Fig. 4
Behavioural measurement of D. simulans and D. mauritiana contrast sensitivity, spatial resolution, and temporal resolution. Gratings of various contrasts (a–c), spatial frequencies (d–f), and temporal frequencies (g–i) were presented to 3 males and 3 females from each of the two species in a rigid tether flight simulator equipped with a wingbeat analyser. The gratings were filtered through a Gaussian window and remained still for .2 s before moving to the left or right, indicated by the dotted line. For each subject, responses to leftward moving gratings were averaged with responses to the same grating moving rightward so that positive values represent mean steering in the direction of the grating (red or blue) and negative represents counter steering (grey). Mean normalised responses taken between .5 and 1.25 s were baseline corrected, subtracting the mean response during the .1 s before the onset of motion. Two of these ranges are indicated by annotations in a and b. connected by dashed arrows to their mean in c. Sample sizes are indicated in the bottom left corner of the colourmaps. The images of gratings in the bottom of c, f, and i are meant to give a sense of the change in the stimulus along the x-axis. Green arrows indicate the change in speed of the grating, ft/fs, which remains constant in the contrast experiment, decreases in the spatial frequency experiment, and increases in the temporal frequency one. a–c As contrast increases, RED3 begins responding significantly at .14 (red arrow in c) and M3 at .27 (blue arrow in c). d–f As spatial frequency increases and therefore rotational velocity decreases, mean responses decrease gradually until the Nyquist limit determined theoretically by the IO angle, reducing the contrast for higher frequencies as a result of aliasing. This limit differed between the two species, with RED3 responding significantly to spatial frequencies as high as .08 CPD (red arrow in f) and M3 as high as .1 CPD (blue arrow in f). g–i As temporal frequency and therefore rotational velocity increases, mean responses increase until they reach the Nyquist limit determined by the temporal resolution of the optomotor response, reducing the contrast for higher frequencies. M3 demonstrated higher temporal acuity, responding significantly to frequencies as high as 50 Hz (blue arrow) while RED3 stopped at 20 Hz (blue arrow). Data provided in Fig. 4_share.zip on figshare [29]