This chapter has examined the behavior of CE visual systems of certain insects and crustaceans. The "front-end" of a CE is the photoreceptor array, consisting of the densely packed ommatidia (lenses, rhabdoms, and retinula cells). The ommatidia make up a spatial sampling array in which each ommatidium "looks" at a slightly different portion of the visual object. Light intensities from each portion of the object are weighted by the directional sensitivity functions of the ommatidia. The retinula cell photoreceptors in any ommatidium respond to their absorbed light intensity, which is proportional to the real convolution of the two-dimensional intensity distribution of the object with the ommatidium DSF. The six or so retinula cells in the ommatidium that absorb this light, depolarize. They send this depolarization elec-trotonically on their axons to the lamina ganglionaris of the OL where visual processing begins.
All CE systems perform feature extraction. An OL neuron showing feature extraction can respond as simply as a burst of spikes for ON of general illumination, or as complexly as a multimodal unit that fires for novel jittery motion of a small black object, as well as for mechanical stimulation of the animal's feet, or a sound. One of the most ubiquitous of the feature extraction responses in all CEs is the DS, movement-sensing neuron. DS neurons can have a slow, random background firing rate in the absence of stimulation. When an object is moved in the preferred direction, it fires faster; when the object is moved in the opposite (null) direction, the firing can fall below the background rate. Most DS units have a broad response for motions away from the preferred direction (perhaps a cosine(9) directional sensitivity pattern), and an optimum object velocity that will give the highest spike frequency. (If the object is moved faster than the optimum velocity, the DS neuron response falls off.) Object shape, size, and contrast also appear to be selected for. Most DS units respond most strongly to a long black stripe moved in its preferred direction, but the author has found DS units that referred black spots about 5° in diameter. There are also DS units that appear to like white stripes, as well. The many DS units found in flying insects probably are involved with flight stabilization and obstacle avoidance.
Mathematical models were developed to account for anomalous resolution in CE visual systems, as well as to describe spatial high-frequency enhancement caused by lateral inhibition. A synthetic aperture model was also developed to demonstrate how increased spatial resolution of objects could occur. These models underscore the importance of signal interactions in the CE array that can lead to improved performance over a single photoreceptor.
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