Anomalous Resolution

The limiting resolution of the CE-to-descending contralateral movement detector (DCMD) neuron system in the locust Schistocerca gregaria and in the lubber grasshopper Romalea microptera has been widely investigated, both optically and electrophysiological^. The DCMD neuron axons (one in each side of the ventral nerve cord) are large, produce large spikes, and are easily recorded from with simple hook or suction electrodes. The operation performed by the CE-to-DCMD neuron system is the detection of small, novel movements of small, dark test objects anywhere over the contralateral eye. DCMD units habituate quickly, and need several minutes to recover their maximum response to novel object movements.

The DCMD neuron has been traced electrophysiologicaly from its origin in the lateral, ipsilateral midbrain, down and across the contralateral esophageal connective to the contralateral ventral nerve cord (CVNC), where it descends as far as the third thoracic ganglion. The DCMD neuron is driven through a rectifying electrical synapse in a 1:1 manner from the lobular giant movement detector (LGMD), neuron which is located in the lobula of the OL. The LGMD neuron also drives by a chemical synapse a descending ipsilateral movement detector (DIMD) neuron in the ipsilateral VNC. (Details of the locust movement detector system can be found in a series of definitive papers by Rowell and O'Shea, 1976a; 1980); Rowell et al., 1977, O'Shea and Williams, 1974; and O'Shea and Rowell, 1975, 1976).

The resolution of the DCMD system has been called "anomalous" because a significant increase in the firing rate of a DCMD unit will occur for novel movements of a dark test object described by spatial frequencies apparently higher than the cutoff frequency that ommatidial optics and DSFs predict. Any test of the spatial resolution of DCMD neurons is made more difficult by the fact that these units respond to novel movements of the test object in their visual field (the entire contralateral CE), and soon habituate for repeated object motion. Thus, tests of

DCMD unit visual resolution must be spaced far apart enough in time for the DCMD system to "unhabituate," i.e., recover its maximum sensitivity to novel object motion (Grossman and Northrop, 1976). Interstimulus intervals used by the author in DCMD system resolution tests with Romalea were never less than 2 min. It was observed by Horn and Rowell (1968) for Schistocerca, that a 2-min recovery time was necessary to avoid response desensitization by habituation. In testing DCMD unit resolution using small, "jittery spot objects," the author has found that while the entire eye is capable of responding to the jittery spot, it is the group of ommatidia directly under the spot that apparently habituate. If the spot is moved over an unstimulated part of the eye, and jittered without delay, little habituation is found at first. If the spot is jittered over the whole eye, then a 2-min rest is required to restore sensitivity to the whole eye.

The first report of anomalous resolution was by Burtt and Catton (1962). They showed that the DCMD unit would give a significantly increased number of spikes when the eye was presented with moving black and white stripes, seen through a rectangular window. Responses were seen down to stripe periods of ~0.3°. The 0.3° limit was considered "anomalous" by other workers because it exceeded the Nyquist limit for spatial sampling by a factor of about 6.67 in the vertical plane of the locust eye, and, also, it is beyond the "resolving power" of the apertures of the individual ommatidia as determined by the Rayleigh criterion (classical optical theory). To try to explain Burtt and Catton's "anomalous" results, other workers suggested that the anomalous response was due to imperfections in the pattern (i.e., spatial subharmon-ics; McCann and MacGinitie, 1965) or was due to a subharmonic generated as stripes emerged from and then passed behind the straight edges of the mask (Palka, 1965). Burtt and Catton (1966, 1969) countered these criticisms by making a precision, radially striped, "wheel" pattern, viewed by the insect in an annular window. This pattern obviously had no edge effects. Burtt and Catton again found a limiting resolution of ~0.3°.

The author has also examined the threshold resolution of DCMD units in Schistocerca and in Romalea using the radial striped wheel pattern used by Burtt and Catton, as well as a fine, rotating checkerboard pattern in an annular window, and a single, black, jittery spot. The jittery black spot was held by a magnet on the gray inside surface of a thin fiberglass hemisphere, 50 cm in diameter. The hemisphere with spot was centered over the CE under test, and illuminated from the sides by diffuse white light. The spot was jittered manually by moving a corresponding magnet on the outside of the hemisphere by hand in a manner (position, direction, speed) to maximize the number of spikes elicited on the DCMD fiber over the test period (generally 1 min). DCMD units have a size preference for jittered spots; a spot of ~5° appears to give the strongest response. Smaller spots obviously give a reduced response, as do spots larger than 5°. White spots also give about half the response of black spots, other factors being equal. Figure 5.2-8 illustrates the fact that the DCMD system can confidently resolve a jittered spot as small as 0.4°. A rotating, black/white, checkerboard object viewed through an annular window (8.2 cm inside diameter; 14.5 cm outside diameter) was found to be the most potent test object for the DCMD system (Northrop, 1974). Rotation of this pattern could elicit

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