NUMEROSITY DIFFERENCES AND EFFECTS OF STIMULUS DENSITY ON PIGEONS DISCRIMINATION PERFORMANCE

Two experiments are described in which pigeons were trained in a simul taneous conditioning procedure to discriminate small arrays of dots th at differed in numerosity. The birds successfully learned to choose th e array of each pair that contained fewer dots when these choices were reinforced and choices of the array with more dots led to timeout. Fo r the majority of numerosity values tested, discrimination performance for a fixed S+ value was better when the numerical difference between S+ and S- values was larger rather than smaller. This effect was seen in the first experiment when the numerical difference value was shift ed between training trials and novel test trials. In the second experi ment, too, performance level depended on the size of the numerosity di fference when the birds were concurrently trained with two difference values that varied across trials within sessions. However, discriminat ion accuracy was influenced secondarily by variations in the density, or interdot spacing, of the stimulus arrays. In order to explain the l atter finding, it is suggested that a tendency to ''scan'' a low-densi ty array incompletely might alter the probability of accepting it as t he smaller numerosity (S+) stimulus. This would increase error rates w ith S- arrays in which the dots are more widely spaced.