Fig. 7. Frequency response curves of DON (A,B) and tectal units (C,D). The response was measured either as the overall spike rate change during stimulation (A,C) or as phase coupling (B,D). Filled circles indicate significant phase coupling (P<0.01). DON units are very uniform in their response and show a linear relationship between the amount of phase coupling and stimulus frequency at lower frequencies (B) with best frequencies of either 10 or 20 Hz. Tectal cells are more variable in their response, but in many units phase coupling does not decrease as much as in DON units with lower frequencies (D).