p567fig15.38 (a) The Gated Pacemaker model for the control of circadian rythms is a recurrent shunting on-center off-surround network whose excitatory feedback signals are gated by habituative transmitters.  Tonic arousal signals energize the pacemaker.  Diurnal (left) and nocturnal (right) pacemakers are determined by whether phasic light signals turn the pacemaker on or off.  An activity-dependent fatigue signal prevents the pacemaker from becoming overly active for too long.  (b) Two simulations of circadian activity cycles during different schedules of light (L) and dark (D).  See the text for details.<BR>||  sourceOn-> on-cells (recurrent) <-(-) (-)> off-cells (recurrent) <-sourceOff.  on-cells-> activity-> off-cells.  off-cells-> fatigue.  Diurnal: sourceOn=[light, arousal]; sourceOff=arousal;.  Nocturnal: sourceOn=arousal; sourceOff=[arousal, light];.