saccadic eye movements has been found to increase subsequent episodic memory retrieval and also to increase subsequent top-down attentional control. We theorise that these effects are related such that saccade-induced changes in attentional processing facilitate memory retrieval. We tested this idea by examining the effect of saccade execution on retrieval conditions that differed in relative ease of consciously accessing episodic memories. Based on recent theories of episodic retrieval, we reasoned that there is a larger role for top-down attention when memories are more difficult to access. Consequently, we expected saccade execution to have a greater facilitative effect on retrieval when memories were more difficult to access. We obtained the expected result in a recall procedure in Experiment 1 and in a recognition procedure in Experiment 2. We also examined an individual difference factor-consistency of handedness-as a possible moderator of saccade execution effects on retrieval. We discuss how our top-down attentional control hypothesis can be extended to explain beneficial effects of saccade execution on other types of cognition, as well as negative effects on retrieval in some cases.
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