Disentangling the relationship between hemispheric asymmetry and cognitive performance Marco Hirnstein a, * , Stuart Leask b , Jonas Rose c , Markus Hausmann a a Department of Psychology, University of Durham, United Kingdom b University of Nottingham, Department of Psychiatry, United Kingdom c Biopsychology, Institute of Cognitive Neuroscience, Ruhr-University of Bochum, Germany article info Article history: Accepted 12 April 2010 Available online 15 May 2010 Keywords: Lateralization Cognition Visual half-field task Sex differences abstract It is widely believed that advantages of hemispheric asymmetries originated in better cognitive processing, hence it is often implied that the relationship between hemispheric asymmetry and cognitive performance is linearly positive: the higher the degree of lateralization in a specific cognitive domain, the better the performance in a corresponding task. Yet, the empirical evidence for this notion is mixed and the statistical methods to analyze this relationship have been criticized. The present study therefore investigated the relationship between hemispheric asymmetries and cognitive performance in two behavioral tasks (a left-lateralized word-matching task and a right-lateralized face-decision task) in 230 participants (140 women, 90 men) by using two different approaches. Both methods correspondingly revealed that a relationship between hemispheric asymmetries and cognitive performance does exist. Contrary to a positive (linear) relationship however, the data could be best described by an inverted U-shaped curve. Although the optimal degree of lateralization seemed to be task-specific, a slight or mod- erate degree of hemispheric asymmetry achieved best cognitive performance in all tasks. Moreover, per- formances deteriorated towards extreme ends of lateralization (i.e., participants with either extreme left or right hemispheric biases). Taken together, the present study provides evidence against the notion that higher lateralization is related to enhanced cognitive performance. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction For more than 100 years now, hemispheric asymmetries are known to be a basic principle of human brain organization. Partic- ularly in the last decade, however, hemispheric asymmetries have also been reported in many other species, comprising vertebrates such as mammals, birds, reptiles, amphibians and fish (for review see Rogers & Andrew, 2002; Vallortigara & Rogers, 2005) and inver- tebrates such as insects (Letzkus et al., 2006) and octopuses (Byrne, Kuba, Meisel, Griebel, & Mather, 2006). Since lateralization is such a wide-spread phenomenon it likely contains a selection advantage over a symmetric brain. For example, it has been suggested that a lat- eralized brain prevents conflicts between the two hemispheres (Vallortigara, 2000), eliminates functional incompatibility between processing familiar events and producing novel behavior (Vallortig- ara, Rogers, & Bisazza, 1999) or leads to a ‘de-duplication’ of func- tions and increasing neural capacity (Levy, 1969, 1974, 1977). Moreover, lateralization is supposed to enhance parallel processing. While one hemisphere is occupied with a certain task, the other hemisphere can simultaneously perform an additional process (Rog- ers, 2006). Taken together, most of those theories suggest that hemi- spheric asymmetries emerged because its development led to enhanced cognitive processing. Taking interindividual differences in the degree of lateralization into account, it is widely believed that a positive relationship between the degree of lateralization and cog- nitive performance exists (Güntürkün et al., 2000). That is, the high- er the degree of lateralization in a specific cognitive domain, or in other words, the more a cognitive domain is lateralized to a partic- ular hemisphere relative to the other hemisphere, the better is the cognitive performance in a corresponding task. However, empirical evidence for this notion is rather patchy with some studies showing the exact opposite. For example, more lateralized participants were outperformed by less lateralized par- ticipants on a single task (Ladavas & Umilta, 1983) and also when performing two tasks simultaneously (Hirnstein, Hausmann, & Güntürkün, 2008). Furthermore, mathematically gifted partici- pants exhibit a more symmetrical activation of brain regions than those of average math ability (O’Boyle et al., 2005). On the other hand, a recent study (Chiarello, Welcome, Halderman, & Leonard, 2009) found positive correlations between visual field asymmetries and reading performance, but only in young adults 0278-2626/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.bandc.2010.04.002 * Corresponding author. Address: Department of Psychology, University of Durham, Science Laboratories, South Road, Durham DH1 3LE, United Kingdom. Fax: +44 (0) 191 3343241. E-mail address: m.a.hirnstein@durham.ac.uk (M. Hirnstein). Brain and Cognition 73 (2010) 119–127 Contents lists available at ScienceDirect Brain and Cognition journal homepage: www.elsevier.com/locate/b&c