Fig. 2. Effects of ambient light intensity (A) and pattern size (B) on visual pattern discrimination in Gnathonemus petersii. (A) Fish A–C were trained at 9.6 lx (grey bar) to the patterns shown in Fig. 3. The percentage of correct choices in unrewarded tests with the training patterns was determined at the daytime light intensities indicated on the abscissa. Each chosen daytime light level was kept constant for several days until all tests were finished. Testing started each day not earlier than 2 h after light onset in the morning. After a change to a new daytime light level, testing was omitted for one day. Fish A (filled triangles), 100 tests at each of two light levels; fish B (open squares), 200 tests at 9.6 lx, 50 tests each at other light levels; fish C (filled circles), 100 tests at each light level. (B) Tests were performed at standard light intensity (9.6 lx) with fish B (filled and open squares) and fish C (filled circles) in which size-reduced versions of the respective training patterns were shown. Training patterns were as shown in Fig. 3, except for the second series (filled squares) with fish B in which training patterns were as shown in Fig. 6 (top row). Note that, in the first series with fish B (open squares), the two patterns were not of the same size; the abscissa indicates the size (visual angle subtended at the retina) of the larger of the two patterns. Filled circles, 100 tests at each size; open squares, 200 tests at training size, 50 tests at reduced size; filled squares, 100 tests at training size, 50 tests at reduced size.