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Frank Durgin, Elsie Aubry, JJ Balisanyuka-Smith, Çiçek Yavuz; Differential grouping affordances of random spatial and temporal information for accurate number estimation. Journal of Vision 2020;20(11):1659. doi: https://doi.org/10.1167/jov.20.11.1659.
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Magnitude estimates of spatial non-symbolic number are thought to follow a power function, like other psychophysical dimensions. However, existing data identifies a common transition point at about 20 dots: Below 20, mean estimates are typically accurate, with a slope of 1. Above 20, underestimation produces a power function with an exponent of 0.5 to 0.9. Here we ask whether the 20-dot transition point reflects a capacity limitation of a general number mechanism or the limit of a spatial grouping strategy. In a first study, we presented temporal sequences of clicks and/or visual flashes with random timing (minimum SOA 75 ms). The average event rates (6 - 8 Hz) precluded verbal counting. Forty-eight participants estimated the numbers of either auditory, visual, or audio-visual events (numbering from 2 to 58). For visual flashes, there was no evidence of special accuracy: Even in the subitizing range, mean estimates followed a power function with an exponent of 0.83 (R2 = .999). For auditory clicks, only the mean estimates for the numbers 2 and 3 were accurate, while a power function exponent of 0.84 fit the numbers beyond 3 (R2 = 1.000). For audiovisual events, mean performance was accurate only up to 4 events; beyond 4, data were fit with an exponent of 0.82 (R2 = .999). A follow-up study with 15 new participants, used hybrid spatiotemporal stimuli consisting of auditory clicks synched with accumulating visual items in a random spatial array that disappeared after the final click. Accurate mean performance in this hybrid condition reached about 11, beyond which estimates had an exponent of 0.70 (R2 = .998). When verbal counting is prevented and intervals are random, temporal numerosity followed a power function above 4. Random temporal processes less easily afford strategic grouping and subitizing than spatial displays, and this may limit accuracy.
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