Doricchi, F. 1, 2
1 Dipartimento di Psicologia 39, Università degli Studi di Roma “La Sapienza”, Roma Italy
2 Fondazione Santa Lucia - IRCCS, Roma Italy
At the turn of the 19th century, in two Nature issues Francis Galton (1880) first described the introspective reports of humans possessing the strong tendency to see numbers ”raising before the mind’s eye” in “definite and constant arrangements” upon spoken number presentation. Later studies demonstrated that in cultures with left-to-right reading, numbers are prevalently organised along a mental number line (MNL), with small magnitudes located to the left of larger ones. Recently, the possibility that such an introspective arrangement reflects an inherent spatial coding of number magnitudes in the human brain, was apparently endorsed by the bias toward higher numbers displayed by right brain damaged (RBD) patients during the bisection of number intervals, which has been taken as synonymous with spatial-attentional neglect for small numbers on the left side of the MNL. In contrast with this conclusion, we report the results of a series of investigations showing that the numerical bias displayed by RBD patients is functionally and anatomically dissociated from an equivalent attentional bias in visual space. Based on experiments designed to generate a mismatch in the “default” association of small numbers with the left side of space and of high numbers with the right side of space, we demonstrate that RBD have troubles in the mental manipulation of small number magnitudes independently from their mapping on the left or the right side of a mental layout. Parallel studies run in healthy adults and in childrens reveal new properties of the MNL, and support the conclusion that assuming a close phenomenological, functional and anatomical equivalence between orienting in visual space and orienting in mental number space is misleading.