Science Literature

Visual adaptation in humans provides us with the ability to see in a wide range of conditions, from the darkness of night to the brightness of the midday sun. Adaptation means that the signals from our photoreceptors are processed so as to amplify weak signals and also to damp strong signals thereby preventing saturation. "Past physiological work has shown that ganglion cells, the output cells of the retina, adapt at lower light levels than cones and one of the downstream targets of the cones, horizontal cells. Such studies provide evidence for adaptation in the retinal circuitry and in the cone photoreceptors." This mechanism is known as "receptor adaptation". New research reveals a second mechanism where there is a convergence of signals from multiple cones within the retinal circuitry. This is described as "post-receptor adaptation". The two mechanisms are complementary to each other. As light levels increase, the main site of adaptation switches from the retinal circuitry to the cone photoreceptors.
The research paper concludes:

"Receptor and post-receptor adaptation permit the amplification required to see objects in shadows while avoiding saturation from the sky. The combination of these adaptive mechanisms allows the visual system to encode details in a scene with greater fidelity than a standard camera at a single exposure setting. The strategy the retina employs - shifting the dominant site of adaptation to match the reliability of the input signals - demonstrates an elegant principle for accurate information processing in sensory perception."

This blog has, in the past, drawn attention to the differences between tinkering evolution and exquisite design. This new research appears to fall into the latter category.

Light adaptation in cone vision involves switching between receptor and post-receptor sites
Felice A. Dunn, Martin J. Lankheet & Fred Rieke
Nature 449, 603-606 (4 October 2007) | doi:10.1038/nature06150

We see over an enormous range of mean light levels, greater than the range of output signals retinal neurons can produce. Even highlights and shadows within a single visual scene can differ ~10,000-fold in intensity-exceeding the range of distinct neural signals by a factor of ~100. The effectiveness of daylight vision under these conditions relies on at least two retinal mechanisms that adjust sensitivity in the ~200 ms intervals between saccades1. One mechanism is in the cone photoreceptors (receptor adaptation)2, 3, 4, 5 and the other is at a previously unknown location within the retinal circuitry that benefits from convergence of signals from multiple cones (post-receptor adaptation)6, 7. Here we find that post-receptor adaptation occurs as signals are relayed from cone bipolar cells to ganglion cells. Furthermore, we find that the two adaptive mechanisms are essentially mutually exclusive: as light levels increase the main site of adaptation switches from the circuitry to the cones. These findings help explain how human cone vision encodes everyday scenes, and, more generally, how sensory systems handle the challenges posed by a diverse physical environment.