Ben M. Harvey & Serge O. Dumoulin
Nature Human Behaviour 1, Article number: 0036 (2017)
Sensory and motor cortices each contain multiple topographic maps with the structure of sensory organs (such as the retina or cochlea) mapped onto the cortical surface. These sensory maps are hierarchically organized. For example, visual field maps contain neurons that represent increasingly large parts of visual space with increasingly complex responses. Some visual neurons respond to stimuli with a particular numerosity — the number of objects in a set. We recently discovered a parietal topographic numerosity map in which neural numerosity preferences progress gradually across the cortical surface, analogous to sensory maps. Following this analogy, we hypothesized that there may be multiple numerosity maps. Numerosity perception is implicated in many cognitive functions, including foraging, multiple object tracking, dividing attention, decision-making and mathematics. Here we use ultra-high-field (7 Tesla, 7T) functional magnetic resonance imaging (fMRI) and neural-model-based analyses to reveal numerosity-selective neural populations organized into six widely separated topographic maps in each hemisphere. Although we describe subtle differences between these maps, their properties are very similar, unlike in sensory map hierarchies. These maps are found in areas implicated in object recognition, motion perception, attention control, decision-making and mathematics. Multiple numerosity maps may allow interactions with these cognitive systems, suggesting a broad role for quantity processing in supporting many perceptual and cognitive functions.