Lateralization of Attention and Procedural Memory engagement in number processing in the human brain
Brain representations of numbers appear to be spatially organized according to the so-called mental number line. The SNARC (Spatial-Numerical Association of Response Codes) effect refers to the speeded up responses to digits when the reaction side is congruent with the digit position on the mental number line (MNL), e.g. left-hand response to a small magnitude and slowed down responses (inhibited) in the case of incongruity (conflict).
Figure 1. Participants asked to indicate if a digit is odd or even (Fig. 1A) respond faster to large numbers with their right hand, and faster to small numbers with their left hand (compatible trials). Reaction time is much longer, when they have to respond to small numbers with their right hand, and with their left hand to large numbers (incompatible trials). Figure 1B shows the difference in reaction time between compatible and incompatible trials (Gut et al. (2011)).
Despite extensive research in this field, the relationship between numerical cognition and attentional processing is still debated. The neural architecture for numbers and visuo-spatial attention may be partially shared. Moreover, the typical tasks used in order to study the SNARC effect evoke both orienting and executive attention processes. These two attention networks are housed in different brain regions and are linked to the different pattern of brain asymmetry. Especially lateralisation of executive attention has been extensively investigated, but there are no current reports focused on brain asymmetry of the SNARC effect. Despite well-known right hemisphere dominance in spatial-numerical processing, it is worth noting that this asymmetry could be dependent on the number magnitude of the stimulus. It would be in accordance with the right hemisphere role in the comparison or estimation, rather than, e.g. the exact calculation, which is the role of the left hemisphere.
We investigated the neurobiological mechanisms underlying this phenomenon using an orienting attention paradigm during magnetic resonance imaging scanning in event related functional MRI paradigm.
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