  The retina is one of the best known quantum detectors with rods able to
respond to a single photon. However, estimates on the number of photons
eliciting conscious perception, based on signal detection theory, are
systematically above these values. One possibility is that post-retinal
processing significantly contributes to the decrease in the quantum efficiency
determined by signal detection. We carried out experiments in humans using
controlled sources of light while recording EEG and reaction times. Half of the
participants behaved as noisy detectors reporting perception in trials where no
light was sent. DN subjects were significantly faster to take decisions.
Reaction times significantly increased with the decrease in the number of
photons. This trend was reflected in the latency and onset of the EEG responses
over frontal and parietal contacts where the first significant differences in
latency comparable to differences in reaction time appeared. Delays in latency
of neural responses across intensities were observed later over visual areas
suggesting that they are due to the time required to reach the decision
threshold in decision areas rather than to longer integration times at sensory
areas. Our results suggest that post-retinal processing significantly
contribute to increase detection noise and thresholds, decreasing the
efficiency of the retina brain detector system.
