Schematic top view of the head of a fly, showing one eye on each side. A counterclockwise stimulus would tend to drive counterclockwise turning behavior on short timescales. In this schematic, this is driven by front-to-back motion detectors on the left eye and back-to-front motion detectors on the right eye (in black, as opposed to gray detectors, which are not active). The influences from the two eyes sum to contribute to counterclockwise turning. One putative anti-directional turning mechanism is shown in blue: under high-contrast and high-luminance conditions, on long timescales, the regressive motion detectors on the right eye influence turning with the opposite sign, with an influence stronger than the progressive motion detectors on the left eye. Our imaging data show that this reversal is not occurring in T4, T5, horizontal system (HS), or CH neurons (
Figure 5), so must be somewhere parallel to or downstream of those neurons.