Abstract
With this paper we propose a model to simulate the functional aspects of light adaptation in retinal photoreceptors. Our model, however, does not link specific stages to the detailed molecular processes which are thought to mediate adaptation in real photoreceptors. We rather model the photoreceptor as a self-adjusting integration device, which adds up properly amplified luminance signals. The integration process and the amplification obey a switching behavior that acts to locally shut down the integration process in dependence on the internal state of the receptor. The mathematical structure of our model is quite simple, and its computational complexity is quite low. We present results of computer simulations which demonstrate that our model adapts properly to at least four orders of input magnitude.
Highlights
There is agreement that adaptation is important for the function of nervous systems, since without corresponding mechanisms, any neuron with its limited dynamic range would stay silent or operate in saturation most of the time [1]
The retina must cope with intensity variations which may span about one [3, 4] to about two orders of magnitude (2 including shadows according to [3], 2-3 according to [5])
Many of the data were gained from rod photoreceptors because they are more amenable to analysis
Summary
There is agreement that adaptation (i.e., the adjustment of sensitivity) is important for the function of nervous systems, since without corresponding mechanisms, any neuron with its limited dynamic range would stay silent or operate in saturation most of the time [1]. This range of intensities has to be mapped onto less than two orders of output activity of retinal ganglion cells [10], implying some form of compression of the scale of intensity values The retina achieves this by making use of a cascade of gain control and adaptation mechanisms, respectively (e.g., [11,12,13,14]). Horizontal cells are coupled with gap junctions (forming a syncytium), whose connectivity or permeability decreases with increasing differences between the inputs of adjacent cells [23, 24] In other words, their horizontal cell network establishes current flows inside of regions that are EURASIP Journal on Advances in Signal Processing defined by low contrasts, whereas no activity exchange occurs between regions which are separated by high contrast boundaries (very similar to an anisotropic diffusion mechanism [25]). Notice that our model lacks the latter stage, and only simulates the photoreceptor adaptation
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