RELATION BETWEEN PREY AND PREDATOR — ROEDER 29I 



The simplest form into which a change in the external world can 

 be coded by a sense organ is a single propagated impulse in one nerve 

 fiber. Such information reaching the nervous system could initiate 

 only an all-or-nothing response just as the sounding of the alarm in 

 the station house can initiate only a maximum effort in the fireman 

 irrespective of the size of the fire. While this condition may be ap- 

 proached in some of the alarm systems of insects, most sense organs, 

 even those composed of one or a few units, are able to transmit to 

 the nervous system some impulse-coded information on stimulus in- 

 tensity, form in time or space, and quality. 



This principle is illustrated here by electrical recordings of nerve 

 impulses in the tympanic nerve of a noctuid moth, Prodenia eridania. 

 When the ear is exposed to a continuous pure tone (pi. 2, fig. 2) the 

 most sensitive of the 2 acoustic sensilla responds with a short burst 

 of impulses due to the abrupt onset of the tone or switching transient. 

 It then continues to fire with a falling frequency which depends upon 

 the intensity of the tone. At a sound intensity of 23 decibels above 

 that producing the threshold response the second less sensitive sen- 

 sillum adds its electrical response to complicate the record. If the 

 sound is a very brief click (pi. 3, fig. i) its intensity is represented in 

 the nerve response by the number of impulses in the after-discharge, 

 which may continue for 10 to 12 milliseconds after the cessation of 

 the stimulus, and by the response of one or of both acoustic units. 



A third temporal dimension of the response which varies with 

 stimulus intensity is the latency. This interval between stimulus and 

 response becomes shorter at higher sound intensities, decreasing by 

 about 2 milliseconds between A and D in plate 3, figure i. 



In this simple system the dimension of intensity in the external 

 change is coded as a dimension in time (duration, latency, or fre- 

 quency of discharge), the form in which information is most fre- 

 quently dealt with by the nervous system. If the population of sense 

 cells consists of two or more with differing thresholds the stimulus 

 intensity could be centrally represented in a spatial form as well, when 

 discrimination of stimulus quality and direction also becomes possible. 

 By observing on the oscilloscope the response patterns and the relative 

 latencies recorded simultaneously from the right and left ears of a 

 moth exposed to a movable source of clicks it is quite possible to tell 

 whether the source is located on the right or left sides or in the 

 median plane of the moth (Treat and Roeder, unpublished). The 

 most obvious difference between responses of the two ears (pi. 3, 

 fig. 2) is the shortening of the latency on the side receiving the 



