Nine experiments of timed odd-even judgments examined how parity and number magnitude are accessed from Arabic and verbal numerals. With Arabic numerals, Ss used the rightmost digit to access a store of semantic number knowledge. Verbal numerals went through an additional stage of transcoding to base 10. Magnitude information was automatically accessed from Arabic numerals. Large numbers preferentially elicited a rightward response, and small numbers a leftward response. The Spatial-Numerical Association of Response Codes (SNARC) effect depended only on relative number magnitude and was weaker or absent with letters or verbal numerals. Direction did not vary with handedness or hemispheric dominance but was linked to the direction of writing, as it faded or even reversed in right-to-left writing Iranian Ss. The results supported a modular architecture for number processing, with distinct but interconnected Arabic, verbal, and magnitude representations.How are numbers mentally represented and manipulated? Despite recent advances in the cognitive psychology and neuropsychology of numerical abilities, theories of the basic architecture of number representations have remained highly controversial. Both modular and interactive views have been proposed to account for the same set of data. On the modular side, McCloskey, Caramazza, and Basili (1985; see also Mc-Closkey, 1992;McCloskey & Caramazza, 1987;McCloskey, Sokol, & Goodman, 1986) have postulated a central amodal and abstract representation of numbers, which would constitute a bottleneck entry to calculation routines and to stored number knowledge and that would be interfaced by notationspecific comprehension and production modules. On the opposite, interactive side, Campbell and Clark (1988; see also Clark & Campbell, 1991) have denied the existence of a central abstract representation and suggested that "visuospatial, verbal, and other modality-specific number codes are associatively connected as an encoding complex" (p. 204) and activate each other during retrieval and calculation. Somewhat intermediate models have been proposed by Dehaene (1992) and by Noel and Seron (in press). Dehaene's (1992) triple-code model postulates three cardinal representations of number-verbal, Arabic, and magnitude-each of which supports specific procedures such as number comparison or mental multiplication. Noel and Seron (in press), on the other hand, have hypothesized a unique preferred entry code, which would be used to access number knowledge and calculation routines but which may be either verbal or Arabic depending on the subject's idiosyncrasies.

Did evolution endow the human brain with a predisposition to represent and acquire knowledge about numbers? Although the parietal lobe has been suggested as a potential substrate for a domain-specific representation of quantities, it is also engaged in verbal, spatial, and attentional functions that may contribute to calculation. To clarify the organisation of number-related processes in the parietal lobe, we examine the three-dimensional intersection of fMRI activations during various numerical tasks, and also review the corresponding neuropsychological evidence. On this basis, we propose a tentative tripartite organisation. The horizontal segment of the intraparietal sulcus (HIPS) appears as a plausible candidate for domain specificity: It is systematically activated whenever numbers are manipulated, independently of number notation, and with increasing activation as the task puts greater emphasis on quantity processing. Depending on task demands, we speculate that this core quantity system, analogous to an internal "number line," can be supplemented by two other circuits. A left angular gyrus area, in connection with other left-hemispheric perisylvian areas, supports the manipulation of numbers in verbal form. Finally, a bilateral posterior superior parietal system supports attentional orientation on the mental number line, just like on any other spatial dimension.

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