Fig. 6. Responses of DON units (A,B) and tectal units (C,D) to sinusoidal stimuli at different amplitudes. A and C show the overall rate change, i.e. the mean spike rate during stimulation minus the spontaneous rate before stimulus onset. B and D show the phase coupling as measured by D (filled circles indicate significant phase coupling, P<0.01). In the DON, the greatest sensitivity is observed from 0.05 to 3 µV cm^–1 (B). However, phase coupling is significant only above 0.3 µV cm^–1. In this range, there is no change in overall spike rate (A). Tectal units show phase coupling down to 0.05 µV cm^–1 (D).