  We measure the reliability of signals at three levels within the blowfly
visual system, and present a theoretical framework for analyzing the
experimental results, starting from the Poisson process. We find that blowfly
photoreceptors, up to frequencies of 50-100 Hz and photon capture rates of up
to about 3*10^5/s, operate well within an order of magnitude from ideal photon
counters. Photoreceptors signals are transmitted to LMCs through an array of
chemical synapses. We quantify a lower bound on LMC reliability, which in turn
provides a lower bound on synaptic vesicle release rate, assuming Poisson
statistics. This bound is much higher than what is found in published direct
measurements of vesicle release rates in goldfish bipolar cells, suggesting
that release statistics may be significantly sub-Poisson. Finally we study H1,
a motion sensitive tangential cell in the fly's lobula plate, which transmits
information about a continuous signal by sequences of action potentials. In an
experiment with naturalistic motion stimuli performed on a sunny day outside in
the field, H1 transmits information at about 50% coding efficiency down to
millisecond spike timing precision. Comparing the measured reliability of H1's
response to motion steps with the bounds on the accuracy of motion computation
set by photoreceptor noise, we find that the fly's brain makes efficient use of
the information available in the photoreceptor array.
