Figure 3. Quantal sampling adapts to provide invariable responses from natural contrast changes.

A, intracellularly recorded locust photoreceptor output to the same naturalistic contrast pattern has a similar waveform at dim (1,500 photons s⁻¹; grey) and bright (1.5 × 10⁵ photons s⁻¹; black) stimulation, implying that the same frequency range is utilised at different illumination. B, this is confirmed by the similar power spectra of the corresponding average responses, or signals (n = 100 repetitions). The arrow highlights the up‐shift in gain with brightening. C, normalised voltage signals (n = 100 repetitions) of both real and simulated Drosophila R1–R6 photoreceptors to the same naturalistic contrast pattern at dim and bright illuminations indicate comparable invariance. D–F, with brightening naturalistic stimulation: D, quantum efficiency (photon‐to‐bump conversion probability) decreases as more of a R1–R6 photoreceptor's 30,000 microvilli becomes refractory (insets), incapable of producing quantum bumps for the next 50–500 ms after their last photon hit. E, however, with more microvilli being activated, sample rate increases (continuous line) until progressive reduction in quantum efficiency D, stabilizes their quantum bump output. Simultaneously, sample size (bump waveform) is attenuated (dashed line). F, a photoreceptor's information transfer rate follows the increase in its quantum bump rate. Together, the adapting quantum bump dynamics ensure that relative changes in voltage responses represent naturalistic light changes (contrasts) accurately, irrespective of the ambient illumination. Although contrast gain in absolute terms (voltage/unit contrast) increases with light intensity, the temporal structure of the transmitted signal remains practically invariable. Data are from Faivre & Juusola (2008) and Song et al. (2012).