Effects of training to low intensity red (R2). (a) Control test (R1; p = 0.56, R2; p = 1.96 × 10−9, binomial test). (b) Test 1 (colour discrimination) with orange (O). R2; p = 2.33 × 10−10, O4; p = 0.60. (c) Test 2 (intensity discrimination) with O. O1; p = 0.32, O3; p = 8.25 × 10−4. (d) Test 3 (simultaneous brightness contrast) with O. Gr1; p = 0.045, Gr3; p = 0.68. (e) Test 1 with purple (P). R2; p = 2.01 × 10−4, P3; p = 0.68. (f) Test 2 with P. P1; p = 0.68, P3; p = 0.27. (g) Test 3 with P. Gr1; p = 0.032, Gr3; p = 0.72. For colour papers, see figure 1. Numbers above each column indicates the number of individuals that visited the disc more than four times of their five visits. Error bars show s.e.