Executive functions in synesthesia Romke Rouw ⇑ , Joram van Driel, Koen Knip, K. Richard Ridderinkhof Brain and Cognition, Dept. of Psychology, University of Amsterdam, The Netherlands article info Article history: Received 9 March 2012 Available online 8 January 2013 Keywords: Synesthesia Visual perception Executive function Cognitive control Stroop effect abstract In grapheme-color synesthesia, a number or letter can evoke two different and possibly conflicting (real and synesthetic) color sensations at the same time. In this study, we inves- tigate the relationship between synesthesia and executive control functions. First, no gen- eral skill differences were obtained between synesthetes and non-synesthetes in classic executive control paradigms. Furthermore, classic executive control effects did not interact with synesthetic behavioral effects. Third, we found support for our hypothesis that inhi- bition of a synesthetic color takes effort and time. Finally, individual differences analyses showed no relationship between the two skills; performance on a ‘normal’ Stroop task does not predict performance on a synesthetic Stroop task. Across four studies, the current results consistently show no clear relationship between executive control functions and synesthetic behavioral effects. This raises the question of which mechanisms are at play in synesthetic ‘management’ during the presence of two conflicting (real and synesthetic) sensations. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction In synesthesia, certain sensations will evoke additional, seemingly unrelated, sensations. One of the most common and well-studied types of synesthesia is grapheme-color synesthesia, where letters or numbers evoke colors (e.g. Baron-Cohen, Wyke, & Binnie, 1987; Ramachandran & Hubbard, 2001b; Simner et al., 2005; Smilek, Dixon, Cudahy, & Merikle, 2001). For example, for a certain synesthete the letter ‘R’ might evoke a bright-blue color. There are many different types of synes- thesia, for example, moving patterns that evoke sounds (Saenz & Koch, 2008) or words that elicit a taste (Ward & Simner, 2003). Different from ‘normal’ (memory) associations, synesthetic experiences have a truly perceptual nature (Dixon, Smilek, Cudahy, & Merikle, 2000; Palmeri, Blake, Marois, Flanery, & Whetsell, 2002; Ramachandran & Hubbard, 2001a; Smilek et al., 2001). Furthermore, synesthetic experiences are consistent (the same associations persist from early child- hood), idiosyncratic, highly specific, and are evoked rapidly and seemingly effortlessly. Synesthesia runs in families, and genetic association studies support the idea of a genetic predisposition (Asher et al., 2009; Barnett et al., 2008; Tomson et al., 2011). The estimated prevalence is about 4%, and the grapheme-color subtype has an estimated prevalence of about 1% of the population (Simner et al., 2006). Synesthesia is not related to drug use, or to psychological, psychiatric or neu- rological disease. Instead, synesthetes generally report that their synesthesia does not bother them, but is pleasant and useful. It is fascinating how synesthetes can have two different, and possibly conflicting, sensations (e.g. the ‘real’ and the syn- esthetic color of a letter) in response to one and the same stimulus. These two experiences sometimes even share the same location in external space (Dixon, Smilek, & Merikle, 2004; Ward, Li, Salih, & Sagiv, 2007). Yet, synesthetes report and display 1053-8100/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.concog.2012.11.008 ⇑ Corresponding author. Address: Brain and Cognition, Dept. of Psychology, University of Amsterdam, Weesperplein 4, 1018 XA Amsterdam, The Netherlands. Fax: +31 (0)20 639 1656. E-mail address: R.Rouw@uva.nl (R. Rouw). Consciousness and Cognition 22 (2013) 184–202 Contents lists available at SciVerse ScienceDirect Consciousness and Cognition journal homepage: www.elsevier.com/locate/concog