Numerosity estimation as a function of stimulus organization.

It was proposed that stimulus organization of a set of homogeneous items can best be measured by using the mean distance to nearest neighbours. Patterns of dots were generated that fell on four different positions along the cluster continuum, from highly regular, through random, to clustered. When thirty subjects were asked to judge numerosity, their estimates decreased significantly as cluster measures increased. Four possible interpretations of the effect of organization on numerosity perception are discussed.     

Low spatial frequencies dominate apparent motion

Experiments are reported which have been designed to establish what features of a pair of figures can be used as an input for apparent motion. The display consisted of a central figure, A, which appeared briefly and was followed immediately afterwards by two figures, B and C, which appeared on either side of the original location of A. Figure A can thus move towards either B or C. When A was a low-pass filtered square it moved towards C (a low-pass filtered square that was similar to A but 'rotated' by 45 degrees) rather than toward B (a high-pass filtered square identical to A in orientation and size). When A was an unfiltered square it moved towards C (a low-pass filtered square of identical orientation) rather than towards B (a high-pass filtered square of identical orientation). Lastly, when A was a 'solid' square it moved towards C (a solid circle) rather than towards B (an outline square). All three experiments suggest that the direction of perceived movement is determined exclusively by low spatial frequencies rather than by similarity of oriented edges, especially when speed of alternation is rapid.