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.
Highlights
Numerical allocation has been shown to be linked with spatial attention mechanisms (Dehaene 1992; Fischer et al 2003), whereby numerical magnitude is superimposed upon a left to right spatially oriented representation termed the mental number line (MNL) (Zorzi et al 2002; Dehaene et al 2003)
Written informed consent was provided as approved by the local ethics research committee. As both vestibular stimulation and switching during rivalryviewing have been shown to shift spatial attention, which in turn can modulate numerical cognition (Rubens 1985; Fischer et al 2003; Paffen and Van der Stigchel 2010; Ferrè et al 2013), we first determined whether either vestibular stimulation alone, or viewing binocular rivalry (BR), induced changes in number pair bisection
We provide the first demonstration of a systematic bidirectional modulation of numerical magnitude toward either lower or higher numbers
Summary
It is postulated that for cultural innovations such as numbers, the brain co-opts evolutionarily older and multifunctional cortical circuits (Hubbard et al 2005; Dehaene and Cohen 2007), invoking fronto-parietal networks, which are repeatedly implicated for the allocation of spatial attention (Corbetta and Shulman 2002), eye movement control (Duhamel et al 1992; Colby and Goldberg 1999), perceptual switching during binocular rivalry (BR; Lumer et al 1998), vestibular cortical processing (Dieterich et al 2003), and numerical cognition (Piazza et al 2004; Cohen Kadosh et al 2008; Knops et al 2009). Numerical allocation has been shown to be linked with spatial attention mechanisms (Dehaene 1992; Fischer et al 2003), whereby numerical magnitude is superimposed upon a left to right spatially oriented representation termed the mental number line (MNL) (Zorzi et al 2002; Dehaene et al 2003). This account is supported by the spatial numerical association of response code (SNARC) effect (Dehaene et al 1993) and the observation that shifts of spatial attention follow number perception. Experiment 1: Physiological Manipulation of Numerical Magnitude and Its Relationship to a Lateralized Spatial Attentional Bias
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