The hunt for SNARC

Abstract The SNARC effect specifically relates small magnitudes to the left hand side and larger magnitudes to the right hand side (e.g. Dehaene et al., 1990; Dehaene et al, 1993). It is certain that cultural characteristics define the SNARC effect: In western cultures small and large numbers are coded in a left-right direction while in Arabic countries magnitude information is coded from right to left (Dehaene et al., 1993; Zebian, in press). In this sense, reading and writing direction have been considered to be the main determinants of the SNARC effect. Indeed, a number of recent studies support the idea that the mastering of a language (and thus reading and writing direction) biases scanning habit in a favourable direction (e.g. Chatterjee et al., 1999; Padakanaya et al., 2002). If this is indeed the case, it is not surprising that the SNARC effect has been found with stimuli other than numbers (Gevers et al., 2004). Related to this, future research could address the question if magnitude and ordinal information are processed by the same mechanism or by different mechanisms with similar properties. However, reading and writing direction alone are not sufficient to explain all SNARC related findings. For instance, it does not allow for an explanation of a vertical SNARC effect as observed by Schwarz and Keus (2004). Apparently, other variables are important in the onset of an association between numbers and space. Furthermore, the existence of a SNARC effect in the vertical dimension could indicate that the number line can be conceptually extended to a number map. Another puzzling question is to what reference frame the SNARC effect is based. Based on the finding of a SNARC effect with both unimanual (Fischer, 2003a) and oculomotor responses (Fischer et al., 2004; Schwarz & Keus, 2004) at this moment an egocentric allocation of the SNARC effect does not seem feasible. On the other hand, both behavioural (e.g. Bachtold et al., 1998) and neuropsychological studies (Veuilleumier et al, 2004) point out that the mental representation of numerical information in itself is of major importance to the SNARC effect. One recent study provided a first step to relate the SNARC effect to other conflict tasks (Mapelli et al., 2003). Directly comparing the SNARC effect with the Simon effect resulted in additive relations between both effects. The question what the exact cause of this additive relation is remains an interesting topic. Furthermore, it could also be worthwhile to contrast the SNARC effect with other conflict tasks such as the Stroop task and the Eriksen Flanker task. To conclude, 10 years of research on the SNARC effect have unravelled interesting issues regarding the spatial coding of numerical information, fingers crossed for the following 10. Key words: SNARC-effect, Spatial Numerical Association of Response Codes, reading and writing direction, mental representation of numerical information The birth of SNARC People have and always had an above average interest in numerical issues. Many reasons can be given for this preoccupation, but one important reason is that numbers provide us a way to describe our environment, both in space (my office measures about 5 by 3 meters, has number 2.06 and you can find it on the second floor) and time (the year is 2003, there is only 30 minutes left to lunch-time, and the deadline for this manuscript is far too close). Notwithstanding the fact that numbers are a very handy tool for describing our surroundings this does not necessarily imply that numbers are spatial by nature. In 1880, Gallon (1880a,b) was the first to notice the apparent relation between the representation of numbers and space. When asked to draw their number representation, participants did so with a large variety of spatially defined numerical representations. After the research done by Galton the matter was not raised anymore for long time, and it is only since ten years that evidence is gathering which directly proves the spatial-numerical relation. …