Neuropsychologia 48 (2010) 2648–2657 Contents lists available at ScienceDirect Neuropsychologia journal homepage: www.elsevier.com/locate/neuropsychologia Hemispheric asymmetries in categorical perception of orientation in infants and adults Anna Franklin a,∗ , Di Catherwood b , James Alvarez a , Emma Axelsson a a Department of Psychology, University of Surrey, Guildford, Surrey, GU2 5XH, England, United Kingdom b Faculty of Education Humanities and Science, University of Gloucestershire, United Kingdom article info Article history: Received 7 August 2009 Received in revised form 30 April 2010 Accepted 5 May 2010 Available online 16 May 2010 Keywords: Spatial Category Lateralization abstract Orientation CP is the faster or more accurate discrimination of two orientations from different categories (e.g., oblique1 and vertical1) compared to two orientations from the same category (e.g., oblique1 and oblique2), even when the degree of difference is equated across conditions. Here, we assess whether there are hemispheric asymmetries in this effect for adults and 5-month-old infants. Experiment 1 identified the location of the vertical–oblique category boundary. Experiment 2, using a visual search task with oriented lines found that adult search was more accurate when the target and distractors were from different orientation categories, compared to targets and distractors of an equivalent physical difference taken from the same category. This effect was stronger for targets lateralized to the left visual field (LVF) than the right visual field (RVF), indicating a right hemisphere (RH) bias in adult orientation CP. Experiment 3, replicated the RH bias using different stimuli and also investigated the impact of visual and verbal interference on the category effect. Experiment 4, using the same visual search task, found that infant search was also faster when the target and distractors were from different orientation categories than the same, yet this category effect was stronger for RVF than LVF lateralized targets, indicating a LH bias in orientation CP at 5 months. These findings are contrasted to equivalent studies on the lateralization of color CP (e.g., Gilbert, Regier, Kay, & Ivry, 2005). The implications for theories on the contribution of the left and right hemispheres of the infant and adult brain to categorical computations are discussed. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction There is converging evidence that the two hemispheres of the brain differ in the extent to which they process information cate- gorically. For example, findings from studies using the visual half field technique, fMRI, rTMS and lesion studies suggest that categori- cal spatial relationships (e.g., on/off, left/right) are computed by the left hemisphere (LH), whilst co-ordinate spatial relationships (e.g., 4 cm, 2 cm) are computed by the right hemisphere (RH: e.g., Baciu et al., 1999; Hellige & Michimata, 1989; Kosslyn et al., 1989; Laeng, 1994; Slotnick & Moo, 2006; Trojano, Conson, Maffei, & Grossi, 2006). A categorical bias for the LH is also found for other types of categorical processing. For example, research suggests that the LH is more efficient at forming category prototypes (e.g., Marsolek, 1995), that the LH encodes the basic-level category whereas the RH encodes the exemplars in object classification and recognition (e.g., Laeng, Zarrinpar, & Kosslyn, 2003; Marsolek, 1999; Marsolek & Burgund, 2008), and that LH activation, as measured by fMRI, increases as abstract categories are learnt (Seger et al., 2000). This ∗ Corresponding author. Tel.: +44 01483 686933. E-mail address: a.franklin@surrey.ac.uk (A. Franklin). body of research has led to a dominant view that the LH processes information in terms of categories, whereas the RH processes infor- mation metrically. Further support for the pervasiveness of the categorical nature of the LH has been provided from evidence that categorical per- ception (CP) of color in adults is stronger in the left than the RH (e.g., Drivonikou et al., 2007; Gilbert, Regier, Kay, & Ivry, 2005). CP is found when stimuli along a physical continuum are parsed into separate categories, and the categorical relationship between stim- uli affects the accuracy and/or speed of stimulus discriminations. For example, for color CP, two stimuli from different hue cate- gories (e.g., B1 and G1) are discriminated faster or more accurately than stimuli from the same hue category (e.g., G1 and G2), even when same- and different-category stimulus hue separation sizes are equated. From a series of visual half field studies, it appears that color CP in adults is stronger for RVF-LH hue discriminations than for those in the LVF-RH. For example, adults are faster at searching for a colored target amongst colored distractors when the target and distractors are from different categories than from the same category, but this category effect is stronger when targets are pre- sented to the RVF (Gilbert et al., 2005). The stronger category effect in the LH than the RH has since been replicated using different tasks and color category boundaries (e.g., Drivonikou et al., 2007; 0028-3932/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.neuropsychologia.2010.05.011