458 ANNUAL REPORT SlSnTHSONIAN INSTITUTION, 1961 



The moth ear responds in this manner to tones from 3 kc. to well 

 over 100 kc, but there is no evidence that it is capable of discriminat- 

 ing between tones of different frequency. It is most sensitive near 

 the middle of its range, that is, to frequencies such as those contained 

 in bat chirps. 



In plate 2, figure 2, it will be noticed that, in each of the recordings, 

 the intervals between the successive impulses increase as the pure 

 tone stimulus continues. In terms of the nerve code outlined above, 

 the A cells report that the sound is declining in intensity with time, 

 although in fact it was kept constant. This adaptation to a constant 

 stimulus occurs in most receptors registering changes in the outside 

 world. In terms of our own experience, the impact of our sur- 

 roundings would be shocking and unbearable if it were not distorted 

 in this manner by sense organs. The brilliance of a lighted room 

 entered after dark w^ould continue to be blinding and the noise of a jet 

 engine would remain unbearable. However, the A cells of the moth's 

 ear adapt very rapidly to a continuous tone, and their full effectiveness 

 as pulse detectors is revealed only when they are exposed to short 

 tone pulses similar to bat chirps. 



In the experiment illustrated in plate 3, figure 1, a tone pulse of 3 

 msec, duration was generated at regular intervals. It is similar to a 

 bat chirp except for its regularity and the absence of frequency 

 modulation. A microphone (upper trace) and moth ear (lower trace) 

 were placed within range, and the intensity of the stimulus pulse was 

 adjusted so that it just produced a detectable response in the most 

 sensitive A fiber (0 db). The intensity was then increased by 5 

 decibel - (db) steps as each recording was made. It will be seen tliat 

 the microphone begins to detect the sound pulse when it is about 10 db 

 above the threshold of the most sensitive A cell in the moth's ear. As 

 before, the increase in frequency of A impulses is evident if the 5 and 

 10 db records are compared, and a response of the less sensitive A cell 

 appears first in the 25 db record where the extra peaks of its action 

 potentials overlap those of the more sensitive A unit. In addition to 

 these two ways of coding intensity, two more can now be recognized. 

 If the interval between detection of the sound by the microphone and 

 by the moth ear is compared at different sound intensities, it will be 

 noticed that the tympanic nerve response occurs earlier and earlier on 

 the horizontal time axis. In other words, the latency of the response 

 decreases with increasing loudness. Also, the sense cells are seen to 

 discharge impulses for some time after the sound has ceased, and tliis 

 after-discharge becomes longer with increasing sound intensity. 



^ The decibel (db) notation expresses relative sound pressures. An intensity of 20 db 

 Is tenfold that of the reference level (0 db), a 40-db sound is a hundredfold the reference 

 level. 



