FIGURE 5.2-8 Normalized DCMD unit responses to a single, jittered, black spot inside a large, gray hemisphere. See text for description of the experimental protocol. The confidence that the DCMD unit responded to the jitter of a 0.46° diameter spot was better than 99.5% (one-sided t-test, 9° freedom). Bar denotes the normalized, random firing rate of the unstimulated DCMD system. (From Northrop, R.B., in The Compound Eye and Vision in Insects, G.A. Horridge, Ed., Clarendon Press, New York, 1975. With permission of Oxford University Press.)

a 95% confident DCMD response down to a checker repeat distance of 0.21°. (A Student t-test was used to validate the responses statistically.)

After demonstrating that anomalous resolution does in fact occur in the DCMD system for a number of different types of test objects, the focus turns to a theoretical model that may describe its mechanism. One strategy that the animal might use is to organize the ommatidia of an eye into a synthetic aperture system, such as used to enhance resolution in radar and radiotelescope systems having more than one antenna (an antenna array). The synthetic aperture approach has also been employed with optical telescopes (Reynolds et al., 1989).

Although it has been suggested that one ommatidium is all that is necessary to sense object movement (Kirschfeld, 1972), higher resolution may be obtained by using two or more closely spaced ommatidia, and processing the rate of change of each vmk(t) so an output is obtained if one vmk(t) is increasing while another vmj(t) is decreasing. No output occurs if both vmk(t) and vmj(t) are changing with the same sign.

The following section describes a hypothetical neural model proposed by the author to account for anomalous resolution.

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