Perception & Psychophysics

, Volume 32, Issue 2, pp 163-170

First online:

A probabilistic model for the discrimination of visual number

  • Michiel P. van OeffelenAffiliated withUniversity of Nijmegen
  • , Peter G. VosAffiliated withUniversity of Nijmegen


This paper proposes a probabilistic model of how humans identify the number of dots within a briefly presented visual display. The model is an application of Thurstone’s law of comparative judgment, and it is assumed that the internal representation of numerosity consists of log-spaced random variables. The discrimination between any two different numerosities is consequently described as a function of max/rain, where max and rain are the larger and smaller numbers, respectively. The model was tested in two experiments in which the Weber fraction for numerosity, corresponding with the critical ratio of max and min, was found to have the value of 162. It was concluded that the classical span of subitizing numerosity is but a special case of the span of discrimination.