Role of feedback loops in the scorpion circadian system

Scorpions are nocturnal animals whose circadian activity rhythm is entraine d via light/dark programs to the median and lateral eyes. Timing of light s ignals conveyed by the median eyes is modified by feedback from the circadi an pacemaker. Using a simulation program, NeuroDynamix, we demonstrated tha t these median-eye feedback loops cause a prolonged dead zone in the phase response curve. Also, entrainment by median eyes during simulated T cycles resulted primarily from brief photoreceptor twilight signals. We propose th at feedback loops reduce entrainment stability, but serve a necessary funct ion by limiting light-generated inputs to the pacemaker to a narrow circadi an gate. (C) 2001 Elsevier Science B.V. All rights reserved.