Abstract
The retina is one of the best known quantum detectors with rods able to reliably respond to single photons. However, estimates on the number of photons eliciting conscious perception, based on signal detection theory, are systematically above these values after discounting by retinal losses. One possibility is that there is a trade-off between the limited motor resources available to living systems and the excellent reliability of the visual photoreceptors. On this view, the limits to sensory thresholds are not set by the individual reliability of the receptors within each sensory modality (as often assumed) but rather by the limited central processing and motor resources available to process the constant inflow of sensory information. To investigate this issue, we reproduced the classical experiment from Hetch aimed to determine the sensory threshold in human vision. We combined a careful physical control of the stimulus parameters with high temporal/spatial resolution recordings of EEG signals and behavioral variables over a relatively large sample of subjects (12). Contrarily to the idea that the limits to visual sensitivity are fully set by the statistical fluctuations in photon absorption on retinal photoreceptors we observed that the state of ongoing neural oscillations before any photon impinges the retina helps to determine if the responses of photoreceptors have access to central conscious processing. Our results suggest that motivational and attentional off-retinal mechanisms play a major role in reducing the QE efficiency of the human visual system when compared to the efficiency of isolated retinal photoreceptors. Yet, this mechanism might subserve adaptive behavior by enhancing the overall multisensory efficiency of the whole system composed by diverse reliable sensory modalities.
Highlights
The first experiments on the sensibility of the human eye to weak, near absolute thresholds, optical signals were conducted in the 1940s (Hecht et al, 1942)
The quantum efficiency (QE) estimated from behavior is very low compared with the absorptive QE estimated from the properties of light photoreceptors at the retina
We decided to set a slightly lower threshold at 50%. This is due to three reasons, (1) we introduced under the form of zero intensity trials a control against dark noise, i.e., detection by chance and (2) we used several naïve subjects and (3) the experimental design introduced a variable delay between the acoustic signal and the photons arrival to prevent anticipation
Summary
The first experiments on the sensibility of the human eye to weak, near absolute thresholds, optical signals were conducted in the 1940s (Hecht et al, 1942) They led to the conclusion that rod photoreceptors can detect a very small number of photons, typically less than 10 during an integration time of about 300 ms (Barlow, 1956). This prediction has been confirmed by several experiments (Rieke and Baylor, 1998) making from the human eye a remarkable light sensitive detector, which can stand a comparison to today’s best man-made detectors (Rieke and Baylor, 1998). Only about 8% of the photons incident on the cornea reach the retina, about 100 photons are required to trigger a neural response even if rod photoreceptors can react to single photons (Rieke and Baylor, 1998)
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