Fig. 3. Average intensity—response curves for day (open circles, N=19) and night (filled circles, N=14). The photo-responsiveness of medulla bilateral neruons (MBNs) was greater during the night than the day. During the day the response was approximately 60% of that during the night. Vertical bars indicate ±1 S.E.M. Significant differences between day and night are indicated by asterisks (P<0.01, t-test).