Fig. 1. Choice frequency for S+ of eight fishes trained to discriminate between two differently shaped objects, symbols of which (taken from Table 1) are shown below each bar along the abscissa. For each object pair, the object on the left depicts the S+ and that on the right the S–. In this and subsequent figures, choice frequency for the object on the left was determined in non-rewarded and non-punished test trials. The total number of choices recorded for each fish was always >100.