(A) Diagram displaying how the negative feedback model works (7 s light in example) (See Online Methods for step-by-step explanation). The model assumes that packets of light information are discrete and are relayed to the PLR circuit to result in pupil constriction at later timepoints. We determined the kinetics of light information relay using a 1-s light pulse-chase. Then, we simply modulate the relative light intensity reaching the retina based on assuming continuous 1-s packets of information. At each new 1-s interval, the model samples the assumed pupil sizes currently driven by each previous packet of light information, uses the maximum value as the current pupil size, and then reduces the stimulus intensity using that pupil size. We then use this new intensity to determine constriction caused at that time. This iterates every second. (B) Putative kinetics of feedback’s impact on PLR at several light intensities (0.0001, 0.001, 0.01, 0.1, 1, 10, 100, 1000, and 10,000 lux). (C) Magnitude of PLR decay caused by feedback as modeled with (D) EC50. Note that our model predicts minor PLR decay as a result of PLR feedback. (E) Experimental investigation of feedback’s role in PLR decay. Atropine was applied to the left eye to inhibit pupil constriction and thus feedback. No enhancement of sustained PLR of the right eye was observed (paired two-tailed t-test).