The effect of stimulus organization on numerosity discrimination

The effect of stimulus organization on numerosity discrimination was studied, where 24, 40, 4, and 13 undergraduate students served as the subjects in four experiments. A test pattern consisting of 1 to 15 dots was presented for 20 to 200 ms. The subject's task was to report the perceived number of dots as soon as possible. The results of Experiment l indicated that the numerosity discrimination was better for the stimulus patterns consisting of four or five spatially separated subgroups of dots, than for the patterns consisting of dots distributed in the whole area with an equal probability. However, Experiments 2 and 3 indicated that this improved numerosity discrimination only occurs when dot-patterns in subgroups are simple and regular. Experiment 4 showed that the improved numerosity discrimination is correlated with shorter reaction times, and suggested that the effect of stimulus organization can be attributed to more rapid processing, by means of a strategy in which stimulus-dots in each group is subitized and then summed up together. Perceptual grouping is not necessarily accompanied by improved numerosity discrimination.