THE DYNAMICS OF SIMPLE CIRCUITS 29 



(a- b) 2 < 4 abaft , 



the roots are complex and the approach to equilibrium is fluctuating 

 with a frequency v satisfying 



-16ji 2 r 2 = (a- b) 2 - 4 abaft . 



In this circuit it is plain that permanent activity is only possible 

 when | 2 > . Thus the simplest circuits which exhibit fluctuation 

 are those consisting of one excitatory and one inhibitory neuron, and 

 a circuit so constituted can maintain permanent activity in the ab- 

 sence of external stimulation only if both neurons originate and termi- 

 nate at the same synapse and A/a > B/b . This is, of course, quite 

 evident intuitively. 



