Arousal shifts in quiescent locusts

Locusts are usually quiescent at night, but this state can be interrupted by spontaneous periods of motor activity, or arousals, that can also be induced by exposure to light stimuli. To investigate whether repeated arousing stimulation has any lasting effect on behaviour, locusts were confronted at night with a series of 1 s light stimuli. Groups of three stimuli at intervals of 60 s were repeated 11 times at 10 min intervals during the first experimental night, and three stimuli at intervals of 90 s were repeated at 15 min intervals during the next night. Arousals and the effects of stimulation were monitored as changes in the spike activity of muscles in the basal part (the scapus) of the right antenna. In the early part of the night preceding the presentation of the light stimuli, neither 60 s nor 90 s periods were present as significant peaks in spontaneous changes in spike activity. The initial stimulus of a series evoked an arousal response that habituated on repetition of the stimulus. The end of the series of stimuli was followed by changes in spike activity that tended to have the same periodicity as the preceding stimuli. Furthermore, a single light stimulus at the end of the night evoked changes in spike activity that again tended to have the same periodicity as the preceding entraining stimuli. Repeated stimulation may therefore establish a memory trace for the period of stimulation that can be recalled either spontaneously or by the application of an appropriate external stimulus.