Perception of Natural Zeitgeber Signals

Abstract

Biological clocks detect timing cues in their natural environment from the daily twilight transitions during dusk and dawn. Compared to lab conditions (lights on/off programs) these natural zeitgeber signals guarantee better external and internal synchronisation with respect to precision (onset of activity), threshold (of effective stimuli) and range of entrainment. This has been shown in humans, various rodent species, birds and arthropods. It is a so far unsolved problem, which sensory and neuronal mechanisms allow for twilight recognition, as image forming eyes extinguish timing cues via their light/dark adaptation. Neurobiological studies on the photoreceptor system of scorpions have lead to a preliminary network model which is mainly based on the re-afference principle. Scorpion eyes are controlled by the circadian clock and have a strong circadian sensitivity rhythm compensating for the external day/night changes. Furthermore, within these eyes there is a second afferent non-visual channel receiving a copy of the efferent circadian signal to correct the impact of the circadian adaptation on the perceived light program and to compare external and internal timing of dusk. This information flow may provide a clear signal to the pacemaker concerning its correct phase angle to the natural light dark program. Sensory models have been proposed in lower vertebrates and insects, where extraretinal and retinal photoreceptor systems may interact to recognise the spectral and intensity changes of twilight. The results and models discussed provide a basis for changing the experimental paradigms for the analysis of entrainment mechanisms in circadian systems.