Differentiating physical and representational neglect to assess the role of spatial attention in mental arithmetic

Although some aspects of quantity representation seem already present in less evolved species, mathematics is too recent in the history of human evolution to benefit of its own representation in dedicated brain structures. It has therefore been suggested that mathematics makes use of evolutionary older circuits subserving visuospatial functions. Several behavioural and neuroimaging studies support this assumption of a link between number and space and have led to the conceptualization of number representation as spatially oriented along a crescent mental number line with small numbers represented on the left side and large numbers on the right side. Accordingly, the solving of subtraction and addition problems would require shifting attention respectively leftward and rightward along the number line. Unilateral spatial neglect as a deficit in attention and awareness of one side of space would provide a paradigmatic case to test the causal role of spatial attention in number processing and mental arithmetic. Previous neuropsychological studies showed numerical biases in left neglect patients that followed the direction of their disorder: they had difficulties to deal with small compared to large numbers and with subtraction compared to addition problems. However, a double dissociation is still lacking in order to establish the functional involvement of spatial attention. The present Master Thesis addressed this issue by investigating number processing and arithmetic performance in a series of patients with left or right physical and/or representational neglect. The latter was specifically investigated since mental arithmetic by definition should be impaired only when attentional deficits extend to a representational level. The results did not allow however to challenge this hypothesis due to a lack of dissociation between both types of neglect. However, results showed a double dissociation between small numbers/subtraction problems and large numbers/ addition problems in left vs. right neglect patients, indicating that attention orientation to one side of space was required to process numbers and solve arithmetic problems. An additional dissociation between the two tasks of number processing suggested that attention orientation may however be involved according to the task used. Our results support the hypothesis of arithmetic as navigation processes along a mental number line and further illustrate how high-level cognition may rely on older evolutionary functions to develop.