Figure 7.
Adaptive Sampling Promotes Contrast Constancy
With brightening naturalistic stimulation:
(A) The sample size (bump amplitude) is attenuated.
(B) More microvilli are activated, increasing sample rate, until progressive reduction in quantum efficiency stabilizes a photoreceptor's bump output.
(C) These dynamics (A and B) reduce the amount of information each sample (bump) carries (D) but ensure that relative changes in voltage responses represent naturalistic light changes (contrasts) accurately, irrespective of the ambient illumination.
Normalized voltage signals (n = 100 repetitions) to the same naturalistic contrast stimulus shown for both real and simulated Drosophila photoreceptors at dim (1,500 photons/s) and bright (1.5 × 105 photons/s) illuminations. Although contrast gain in absolute terms (voltage/unit contrast) increases with light intensity [8, 12, 26, 34, 43], the temporal structure of the transmitted signal remains practically invariable.