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Topographic processing of numerosity in the human brain

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Numerosity, the set size of a group of visually-presented items, is processed by association cortex, but certain aspects mirror properties of primary senses (Dehaene, 1997; Burr and Ross, 2008). Sensory cortices contain topographic maps reflecting the structure of sensory organs such as the retina, cochlea or skin. Is the cortical representation and processing of numerosity organized topographically, even though no sensory organ has a numerical structure? Using high-field fMRI (7T) and custom-built model-based analysis that captures numerosity tuning (Dumoulin and Wandell, 2008), we describe neural populations tuned to small numerosities in human posterior parietal cortex. These neural populations are organized topographically, forming a numerosity map where preferred numerosity increases from medial to lateral cortex. This numerosity map is robust to changes in low-level stimulus features, although numerosity-tuning properties do vary with stimulus features. Furthermore, the cortical surface area devoted to specific numerosities (cortical magnification factor) decreases with increasing numerosity, and the tuning width increases with preferred numerosity. These organizational properties mirror key features of sensory and motor topographic maps. This extends topographic principles to representation of higher-order cognitive processing in association cortex, supports the analogy between numerosity and primary senses, and demonstrates that topographic structures can emerge within the brain.

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