Photoreceptor performance and the co-ordination of achromatic and chromatic inputs in the fly visual system

White noise techniques are used to compare the: two photoreceptor sub-types in blowfly retina, the short visual fibres (R1-6) that code achromatic con trast, and the long visual fibres (R7 and R8) that together code wavelength distribution and polarisation plane. Measurements of signal and noise spec tra and contrast gain, tak:en across a broad intensity range, permit a deta iled comparison of coding efficiency under natural conditions of illuminati on. As a function of excitation (effective photons per photoreceptor per se cond; hv/rec per s), adaptive changes in the long and short visual fibres a re similar, suggesting that post-rhodopsin their phototransduction cascades are identical. Under identical natural daylight conditions (photons per cm (2) per second; hv/cm(2) per s) short visual fibres catch more photons, thu s operating with a higher signal to noise ratio and faster response, to con sistently outperform the long visual fibres. Long visual fibres compensate for their poor quantum catch by having a higher absolute gain (mV/hv) which at low light intensities enables them to achieve a level of contrast gain (mV/unit contrast) similar to the short visual fibres. Differences in signa l to noise ratios are related to known differences in photoreceptor structu re and synaptic frequency among visual interneurons. The principles of matc hing sensitivity and synapse number to quantum catch described here could e xplain analogous differences between chromatic and achromatic pathways in m ammalian and amphibian retinas. (C) 1999 Elsevier Science Ltd. All rights r eserved.