Circadian Inputs Influence the Performance of a Spiking, Movement-Sensitive Neuron in the Visual System of the Blowfly

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Abstract
Long-term extracellular recordings from a spiking, movement-sensitive giant neuron (H1) in the third optic ganglion of the blowfly Calliphora vicina (L.) revealed periodic endogenous sensitivity fluctuations. The sensitivity changes showed properties typical of an endogenous circadian rhythm. This was true for the responses in reaction to intensity changes of visual patterns as well as for the responses elicited by pattern movement. For these two types of stimuli, the circadian fluctuations were comparable, but the envelope in the case of responses to movement was more robust. A circadian fluctuation in responses to movement is, therefore, present at the level of single elementary movement detectors. The tonic activity of the neuron was also shown to be under circadian control. In constant darkness (DD) the fluctuation was circadian, whereas in constant light it was not. The subjective light-dark (LD) transitions in the tonic activity in DD closely followed the LD transitions in the holding cages initially; that is, there was low activity at night and high activity during the daytime. The sensitivity fluctuations in response to visual stimuli led the tonic spike activity fluctuations by several hours.