420 Comparative Animal Physiology 



works by means of measurements of sensory phenomena. It is generally agreed 

 that the' photochemical cycle is responsible for sensation in that it is the mech- 

 anism by which the sense cell is activated. However, Crozier's theory holds 

 that in explaining the shape of the curves for various visual phenomena only 

 the statistical distribution of thresholds need be considered. 



An additional theory is that of Householder and Landahl.^^ j^ this the- 

 ory the responsibility of determining the shape of curves for visual phenom- 

 ena is entirely removed from the sense cell and is placed in the central ner- 

 vous system (or possibly in the neurone layers of the retina). On the basis of 

 assumed dynamics of synaptic transmission, Householder and Landahl have 

 derived equations which describe the relationship between critical flicker, 

 light to dark ratio, and intensity, between relative brightness, light to dark 

 ratio, and frequency, and also between brightness discrimination and inten- 

 sity. Provision is made in the equations for the excitatory and inhibitory pro- 

 cesses at the synapses, and it is implied that different intensities of light 

 cause excitation of completely different sets of neurones. As far as they go, 

 the derived equations offer a reasonably good fit for the experimental data. 



Evaluation of the Householder and Landahl interpretation in terms of other 

 theories or of known facts is very difficult, principally because the equations 

 have been derived on the basis of a few assumptions concerning synaptic 

 transmission within the central nervous system, without reference to photo- 

 chemistry or to statistical distribution of thresholds. The equations are ap- 

 plicable to all types of sensory discrimination, and at present there seems to 

 be no way of proving or disproving them or even of estimating their potential 

 value. 



The Electrical Activity of Photoreceptors. Consideration of the function 

 of vision reveals that the light, which is the agent activating the visual 

 system, goes no further than the photosensitive membrane upon which it 

 is incident. However, the experience of the light, or its sensation, takes place 

 within the central nervous system. The evidences for the existence of a secon- 

 dary reaction in the retina, i.e., the catalysis of a substrate by one or several 

 photodecomposition products, into an activating agent T, have been pre- 

 sented (p. 408). However, this throws no light on the nature of the 

 agent T or the manner in which it initiates nervous activity in the optic 

 pathway. 



The data concerning the electrical activity of photoreceptors reveal certain 

 relationships that indicate its possible importance as a link in the chain of 

 events that constitute the peripheral visual mechanism. 



The Nature of Retinal Electrical Activity. The fact that electrical poten- 

 tials may be measured from the front and back of the vertebrate eye was 

 discovered by Du Bois Reymond in 1849," and the fact that this voltage 

 changes with the intensity of illumination was discovered by Holmgren in 

 1865.^" Since these pioneer observations were made, it has been established 

 beyond doubt that whenever a photoreceptor is illuminated (assuming a 

 viable photoreceptor 'and illumination above a minimal intensity), there oc- 

 curs a fluctuation in the steady potential difference which exists across the 

 eye. This steady potential difference is so oriented (Fig. 115, B, left) that 

 the apical ends of the photosensitive elements (rods and/or cones in verte- 

 brates) are negative to the basal ends. The change in this potential wrought 



