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Unformatted text preview: Limulus Vision in the Marine Environment ROBERT B. BARLOW*, JAMES M. HITT, AND FREDERICK A. DODGE Center for Vision Research, Department of Ophthalmology, Upstate Medical University, 750 Adams Street, Syracuse, New York 13210 Abstract. Horseshoe crabs use vision to find mates. They can reliably detect objects resembling potential mates under a variety of lighting conditions. To understand how they achieve this remarkable performance, we constructed a cell- based realistic model of the lateral eye to compute the ensembles of optic nerve activity (neural images) it trans- mits to the brain. The neural images reveal a robust encod- ing of mate-like objects that move underwater during the day. The neural images are much less clear at night, even though the eyes undergo large circadian increases of sensi- tivity that nearly compensate for the millionfold decrease in underwater lighting after sundown. At night the neural images are noisy, dominated by bursts of nerve impulses from random photon events that occur at low nighttime levels of illumination. Deciphering the eyes input to the brain begins at the first synaptic level with lowpass temporal and spatial filtering. Both neural filtering mechanisms im- prove the signal-to-noise properties of the eyes input, yield- ing clearer neural images of potential mates, especially at night. Insights about visual processing by the relatively simple visual system of Limulus may aid in the design of robotic sensors for the marine environment. Introduction The world is rich with sensory information, and animals are highly efficient at extracting what is essential for their survival. The retina begins the processing of visual infor- mation by transforming patterns of incident light intensity into trains of impulses in optic nerve fibers (Dowling, 1987). The retina encodes information it receives in a reli- able and efficient manner (Rieke et al., 1997) but does not encode all of it. Rather, the retina extracts certain features in the visual scene at the expense of others (Lettvin et al., 1959). An important first step for exploring the neural code the eye transmits to the brain when an animal sees is to understand what an animal can see in its natural habitat. The next step is to examine the retinal coding of natural scenes in activity of optic nerve fibers. Recordings from single nerve fibers have indeed yielded useful insights about reti- nal function; however, it is difficult to infer from them the information transmitted by arrays of optic nerve fibers to the brain about the complex patterns of illumination animals encounter in their natural habitat. Techniques such as multi- electrode arrays (Meister et al., 1994) and voltage-sensitive dyes (Wong et al., 1995) can access patterns of activity generated by ensembles of retinal neurons, but they are not practical for recording from large numbers of optic nerve fibers in behaving animals....
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This note was uploaded on 11/02/2010 for the course PSYC 473B taught by Professor Patriciadilorenzo during the Fall '10 term at Binghamton University.
- Fall '10