Appendix 8—figure 2. The hypothesis predicts R1-R6 output to double-fast moving point-objects.

All the parameters of the simulations are fixed to the experimentally measured values, similar to Appendix 8—figure 1. The stimulus is the same two bright dots, but this time, they cross the cell’s receptive field (RF) double-fast, 409^o/s (i). (A) Again, if the rhabdomere remains stationary during their flyby (corresponding to the classic theory, ii), their light fuses (iii), and the photoreceptor output cannot distinguish the dots (iv). (B) By including the light-induced back-to-front rhabdomere movement (ii) but with the same RF shape, the light input broadens slightly (iii), but cannot separate the two dots. Consequently, the photoreceptor output (iv) shows a slightly narrower single peak than in the previous case in (A). (C) If, however, the rhabdomere contraction (away from the lens’ focal point, ii) moves the RF and actively narrows it (from 8.8^o to 4.9^o), the light input from the dots is transformed into two intensity spikes (iii), which R1-R6 output separates into two peaks (iv). (D) Corresponding intracellular R1-R6 recordings show comparable dynamics to the full model (C). These and other simulations and recordings, which all show good correspondence, establish that photoreceptors’ RFs must move and narrow dynamically during light stimulation.