Photopic motion sensitivity at high temporal frequencies is limited by the dark light of the eye, not quantal noise

摘要

Absolute detection threshold in total darkness is limited by internal noise independent of background intensity caused by spontaneous activity of photoreceptors or retinal cells. This 'dark light of the eye' causes incremental threshold to be independent of very low background intensities (linear law). Under photopic conditions, incremental threshold to low spatiotemporal frequencies is proportional to background intensity (i.e., Weber law; contrast sensitivity independent of background intensity) and is therefore undoubtedly limited by neural noise arising after contrast normalization. For high spatial frequencies, incremental threshold is proportional to the square root of the background intensity (de Vries-Rose law), which is explained by quantal noise resulting from the probabilistic absorption of photons by photoreceptors. For high temporal frequencies, however, incremental threshold is independent of background intensity (i.e., linear law; contrast sensitivity proportional to background intensity) even under photopic conditions, but has been nevertheless attributed to quantal noise, presumably because dark light is expected to have a negligible impact at high background intensities. In the current study, we investigated the properties of the noise limiting photopic motion sensitivity by measuring contrast threshold for a direction discrimination task in absence of noise and in high noise as a function of temporal frequency and background intensity. This method enabled us to derive equivalent input noise, which was found to have a U-shape as a function of temporal frequency. Given that quantal noise is temporally white and is not preceded by any temporal filtering, these results are incompatible with the quantal noise hypothesis. On the other hand, the linear law observed at high temporal frequencies suggests that the limiting noise was independent of background intensity and occurred before contrast normalization. Such an intensity-independent noise needs to result from early spontaneous activity independent of background intensity, namely, the dark light of the eye