1. Extracellular recordings were made from single units in the tritocerebrum of locusts. The units responded to objects moved anywhere in the contralateral visual field. They were relatively insensitive to stationary stimuli and did not respond to auditory or tactile stimulation.

2. Responses to successive movements gradually declined even when an interval of several minutes elapsed between each movement. The number of impulses evoked by a stimulus was strongly influenced by the amount and timing of previous stimulation. Such influences occasionally lasted for several hours.

3. The responses of some cells to the first few groups of stimuli presented at the beginning of an experiment were non-decremental, but the response to subsequent stimulation was always decremental.

4. Many of the units showed directional preferences, discharging more vigorously to movement in one direction than in the reverse direction. In no case was movement inhibitory to the cell.

5. The directional preferences were modified by primacy so that for a number of presentations a unit responded preferentially to the directional characteristics of the first of a group of stimuli.

6. The cells were insensitive to movement of the whole visual field brought about by movement of the whole animal, but still retained their responsiveness to relative movement within the visual field.

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