Title

What the Limulus eye tells the Limulus brain

Date of Award

1997

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical and Chemical Engineering

Advisor(s)

Robert Barlow

Second Advisor

Frederick Dodge

Keywords

neural coding, retinal signals, signal processing

Subject Categories

Neurology

Abstract

Every spring millions of horseshoe crabs invade shallow estuaries along the eastern coast of North America in search of a mate. Vision plays an important role in this behavior. Field studies show that animals reliably detect one another under a variety of illumination conditions despite large differences in the contrast of their carapace. Such performance is remarkable in view of the relatively simple design of their lateral eyes which process visual signals with $\sim$1000 coarsely-faceted ommatidia using just two types of retinal neurons.

To understand how horseshoe crabs use vision to identify potential mates, underwater movies were recorded with a miniature videocamera attached to animals moving in their natural habitat while the activity of single optic nerve fibers was monitored with a microsuction electrode. The movies were presented to a realistic computational model of the eye that simulates its input to the brain for any specified scene. Computed patterns of optic nerve activity were confirmed by single fiber recordings and examined for information about behaviorally-relevant objects. Intracellular recordings were also made from second-order neurons in the brain to characterize properties of neural circuits that process retinal signals.

I find that the Limulus lateral eye is well adapted for meeting the needs of the animal and the challenges of its visual environment. The visibility of underwater objects at mating beaches is limited by water turbidity to $\sim$1.5m in accordance with the visual range of the animal. Within this range, the eye transmits robust signals to the brain about objects having the size, contrast, and motion of potential mates. It encodes these objects with coherent patterns of activity among small clusters of ommatidia. The patterns are enhanced by flickering light from overhead waves to which the eye is maximally sensitive. Wave glitter also evokes large responses in clusters of ommatidia viewing the sand which obscure responses to moving crab-size objects, but integrative properties of central neurons appear to extract only those patterns of activity which encode potential mates. These findings yield insights about the retinal code mediating visually-guided behavior in Limulus and, perhaps, other animals.

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