Abstract
Perception of number and space are tightly intertwined. It has been proposed that this is due to 'cortical recycling', where numerosity processing takes over circuits originally processing space. Do such 'recycled' circuits retain their original functionality? Here, we investigate interactions between numerosity and motion direction, two functions that both localize to parietal cortex. We describe a new phenomenon in which visual motion direction adapts nonsymbolic numerosity perception, giving rise to a repulsive aftereffect: motion to the left adapts small numbers, leading to overestimation of numerosity, while motion to the right adapts large numbers, resulting in underestimation. The reference frame of this effect is spatiotopic. Together with the tuning properties of the effect this suggests that motion direction-numerosity cross-adaptation may occur in a homolog of area LIP. 'Cortical recycling' thus expands but does not obliterate the functions originally performed by the recycled circuit, allowing for shared computations across domains.
Highlights
Our perception of numerosity and space are tightly interrelated, as evidenced by the ‘mental number line’, where small numbers are mapped to the left and large numbers are mapped to the right (Dehaene et al, 1993)
The combination of properties we find in our study, namely the tuning function, receptive field size, and frame of reference of the motion direction-numerosity cross-adaptation effect matches the known properties of area LIP neurons at the first stage of number processing, which are broadly tuned to numerosity (Roitman et al, 2007), have relatively small receptive fields (Ben Hamed et al, 2001), and a spatiotopic frame of reference (Mullette-Gillman et al, 2005)
Motion direction-numerosity cross-adaptation is consistent with studies showing that transcranial magnetic stimulation (TMS) over parietal cortex can disturb number comparisons and motion detection (Salillas et al, 2009), as well as those showing that the number of items in a display can be overestimated when they move at high speed (Afraz et al, 2004)
Summary
Our perception of numerosity and space are tightly interrelated, as evidenced by the ‘mental number line’, where small numbers are mapped to the left and large numbers are mapped to the right (Dehaene et al, 1993). This spatial arrangement of numbers is evident in preverbal infants (de Hevia and Spelke, 2010), non-human primates (Drucker and Brannon, 2014), and even birds (Rugani et al, 2015), suggesting a deep evolutionary heritage of the mental number line, there is evidence that it can be influenced by cultural practice (Shaki and Fischer, 2008).
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