Note Contribution of the right intraparietal sulcus to numerosity and length processing: An fMRI-guided TMS study Vale ´rie Dormal a , Michael Andres a,b and Mauro Pesenti a, * a Centre de Neuroscience Syste `me et Cognition, Institut de Recherche en Sciences Psychologiques, Universite ´ catholique de Louvain, Louvain-la-Neuve, Belgium b Institute of Neuroscience, Laboratory of Neurophysiology, Universite ´ catholique de Louvain, Brussels, Belgium article info Article history: Received 10 February 2010 Reviewed 12 August 2010 Revised 19 October 2010 Accepted 20 May 2011 Action editor Jordan Grafman Published online 1 June 2011 Keywords: Numerosity processing Length processing Intraparietal sulcus Transcranial magnetic stimulation abstract The critical involvement of the left and right intraparietal sulci (IPS) in numerosity and length processing was tested with neuro-navigated repetitive transcranial magnetic stimulation (rTMS). Participants had to categorize linear arrays of dots as containing “few” or “many” dots, and filled rectangles as “short” or “long”. The stimulation sites were determined for each individual participant by identifying the IPS areas showing peak activity during a number comparison task in earlier functional magnetic resonance imaging (fMRI) studies. Compared to the stimulation of the vertex chosen as a control site, rTMS over the right IPS increased the error rate in both tasks. This result indicates that the integrity of the right IPS is a necessary condition not only for discriminating numerosities but also for performing accurate judgements of lengths. We propose that these two processes rely on a common representation and/or mechanism in the right IPS. ª 2011 Elsevier Srl. All rights reserved. 1. Introduction Numerosity and space are often linked in the descriptions people give of their mental representation of numbers (Galton, 1880; Seron et al., 1992) and in the representations left by primitive cultures (Ifrah, 2000). Moreover, one well-known method of teaching arithmetic operations relies on the manipulation of sticks of various lengths (the Cuisenaire rods; Cuisenaire and Gattegno, 1954). This relationship between numerosity and length has also generated a long tradition of research in cognitive neurosciences. Several behavioural studies have shown a functional interaction through the presence of facilitation and interference effects between numerosity and length. In numerosity comparison tasks, it has been shown that long arrays of elements are numerically overestimated by children (Houde ´ and Guichart, 2001; Pufall and Shaw, 1972) and even by adults (Dormal and Pesenti, 2007), and that non-symbolic numerical cues interfere with performance in length bisection (de Hevia and Spelke, 2009) and length comparison (Dormal and Pesenti, 2007) tasks. The neural substrate of numerosity and length estimation has long been investigated in separate experiments that showed right or bilateral parietal activations during non- symbolic numerosity (e.g., Dormal et al., 2010; Piazza et al., 2004) and length comparison judgements (e.g., Fink et al., 2000). Importantly, even for bilateral activations, a right- hemispheric dominance for non-symbolic numerosity pro- cessing is frequently observed in children (Holloway and * Corresponding author. Centre de Neuroscience Syste ` me et Cognition, Institut de Recherche en Sciences Psychologiques, Universite ´ Catholique de Louvain, Place Cardinal Mercier, 10, B-1348 Louvain-la-Neuve, Belgium. E-mail address: mauro.pesenti@uclouvain.be (M. Pesenti). Available online at www.sciencedirect.com Journal homepage: www.elsevier.com/locate/cortex cortex 48 (2012) 623 e629 0010-9452/$ e see front matter ª 2011 Elsevier Srl. All rights reserved. doi:10.1016/j.cortex.2011.05.019