A neuroscientist explores the network of cells in the retina.

The brightness of ambient illumination varies by as much as nine orders of magnitude from night to day. Many models of the visual system regard this variation as merely an inconvenience. However, these changes in illumination reset circadian clocks and influence mood, sleep and even migraine headaches. So how does the eye measure illumination over such a wide range?

Melanopsin-expressing retinal ganglion cells (mRGCs), a recently discovered subtype of the retina's output cells — those that convey signals from the eye to the brain — are dedicated to this task. mRGCs have their own photopigment, melanopsin, so can respond to light directly. Nonetheless, they also receive input from the other light-absorbing cells in the retina — the rods and cones. Two papers show that the cells linking rods and cones to mRGCs are unusual.

A rule of retinal wiring is that those RGC dendrites or projections that receive 'ON' signals — which indicate an increase in illumination — occupy one retinal layer, whereas those receiving 'OFF' signals are in another. Using mice, two independent groups show that mRGCs buck this trend by receiving ON signals in the OFF layer: David Berson and his team at Brown University in Providence, Rhode Island, and Stephen Mills and his colleagues at the University of Texas at Houston (O. N. Dumitrescu et al. J. Comp. Neurol. 517, 226–244; 2009; H. Hoshi et al. J. Neurosci. 29, 8875–8883; 2009).

These data indicate that the retinal networks that carry rod and cone signals to mRGCs follow their own rules. As artificial lights increasingly replace the Sun as our main source of photons, we are faced with the question of what makes a 'good' light for our physiology. The answer depends on which photoreceptors we want to stimulate and so requires a deeper understanding of these networks.

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