Information Storage in Nerve Cells 199 



Background electrical activity in such neuronally isolated regions 

 is much less prominent than in normal cortex even when the 

 animal is awake. The pattern more closely approximates the 

 deeply anesthetized state. The cells perhaps are not so busy doing 

 other things and are therefore more a\^ailable for recruitment by 

 an active pacemaker. Thus it is not surprising" that the responses 

 obtained are similar to those of Figure 7A. Nine per second shocks 

 before tetanization produced only a small DCR. After tetanization 

 a new response emerged which was tightly locked to the stimulus 

 frequency. Changing the stimulus frequency to three per second 

 perturbed the system somewhat but clear responses did appear 

 at the new frequency. When the stimulus voltage was reduced 

 by about half (Fig. 8A) the responses were also reduced and were 

 less reliably evoked. Further reduction of stimulus voltage abolished 

 all response (Fig. 8B) but returning to the pre-tetanization voltage 

 restored the stimulus-locked response in full (Fig. 8C). When the 

 stiinulus was turned off (Fig. 8D) there was no trace of an after- 

 discharge. 



The variations in stimulus voltage just described indicate that 

 the response arising after tetanization depends upon the specific 

 stimulus. It cannot be explained as an apparent coherence re- 

 sulting from emergence of a background rhythm, paroxysmal or 

 not, having frecjuency characteristics close to those used to stimu- 

 late. Finally, as suggested above, this experiment supports the 

 notion that the tetanization effect is transmitted to the cells at 

 the recording site by non-synaptic means. 



Figure 9 illustrates an experiment similar to that just described. 

 Electrodes were arranged exactly as in the diagram of Figure 8 

 within and beside a neuronally isolated cortical region. Direct 

 your attention particularly to Figure 9B. In this experiinent the 

 pre-tetanization stimulus frecjuency was nine per second. After the 

 tetanus an altered but stimulus-locked response was evident at 

 nine per second. At that point during a period coincident with 

 the second tracing the experimenter slowly changed the stimulus 

 frequency from nine to three per second. The response pattern 

 reflects the changing temporal character of the signal and locks 

 in at three per second (third tracing). But something else has 

 happened as well. Between the major three per second deflections 



