Bidirectional Modulation of Numerical Magnitude

Abstract

Numerical cognition is critical for modern life; however, the precise neural mechanisms underpinning numerical magnitude allocation in humans remain obscure. Based upon previous reports demonstrating the close behavioral and neuro-anatomical relationship between number allocation and spatial attention, we hypothesized that these systems would be subject to similar control mechanisms, namely dynamic interhemispheric competition. We employed a physiological paradigm, combining visual and vestibular stimulation, to induce interhemispheric conflict and subsequent unihemispheric inhibition, as confirmed by transcranial direct current stimulation (tDCS). This allowed us to demonstrate the first systematic bidirectional modulation of numerical magnitude toward either higher or lower numbers, independently of either eye movements or spatial attention mediated biases. We incorporated both our findings and those from the most widely accepted theoretical framework for numerical cognition to present a novel unifying computational model that describes how numerical magnitude allocation is subject to dynamic interhemispheric competition. That is, numerical allocation is continually updated in a contextual manner based upon relative magnitude, with the right hemisphere responsible for smaller magnitudes and the left hemisphere for larger magnitudes.

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

“…In agreement with the latter patient studies, our findings in healthy individuals demonstrate numerical magnitude allocation is subject to dynamic interhemispheric competition between fronto‐parietal networks independently of any spatial attention influences 10. That is, the right hemisphere preferentially encodes smaller magnitudes, whereas the left hemisphere is responsible for encoding relatively larger magnitudes, and this is continually updated in a contextual manner 10.…”

“…That is, the right hemisphere preferentially encodes smaller magnitudes, whereas the left hemisphere is responsible for encoding relatively larger magnitudes, and this is continually updated in a contextual manner 10. Despite our finding, whether neurological dysfunction of the left hemisphere can systematically induce pathological biases in magnitude allocation toward smaller magnitudes remains unknown.…”

“…Participants were required to estimate the midpoint (within 6 sec to avoid calculation) of the two numerical magnitudes across the following 20 trials presented in the following temporal sequence (randomized order) (33‐87), (32‐89), (37‐91), (93‐39), (66‐41), (68‐44), (47‐90), (48‐92), (52‐91),(92‐56), (89‐57), (87‐59), (61‐99), (63‐97), (67‐95) (99‐67), (58‐124), (131‐59), (131‐55), and (58‐132) 5, 10, 18. Note, as evident from the trials above, the number presented on the left of the pair varied from being either the larger or the smaller value, to avoid any effects associated with either spatial or temporal biasing.…”

“…Bisection errors were calculated by subtracting the arithmetical midpoint from the participant‐reported midpoint which we converted into percentage bisection errors by dividing the errors with the number interval size. Positive mean % bisection errors denote an overestimation, whereas negative mean % bisection errors denote underestimation from the actual midpoint 5, 10, 18…”

“…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.…”