Ricerc@Sapienza

Whether the semantic representation of numbers is endowed with an intrinsic spatial component, so that smaller numbers are inherently represented to the left of larger ones on a Mental Number Line (MNL), is a central matter of debate in numerical cognition. To gain an insight into this issue, we investigated the performance of right brain damaged patients with left spatial neglect (N+) in a bimanual Magnitude Comparison SNARC task and in a uni-manual Magnitude Comparison Go/No-Go task (i.e. "is the number smaller or larger than 5?").

Humans are prone to mentally organise the ascending series of integers according to reading habits so that in western cultures small numbers are positioned to the left of larger ones on a mental number line. Despite 140 years since seminal observations by Sir Francis Galton (Galton, 1880a, b), the functional mechanisms that give rise to directional Space eNumber Associations (SNAs) remain elusive.

Inspecting or transforming the position of Arabic numbers in mental space helps everyday mathematical calculations. Nonetheless the neural and functional bases of this ability are poorly understood. Here we show that imagining the position of Arabic numbers on a horizontal mental number line speeds up the detection of targets appearing at corresponding positions in visual space. No similar advantage is found when numbers are merely perceived or classified according to their magnitude.

The SNARC effect consists of faster reaction times to small numerical magnitudes when manual responses are delivered in the left-side of space and to large magnitudes when responses are delivered in the right-side. This spatial compatibility effect points at the interaction between the representations of space and that of numbers. Several studies have highlighted that an important determinant for the production of the SNARC is the use of contrasting left/right spatial codes in the selection of motor responses. In these studies, one spatial code for response selection, e.g.

In a series of five experiments with unimanual Go/No-Go tasks and intermixed Arabic numerical, i.e. numbers lower or higher than 5, and directional targets, i.e. arrows pointing to the left or to the right, we explored whether spatial codes used in isolation inherently evoke the left-to-right representation of number magnitudes, i.e. Space-to-Number congruency effect, and, vice-versa, whether number-magnitude codes used in isolation inherently evoke the conceptual activation of left/right spatial codes, i.e. Number-to-Space congruency effect.