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Alexandra Tikidji-Hamburyan, Thomas A. Münch; Light Adaptation at Distinct Intensity Levels within the Photopic Regime. Invest. Ophthalmol. Vis. Sci. 2012;53(14):6913.
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© ARVO (1962-2015); The Authors (2016-present)
Adaptation of the visual system to different ambient light intensities is an essential property of vision. There are three discrete light intensity ranges in which the retina operates in different "modes": scotopic range (night vision, light-sensitivity mediated by rods), mesopic range (mediated by rods and cones), and photopic range (day vision, mediated by cones). Here, we investigated if there are distinct and discrete adaptational states of the retina within the scotopic, mesopic, and photopic intensity ranges.
Mice were dark adapted prior the experiment for at least 4 hrs. Multi-electrode-array recordings were made from ganglion cells of flat-mounted retinas in response to light stimuli (2-sec full-field flashes, presented every 10 seconds). Every ~30 minutes, we increased the luminance by 1 log unit, while keeping the contrast of the full-field flashes the same. In total, the experiment was performed over 10 log units, ranging from scotopic to high photopic levels.
As expected, we observed adaptational changes of the responses when stepping into the mesopic and into the photopic ranges. These adaptational changes were characterized by supralinear increases of response magnitudes when entering the mesopic range (presumably due to simultaneous activation of rods and cones), changes of sensitivity, and adjustments of gain and dynamic range of the responses. These processes occurred at different temporal scales, taking up to several minutes. Surprisingly, we found that two distinct intensity levels within the photopic range, separated by 2 intensity log units (corresponding to about 10^4 and 10^6 isomerizations/rod•sec) also triggered specific adaptational changes that took about 15 minutes to stabilize. They were mainly characterized by a reduction of the response gain of up to 60%. Interestingly, those specific changes were only present in the OFF responses, whereas the ON responses adapted moderately like at all other light level transitions within the photopic range.
It is unlikely that these adaptational changes can be explained by photoreceptor saturation, as this should have symmetric effects on retinal circuits. Furthermore, the responses increase again and are stable at the next higher light levels. Instead, our observations indicate a deployment of some asymmetrical mechanisms (different in the ON and OFF retinal pathways) to adjust the dynamic range of cells within the photopic range.
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