The term biological clock has been given to the phenomenon displayed by organisms, both plants and animals, that pace their functions in a cyclic fashion relat~d in some way to environmental stimuli. In this discussion, we shall be dealing primarily with formulation of a model for the timing mechanism by which organisms mark the passage of the hours of the day and with the external stimuli which influence this timing mechanism.
The electronic model presented in this report has attempted to circumvent these difficulties. This model parallels very closely the mathematical model in relation to determining the times for onset and termination of activity as separate problems.
A general block diagram of the proposed electronic model is shown in Fig. 14. Basically, the model consists of three functional units: (1) an endogenous clock unit, (2) a conditional response unit, and (3) decision units. The endogenous clock unit includes the master clock and trigger, the activity control, and the animal state circuits. These circuits are used to pace the animal’s internal rhythms and to supply information concerning the animal’s present state to the conditional response and decision units. The conditional response unit comprises the analog portion of the model and is used to determine the amount of phase shift that should be applied, while the decision or logic units make all of the required decisions for the model in terms of past and present information concerning the endogenous clock unit and external stimuli.
In typical operation of the model, an advancing or delaying signal in the form of a positive or negative voltage level will be added with a voltage sweep generated by the master clock. When the combination of the two voltages reaches some preset value, the output of a comparitor indicates the onset of activity. If no advancing or delaying signal is present, the time between any two successive onsets of activity will be equal to the endogenous period of the animal. At an onset of activity signal, the activity control begins counting the animal’s normal and minimum active periods. The logic circuits determine whether the animal should advance or delay his onset of activity or become inactive. After the logic circuits have determined that a delay or advance should be made, the conditional response controller determines the amount of phase shift to be applied to the endogenous clock unit.