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Ben Harvey, Alessio Fracasso, Natalia Petridou, Serge Dumoulin; Overlapping topographic representations of numerosity and object size in human parietal cortex. Journal of Vision 2015;15(12):1283. doi: 10.1167/15.12.1283.
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© 2017 Association for Research in Vision and Ophthalmology.
Sensory cortical areas contain topographic maps reflecting the structure of sensory organs such as the retina, cochlea or skin. We recently demonstrated tuning for numerosity (the set size of visually-presented objects) in a human parietal topographic map, where numerosity preferences change gradually across the cortical surface (Harvey et al, Science 2013). This demonstrated that topographic maps can emerge within the brain to optimize processing of quantities. This map has organizational properties common across sensory cortices: more of the cortex processes small numerosities than larger numerosities, and does so with finer tuning widths. We make decisions based on numerosity, but other quantities such as object size may affect these decisions. The size of the presented objects can also affect numerosity perception. Here we measure object size-selective responses using 7T fMRI and population receptive field (pRF) modeling (Dumoulin & Wandell, Neuroimage 2008). We use stimuli that minimize correlations between object size and visual field position. We report object size preferences that are also organized as topographic maps. However, small sizes are not over-represented like small numerosities: the representation of small and large sizes covers similar amounts of cortex, and with similar tuning widths. Furthermore, non-preferred sizes produce surround inhibition. This object size map largely overlaps with the numerosity map. Size preferences and numerosity preferences are well correlated. Although both numerosity and size maps partially overlap with the IPS visual field maps, neither numerosity nor size preferences are correlated with visual field position preferences (specifically pRF eccentricity). As such, size tuning here is separated from the position preferences seen in visuo-spatial receptive fields. Instead, this size tuning is linked to preferences for other quantities. Overlapping representations of object size and numerosity may explain why their perceptions interact. These overlapping maps suggest a cognitive representation of quantity that generalizes between numerosity and size.
Meeting abstract presented at VSS 2015
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