Pushing it to the Limit: Adaptation with Dynamically Switching Gain Control
- PDF / 1,595,924 Bytes
- 10 Pages / 600.03 x 792 pts Page_size
- 62 Downloads / 187 Views
Research Article Pushing it to the Limit: Adaptation with Dynamically Switching Gain Control Matthias S. Keil1 and Jordi Vitria` 1, 2 1 Centre
de Visi`o per Computador, Edifici O, Campus UAB, 08193 Bellaterra, Cerdanyola, Barcelona, Spain Science Department, Universitat Aut`onoma de Barcelona, 08193 Bellaterra, Cerdanyola, Barcelona, Spain
2 Computer
Received 1 December 2005; Revised 11 July 2006; Accepted 26 August 2006 Recommended by Maria Concetta Morrone 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 shut down locally 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. Copyright © 2007 M. S. Keil and J. Vitri`a. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
1.
INTRODUCTION
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]. Because neurons are noisy devices, reliable information transmission is only granted if the distribution of levels in the stimulus matches the neuron’s reliable operation range [2]. Consider, for example, the mammalian visual system, with the retina at its front end. When performing saccades, 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]). From one scene to another (e.g., from bright sunlight to starlight), the range of intensity variations may well span up to ten orders of magnitude [6–9]. 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., [11–14]). Specifically, cone photoreceptors may decrease their sensitivity proportionally to background intensity, over
about 8 log units of background intensity [15]. This relationship is known as Weber’s law (e.g., [16]). Adaptation in photoreceptors is achieved by subtly balanced network of molecular processes (s
Data Loading...
