No inherent left and right side in human ‘mental number line’: evidence from right brain damage

Abstract

Spatial reasoning has a relevant role in mathematics and helps daily computational activities. It is widely assumed that in cultures with left-to-right reading, numbers are organized along the mental equivalent of a ruler, the mental number line, with small magnitudes located to the left of larger ones. Patients with right brain damage can disregard smaller numbers while mentally setting the midpoint of number intervals. This has been interpreted as a sign of spatial neglect for numbers on the left side of the mental number line and taken as a strong argument for the intrinsic left-to-right organization of the mental number line. Here, we put forward the understanding of this cognitive disability by discovering that patients with right brain damage disregard smaller numbers both when these are mapped on the left side of the mental number line and on the right side of an imagined clock face. This shows that the right hemisphere supports the representation of small numerical magnitudes independently from their mapping on the left or the right side of a spatial-mental layout. In addition, the study of the anatomical correlates through voxel-based lesion-symptom mapping and the mapping of lesion peaks on the diffusion tensor imaging-based reconstruction of white matter pathways showed that the rightward bias in the imagined clock-face was correlated with lesions of high-level middle temporal visual areas that code stimuli in object-centred spatial coordinates, i.e. stimuli that, like a clock face, have an inherent left and right side. In contrast, bias towards higher numbers on the mental number line was linked to white matter damage in the frontal component of the parietal-frontal number network. These anatomical findings show that the human brain does not represent the mental number line as an object with an inherent left and right side. We conclude that the bias towards higher numbers in the mental bisection of number intervals does not depend on left side spatial, imagery or object-centred neglect and that it rather depends on disruption of an abstract non-spatial representation of small numerical magnitudes.

Biased numerical cognition impairs economic decision‐making in Parkinson's disease

“…Furthermore, our findings are in line with previous studies that illustrate dissociation between numerical and spatial mechanisms6, 7, 8, 9, 10 as no relationship was observed between numerical magnitude biases and any lateralized spatial attention bias. Thus, our findings support the generalized notion of hemispheric allocation of numerical magnitude 8, 10…”

“…Previous neuropsychological studies have demonstrated that right hemisphere fronto‐parietal lesions that invoke a rightward spatial attentional bias can induce an isomorphic pathological bias toward larger magnitudes 5. Critically, subsequent research has illustrated that such pathological biases in numerical magnitude allocation are dissociated from any spatial attention bias 6, 8…”

“…Currently, however, the precise neural mechanisms that underpin numerical magnitude allocation remain unclear 5, 6, 7, 8, 9, 10. Previous neuropsychological studies have demonstrated that right hemisphere fronto‐parietal lesions that invoke a rightward spatial attentional bias can induce an isomorphic pathological bias toward larger magnitudes 5.…”

The temporary nature of number—space interactions.

“…By now several studies have reported that there is no correlation (e.g. Aiello et al, 2012;Ashkenazi & Henik, 2010). Of course null findings are hard to interpret.…”

Commonalities for Numerical and Continuous Quantity Skills at Temporo-parietal Junction

“…However, evidence for this common processing is mixed. For example, supporting data come from the similarity of performance in number, time, or space processing (e.g., Brannon, Suanda, & Libertus, 2007;Zorzi, Priftis, & Umiltà, 2002), but the opposite possibility-that number, time, and space may each be fully independent-is supported by TMS and lesion studies showing dissociations among these dimensions (e.g., Aiello et al, 2012;Dormal, Seron, & Pesenti, 2006;Doricchi, Guariglia, Gasparini, & Tomaiuolo, 2005). A third, intermediate, possibility is that number, time, and space are only partly independent (e.g., Walsh, 2003).…”