PERCEIVED NUMEROSITY OF SPATIOTEMPORAL EVENTS

Numerosity discrimination was examined when items were varied in space -time position rather than in space only. Observers were instructed to indicate which of two adjacent streams of visual events contained mor e items. The precision of numerosity discrimination of dynamic events was not remarkably different from that of static patterns. Two basic n umerosity biases previously found for static dot patterns-inhibitory o verestimation and satellite underestimation-were demonstrated for item s distributed randomly over a spatiotemporal inter-val. It was also de monstrated that two streams, equated in the number and luminous energy of items, are not judged equal in their visible number if items in on e of these two streams have longer duration than items in the second s tream. These findings can be accounted for by the occupancy model of p erceived numerosity (Allik & Tuulmets, 1991a) if it is supposed that t he impact that each element has on its neighborhood is spread along bo th spatial and temporal coordinates. Perceived numerosity decreases wi th both spatial and temporal proximity between the visual items. Space and time have interchangeable effects on perceived numerosity: the am ount of numerosity bias caused by the spatial proximity of items can a lso be produced by the properly chosen temporal proximity of items.