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David Burr, Guido Cicchini, Giovanni Anobile; Spontaneous perception of numerosity in humans. Journal of Vision 2017;17(10):744. doi: 10.1167/17.10.744.
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© ARVO (1962-2015); The Authors (2016-present)
Introduction: Humans and many other species can make rapid, nonverbal estimates of numerosity. However, it is still unclear whether numerosity is detected spontaneously by dedicated mechanisms, or calculated indirectly via texture-density mechanisms. Methods: We used an "odd-one-out" technique to measure the equivalent of "MacAdam Ellipses" in a two-dimensional logarithmic "numerosity space", defined by density and area (with numerosity given by the positive diagonal in the space). Results: For stimuli of moderate densities, discriminations were well described by an elongated ellipse, with the short axis (maximum sensitivity) aligned to the numerosity diagonal. Sensitivity along this axis was 16 times that of the orthogonal direction, showing that numerosity is the most sensitive dimension, and can not be predicted from the independent thresholds for area and density. We also measured discrimination ellipses with a subjective technique, where subjects were asked to make explicit judgments about numerosity, density or area. Psychometric functions for number judgments were aligned to the numerosity direction (45°), but those for area and density judgments were not aligned to their axes (0 and 90°), but strongly biased towards number. This suggests that area and density judgments may be at least partially mediated by numerosity mechanisms, rather than the other way round. At high densities, where items were too crowded to be perceptually segregated, the discrimination ellipses became more circular, and are well predicted by independent encoding of area and density. Discussion: The results clearly point to the existence of mechanisms selectively encoding numerosity. The clear differences in processing of sparse and dense arrays shows that these specialized numerosity mechanisms operate only under conditions where the items can be perceptually segregated.
Meeting abstract presented at VSS 2017
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