MM units were the most frequently encountered unit class (46% of the 181 units studied). These neurons fired erratically in both steady-state light (LSS) and dark (DSS) in the absence of a moving object, their average rate slightly higher in LSS. There was always an MM burst at ON and at OFF of general illumination. The receptive field of a MM unit was the entire ipsilateral eye. Maximum response was elicited by small, jerky object motions in random directions. MM units responded maximally to motions of a black disk, square, or triangle from 3° to 5° in diameter, although good responses could be obtained from white test objects of the same size range. Continuous, smooth motion of a spot in one direction did not give a sustained response. If a spot were jittered in a restricted region of the hemisphere, an MM unit would quickly adapt; if the same spot were moved to an area viewed by an unstimulated area of the eye, the MM response would again return, then adapt. The moving stripe of the "ice cream scoop" object would cause an MM unit burst only at the onset of motion. There was no motion directional sensitivity for any object to which an MM unit responded.
MM units behaved similarly in their responses to visual objects as the descending contralateral movement detector (DCMD) neuron axons in the ventral nerve cord. However, MM units were unique in that they could be caused to fire faster when the animal was given tactile stimuli (e.g., stroke the legs and abdomen with a paintbrush or fine wire), or acoustic stimuli (e.g., loud clicks). Tactile and acoustic stimuli adapted far more slowly than responses to jittered spots. Mechanical and visual stimuli given close in time generally revealed no particular response poten-tiation of one vs. the other.
Northrop and Guignon (1970) used radio-frequency lesioning to estimate the recording sites of OL units. Of 30 MM units lesioned, 8 were found on the proximal, distal, and lateral surfaces of the medulla, 3 were in the outer chiasma (between the lamina and the medulla), 2 were in the lamina, 5 were in the volume of the medulla,
7 were in the lobula, and 5 were in the inner chiasma (between the medulla and lobula). Receptive fields of the MM unit were both CEs (ipsilateral and contralateral).
MM units were also found in Locusta by Horridge et al. (1965) (type CE) and in moths by Blest and Collett (1966). Over 30 years have past, and the role of these ubiquitous units is still not known. A good guess is arousal, a sort of sensitivity control triggered by high-spatial-frequency object's new movement anywhere in the animal's visual field, or by a novel sound or touch.
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