24 



R. C. GESTELAND AND OTHERS 



Strength. The usual effect of changing the strength of the stimulus is 

 shown in Fig. 4. (We define stimulus strength as the amount of odorous 



Fig. 3. Responses to two puffs of butyric acid. The lower trace was taken less 

 than 1 min. after the upper. Sweep length, 10 sec. 



Fig. 4. Responses to increasing stimulus strengths. The top trace is the 

 smallest ; the lower, the largest puff of //-butanol. Sweep length, 10 sec. 



substance arriving per unit time.) Tt shows a unit responding to three puffs 

 of /2-butanol. The smallest is at the top. The three records were taken 

 far enough apart in time so that there is minimal effect of reduced sensi- 

 tivity because of the preceding stimulus. The pattern of the response is 

 strikingly similar in all three cases, even though the number of spikes 

 increases with stimulus strength. There is always a threshold effect, and 

 for a unit that has not had its threshold raised by multiple preceding 



