2 : 4/ Light and the Eye 43 



Chapter 4. For the purposes of this chapter, one should note that the 

 neurons are the functional units of the nervous system. Each consists 

 essentially of a cell body, a long process called an axon, and shorter 

 processes called dendrites. The rod and cone cell bodies are similar to 

 neuron cell bodies except that they are attached to photoreceptors in 

 lieu of axons. 



It should be emphasized that light goes through layers 10, 9, 8, 7, 6, 

 5, 4, 3 before being useful for vision in layer 2. The arrangement of two 

 layers of neuron cell bodies, with their connections to the rod and cone 

 cell bodies, as well as almost innumerable connections between neuron 

 cell bodies, is indeed complex. To those who have looked behind the 

 front panels of an electronic digital computer, the retinal structure 

 suggests strongly that the output of the rods and cones is analyzed in a 

 computerlike fashion by these layers of nerve cell bodies. And indeed, 

 it will be shown in Chapter 7 that electrophysiological evidence supports 

 this suggestion. 



Within the layers of nerve cell bodies, a number of different types of 

 cells have been discovered. In discussing the mechanism of color 

 vision, it is important to include these different types. A discussion of 

 their forms will be deferred until Chapter 7. 



4. Thresholds and Acuity 



In this section, three different types of measures of the sensitivity of the 

 eye are discussed. The first is the quantum threshold, that is, the 

 minimum number of photons necessary to elicit a sensory response. The 

 second is the relative sensitivity of the eye to light of varying wavelengths. 

 The last measure, the acuity, represents the keenness of vision and is 

 measured by the minimum separation of two objects that can just be 

 discriminated as two and not one. 



A. Quantum Thresholds 



Vision occurs when light is absorbed by the photosensitive rods and 

 cones. At the threshold of vision, only a minimum of light is necessary. 

 The absorption of light is best described in terms of quantum theory. A 

 natural question then is: How many photons must be absorbed by a 

 visual receptor (rod or cone) for the subject to see a flash of light? This 

 problem was first investigated in detail by the biophysicist S. Hecht. 



His first approach was to use light of wavelengths to which the eye 

 was most sensitive and to expose the eye to short flashes. The eyes were 

 dark-adapted to make their sensitivity a maximum. The number of 

 photons striking the cornea for a just noticeable flash was measured. 



