Neuronal Basis of A Sensory Analyser, the Acridid Movement Detector System

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Abstract
The Lobular Giant Movement Detector neurone (LGMD) of Schistocerca responds with spikes when small areas of the visual field change in luminance. Previous work has shown that changes of ± 1 log10 unit are enough to produce maximal ON and OFF responses. Using a 5° test area, it is shown that the number of spikes generated by such a stimulus depends on the luminance of the surrounding area. When the surround is dark, the response is maximal; when it is brightly lit, the response is minimal. Intermediate intensities produce intermediate values of response. A × 2 change in response is produced by about 3 log10 units change in surround intensity. A bright annulus, with diameters of 10·5° and 25·8°, inhibits both ON and OFF responses when concentric with the 5° test area, but not when it is 30° eccentric to the test area. The inhibitory effect shows no decrease after 4 min. These results are interpreted to indicate a tonic lateral inhibitory network, sited peripherally in the optic lobe prior to the divergence of the separate ON and OFF channels found in the projection from the medulla to the LGMD. It is probably identical with that described for the lamina by previous workers.