The grasshopper Romalea microptera is a large, flightless insect native to the Everglades of Florida and other swamps and marshes in the southeast U.S. It is a pleasure to work with because it is large (adult females can be 7.5 to 10 cm in length), and consequently its head, CEs, and OLs are proportionally large. Inspired by the work of Lettvin et al. (1959) on the frog's retina and Horridge et al. (1965) on the locust's OLs, Northrop and Guignon (1970) and, later, Northrop (1974) examined single units from the OLs of Romalea for feature extraction operations on simple, contrasting visual objects, moving and stationary, presented on the inside surface of a hemisphere concentric with the eye under test.
Hemispheres 50 and 28 cm in diameter were used. The insides of the hemispheres, viewed by the insect, were painted light gray so either white or black test objects could be used. Black and white disk and square objects were 14, 28, and 56 mm in diameter, or on a side; these subtended 3.2, 6.4, and 12.8° on the 50-cm-diameter hemisphere, and 5.7, 14.5, and 22.9° on the 28-cm hemisphere, respectively. Spot objects were moved manually by magnets from outside the hemispheres, or by a Ledex rotary solenoid outside the large hemisphere. A flat spring arm was connected to the solenoid; a magnet was glued to the end of the arm, which moved a spot in an arc inside the hemisphere. One 28-cm hemisphere was constructed like a giant ice cream scoop; it had a 2.5 cm wide (10.2°), flat band pivoted at the rim by two bearings. The band was moved manually; its angle could be read on a protractor, and also sensed electrically by a potentiometer that moved with the band. The band could be made black or white, and its width could be made greater than 10.2°. Objects inside the hemispheres were illuminated by a 6.3 V dc, 24 W, high-intensity tungsten lamp. Illumination was nearly uniform on the object surface.
A modified ophthalmoscope was used to project a microbeam of intense white light directly on the eye's surface; as few as seven ommatidia could be illuminated at a time with this device to test for simple, local ON and OFF responses
Neurophysiological recordings from OL units were extracellular, made with electrolytically sharpened Pt-30% Ir microelectrodes. About 3 to 5 (m of Pt-black-coated tip was exposed; the rest of the electrode shank was glass-insulated. Conventional low-noise amplification was used to condition spike signals.
Northrop and Guignon (1970) approached the problem of unit classification from a parsimonious point of view. They were primarily interested in finding if the insect visual system could extract object size information (i.e., responded selectively to objects containing a restricted range of spatial frequencies), as well as to the direction of moving objects. Unit responses to simple changes in general and local illumination were not considered to be as important as any potential, spatiotemporal filtering operations. On the basis of tests used, Northrop and Guignon (1970) were able to claim five OL unit classes, described below:
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