VISUAL RECEPTORS AS BIOLOGICAL TRANSDUCERS 35 



the number of impulses in a group. Amplitude variations are effectively re- 

 moved by the "all or nothing" mechanism. 



One of the basic problems in receptor physiology is to determine how, in a 

 given receptor, the energy released by the initial mechanical, chemical or 

 thermal event is amplified and converted into the kind of information carried 

 by nerves. 



In the case of the photoreceptors, beyond the initial photochemical process, 

 the mechanism of transduction is largely unknown; whereas, the mechanism 

 by which nerve impulses are propagated is getting to be understood. The bio- 

 chemists working forward from the initial chemical events in the receptor and 

 the electrophysiologists working backwards from the propagated impulses in 

 the attached nerve fiber are trying hard to close the gap. Being an electro- 

 physiologist, I am going to tell you how far backward Hartline 1 , Wagner 2 , 

 and myself have gotten on one particular receptor 3 . Let us start with the nerve 

 impulse. 



Fig. 1 is a copy of one of Hartline's old records which has been reproduced 

 in the literature many times. It shows the coded message from a single optic 

 nerve fiber of the eye of the horseshoe crab, Limuhis. The black band at the 

 lower part of each trace indicates the duration of the stimulating light and the 

 time marks at the bottom of each trace indicate fifths of a second. Each record 

 was taken with a light intensity a factor of ten greater than the one immediately 

 underneath it. It is evident that there is a latent period during which no re- 

 sponse occurs, an initial burst of impulses signaling a change in illumination, 

 a maintained discharge which is a function of the intensity of illumination and 

 a cessation of the discharge a short time after the stimulus is removed. 



Analysis of similar records shows an approximately logarithmic relationship 

 between the intensity of the light and the number of impulses discharged in a 

 given time interval. This is just an expression of the familiar Weber- Fechner 

 law of psychology. The important point is that the logarithmic relation is 

 determined by the receptor itself and not further along in the central nervous 

 system. An engineer would refer to this logarithmic response as volume com- 

 pression which is often used to prevent overloading of a communication chan- 

 nel, and, indeed, probably serves that purpose here. 



Hartline and his associates have studied many of the properties of the 

 Limulus photoreceptor by measuring the output under a wide variety of input 

 conditions and though the response is too complicated and too variable to per- 

 mit the deriving of an exact mathematical transfer function for the system, as 



1 H. K. Hartline, Rockefeller Institute for Medical Research, New York. 



2 H. G. Wagner, Commander M. C, USN. 



3 The author assumes full responsibility for the views herein expressed since, due 

 to the pressure of time, his colleagues have not had the opportunity of editing his 

 remarks. 



