Dishabituation and Arousal in the Response of Single Nerve Cells in an Insect Brain

In previous papers (Rowell & Horn, 1967; Horn & Rowell, 1968) the behaviour of certain neurones in the tritocerebrum of the desert locust have been described. Among other characteristics, these cells respond briskly to the movement of small dark objects in the visual field of the contralateral eye, irrespective of the movement of the animal itself. If a given movement is repeated, the response wanes (habituation), though the original level may be regained after a period of rest. Response decrement is not due to changes affecting the recorded cell directly, for it responds at the original level to an identical stimulus presented elsewhere in the visual field. The time courses of habituation and of recovery from it in these cells are influenced in a complex fashion by various parameters of the stimulus routine. These influences are discussed elsewhere (Horn & Rowell, 1968). In this paper we report on a further group of factors affecting response level of tritocerebral neurones. For example, activity in regions of the central nervous system remote from the recorded cell can produce medium and long-term changes in responsiveness. These influences can be grouped together as arousal phenomena. Arousal has been little studied at the individual neurone level in invertebrate nervous systems (but see Kennedy & Preston, 1963), though there is a large body of work, concerned mainly with the reticular activating system, on arousal in vertebrates. The events treated here are (1) dishabituation brought about by electrical stimulation of the nerve cord and (2) 'spontaneous' and diurnal changes in responsiveness.