Nd1.0 neutral density filter

Visual stimuli were presented on a 9 cm–by–9 cm rear projection screen in front of the fly covering a visual angle of ~80° in azimuth and ~55° in elevation. To cover a larger part of the horizontal visual field of ~168°, we rotated the fly with respect to the screen two times by 45° and recorded each fly at three positions relative to the screen (fig. S1A). In total, we thus stimulated an area of the visual field ranging from −34° to 134° in azimuth and −17° to 36° at the closest point of the screen to the fly in elevation (fig. S1A). Note that results are just plotted in a range between −23° and 120° in azimuth, as no neuronal responses were measured to the stimulus beyond that visual area. Stimuli were filtered through a 482/18 bandpass filter (Semrock) and ND1.0 neutral density filter (Thorlabs) and projected using a LightCrafter 4500 DLP (Texas Instruments, Texas, USA) with a frame rate of 100 Hz and synchronized with the recording of the microscope as described previously (54 (link)). Visual stimuli were generated using custom-written software using C++ and OpenGL. To correct for distortions due to the fly’s viewing position relative to the screen, stimuli were drawn on a virtual cylindrical surface and perspective-corrected using frustum.