In¯uence of cognitive strategies on the pattern of cortical activation during mental subtraction. A functional imaging study in human subjects Pierre Burbaud a, * , Olivier Camus b , Dominique Guehl a , Bernard Bioulac a , Jean-Marie Caille  b , Miche Á le Allard b a Laboratoire de Neurophysiologie CNRS UMR 5543, Universite  Victor Segalen, 146 rue Le Âo Saignat, 33076 Bordeaux, France b Service de Neuroradiologie, CHU de Bordeaux, Place Ame  lie Raba Le  on, 33076 Bordeaux, France Received 12 April 1999; received in revised form 2 May 2000; accepted 3 May 2000 Abstract Functional magnetic resonance imaging (fMRI) at 1.5 T was used to investigate the in¯uence of cognitive strategies on cortical activation during mental calculation. Twenty-nine right-handed subjects performed a serial subtraction of prime numbers. Even though a common corpus of brain areas was activated during this mental calculation, differences appeared between subjects in function of their spontaneous cognitive strategy. In subjects using a so called verbal strategy (n  15), the main activation was located in the whole left dorsolateral frontal cortex with a little activation of the inferior parietal cortex. In subjects using a so called visual strategy (n  14), a bilateral activation in the prefrontal cortex and a high activation in the left inferior parietal cortex were observed. These results demonstrate that numbers are processed through a distributed network of cortical areas, the lateralization of which is clearly in¯uenced by subject strategy. q 2000 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Mental subtraction; Functional magnetic resonance imaging; Cognitive strategies; Verbal and visual components Many cognitive theories on number processing have been proposed during the last decade [4,6,14,15], but the func- tional anatomy of mental calculation is still poorly docu- mented. In functional imaging studies performed during mental calculation tasks, a pattern of bilateral activation in the prefrontal, premotor and parietal cortices has been observed [2,3,8,19,20]. However, the involvement of speci- ®c modules may be in¯uenced by the nature of the arith- metic procedure [7], task complexity [14,18] or strength of lateral dominance [2]. Above all, cortical activation may be in¯uenced by various individual cognitive strategies, such as initial subtraction to the nearest ten to simplify calcula- tion, mental visualization of the arithmetic procedure, or verbal rehearsal of data during the calculation process [20]. Thus, inter-subject variability in cortical activation might result in part from this variety of individual strategies. To date, the role of individual strategies on the pattern of brain activation during cognitive tasks has rarely been examined in functional imaging studies since most of them rely on averaged data from groups of subjects. Func- tional magnetic resonance imaging (fMRI) offers a high spatial resolution and allows investigation of inter-indivi- dual variability during a given cognitive task. The aim of the present experiment was to assess with fMRI the in¯uence of individual cognitive strategies of number processing on the pattern of cerebral activation during mental calculation. Thirty-three right-handed subjects (25 male/eight female, age 25.2 ^ 3.0 years) participated in this study. Lateral dominance was evaluated with an adapted form of the Harris test [2,3,11]. The task used in the present study was a serial subtraction of number. A given prime number, 13 or 17, was subtracted from an initial prime number greater than 500, then from the result, and so on serially, during each activation period (90 s). It was not possible to check for task performance of subjects once inside the scanner, as speak- ing generates movement artefacts. Consequently, before actual scanning, they performed two similar series of calcu- lations in the scanner and provided answers in a loud voice. This permitted an estimation of the number of operations that subjects performed during the 90-s activation periods (Table 1). The task was then carried out purely mentally and without vocalization during functional scanning. After Neuroscience Letters 287 (2000) 76±80 0304-3940/00/$ - see front matter q 2000 Elsevier Science Ireland Ltd. All rights reserved. PII: S0304-3940(00)01099-5 www.elsevier.com/locate/neulet * Corresponding author. Tel.: 133-557-57-37-35; fax: 133-557- 57-37-50. E-mail address: bdneuro@umr.5543-u.bordeaux2.fr (P. Burbaud). RETRACTED