04.12.2013
Pressemitteilung: Kein blauer Himmel für Mäuse
Guppys, Hyänen und Mäuse haben eine bestimmte Spezialisierung der Netzhaut in ihrem Auge gemeinsam: Fotorezeptoren ("Zapfen"), die für "grünes" Licht empfindlich sind, befinden sich hauptsächlich in der oberen Hälfte des Auges, während Zapfen, die für "blaues" Licht empfindlich sind, die untere Hälfte dominieren. Da die Linse das Bild beim Eintritt in das Auge invertiert, scheint diese Anordnung intuitiv sinnvoll zu sein: Blaues Licht vom Himmel wird von den blauen Zapfen erfasst, während das grünliche Licht vom Boden auf die grünen Zapfen fällt. Wissenschaftler um Thomas Euler am Werner-Reichardt-Zentrum für Integrative Neurowissenschaften der Universität Tübingen haben diese Spezialisierung der Netzhaut bei Mäusen untersucht (Press release in English only).
In their study, just published in the journal Neuron, they show that this arrangement is not an adaptation to the predominating ‘colors’ of the sky and the ground, as was previously thought. Their experiments showed that the apparent match between ‘color’ and differential cone distribution brings the animals no advantage. “The green cones would ‘see’ the light in the sky just like the blue cones”, explains Thomas Euler. Instead, this specialized distribution of cones appears to subserve a much more fundamental aspect of vision: the detection of dark-light contrasts.
As photographers will know, there is a very substantial difference between the sky and the ground in terms of brightness and contrast. On the ground average brightness is rather low in comparison with the sky, and light is reflected from things such as leaves and the earth, so that contrasts of bright and dark are roughly equally frequent. Light from the sky, however, is usually direct, with objects appearing as dark silhouettes against a bright background. So it would make sense for different parts of the eye to be adapted to the predominant distribution of contrast below and above the horizon.
“Indeed, this is the case: Green cones in the mouse retina respond similarly to dark and light stimuli, but blue cones respond more strongly to dark ones”, clarifies the scientist. The ‘blue’-sensitive, sky-oriented lower half of the mouse retina is tuned to spot dark objects against a bright background. These objects can be, for example, birds of prey. The results match very well with two recent studies in which it was shown that mice either freeze or run and hide as soon as something dark appears above them. This instinctive reaction occurs within less than 200 milliseconds, which is too short a time to allow for a thorough ‘assessment’ of the situation. Therefore, the researchers think that this behavior is based on very fast visual processing, as it might be provided by a signal path specializing in dark contrasts that begins at the first synapse, with the cones.
Publication: Tom Baden, Timm Schubert, Le Chang, Tao Wei, Mariana Zaichuk, Bernd Wissinger, Thomas Euler (2013): A Tale of Two Retinal Domains: Near-Optimal Sampling of Achromatic Contrasts in Natural Scenes through Asymmetric Photoreceptor Distribution. Neuron 80, 1–12, December 4, 2013.
dx.doi.org/10.1016/j.neuron.2013.09.030
Organization:
- Werner Reichardt Centre for Integrative Neuroscience
Reseach Group: Ophthalmic Research
Contact: Prof. Dr. Thomas Euler