Figure 7.

Motion nulling reveals contributions of lamina neurons to contrast sensitivity as a function of stimulus speed. (A) Nulling stimuli consist of two superimposed square-wave gratings (45° spatial period): a constant reference stimulus, and a test stimulus whose contrast is varied across trials. At low test contrast, flies follow the reference stimulus (ΔWBA < 0); at high test contrast, flies follow the test stimulus (ΔWBA > 0). The null contrast is the contrast of the test stimulus needed to cancel, or “null”, the reference stimulus. (B) Example of a motion nulling phenotype: silencing L3 neurons alters fly contrast sensitivity at low speeds. The reference stimulus has a relative contrast of 0.27 and rotates counterclockwise at 4 Hz, while the test stimulus rotates clockwise at 1.33 Hz and the contrast is varied across trials. Control flies follow the high contrast test stimulus on the last trial, while flies with L3 silenced follow the reference stimulus. (C) Tuning curves of contrast sensitivity (1/null contrast) measured over a range of test stimulus temporal frequencies. Silencing four of the lamina monopolar cells, L1–L4, alters contrast sensitivity tuning at both high and low frequencies. (D) Same as in F, but for feedback neurons which contribute significantly to contrast sensitivity tuning. The neuron classes shown here represent all of the individual cell types for which the null contrast of at least 2 temporal frequencies is significant at the p < 0.1 level or lower (details in Supplemental Experimental Procedures). (E) Summary of the changes in temporal tuning: inactivating L1, L2, and L4 lead to enhanced contrast sensitivity at lower frequencies and reduced sensitivity at higher frequencies, L3 inactivation leads to the opposite phenotype, and the feedback cells only affect the flies’ contrast sensitivity at lower frequencies. (F) A model for lamina processing with parallel inputs to an HR-EMD, representing the L1, L2/L4, and L3 pathways. L1 and L2/L4 preprocessing were modeled as fast pathways (with identical low-pass filters with time constants of τ = 4 ms) while the L3 input was modeled as a much slower pathway (low-pass filter with τ = 80 ms). The remainder of the model is a standard HR-EMD (with the delay implemented as a low-pass filter with τ = 18 ms; see Supplemental Experimental Procedures for details of the simulation). (G) Simulated responses of this model to the identical stimuli used in the motion nulling behavioral experiments capture the general changes in temporal tuning seen in C. Removing either L1 or L2/L4 pathway input resulted in enhanced contrast sensitivity to low frequency stimuli and a reduction in the high frequency sensitivity, while removing the L3 input leads to the opposite phenotype.