128 Neural Aspects of Vision \1 : 2 



Similar experiments, using much smaller test fields, show that in the 

 photopic eye the central 20' of the fovea lacks blue, and the central 15' 

 lacks both blue and yellow. This is to be expected from the model 

 under discussion for the fovea contains no rods. In the absence of rods, 

 there would also be no d or m type cells. At 570 m^u, a wavelength in 

 the yellow, the central 15' of the fovea give a gray sensation. This 

 supports the antagonistic roles of green and red used in the model at the 

 p type cells. (Note that yellow would normally be sensed by the m type 

 cells, supposedly missing from the fovea.) 



Time" measurements have fascinated many biophysicists. In the eye, 

 one can measure kinetic curves of recovery rate to bright illuminations 

 of various durations. At least four different time constants can be 

 found by these experiments. For short flashes of 0.02-0.10 sec, there is 

 a very rapid recovery. For longer exposures, there is an after image 

 for 0.5 to 5 sec, a recovery of the cone threshold from 20 to 200 sec, and 

 a recovery of the rod threshold (dark adaptation) between 4 and 40 

 minutes. Talbot's model with three receptors, k, /, and i cells, all 

 contributing to the time constants, predicts the existence of several 

 kinetic curves, more so than the above experiments reveal. 



Additional kinetic constants can be found from stimulating the eye by 

 means other than light. A wide variety of stimuli, such as magnetic 

 fields of 60 cps, electrical stimuli, and excess pressure, all produce visual 

 sensations. In the dark-adapted eye, the sensation is reported to be 

 blue, corresponding to the fact that the large fibers of the d-m system 

 are easiest to stimulate. In a series of experiments, eyes were light- 

 adapted and then the electrical threshold stimulus needed to elicit a 

 light sensation was determined. Under these conditions, a series of 

 kinetic recovery curves is obtained which are more rapid than the times 

 for recovery of rod and cone vision. Hence, the neurons themselves 

 must be stimulated. A final conclusion from these experiments is that 

 fatiguing or blocking does occur at the neural level within the retina, 

 so that the 1° and 2° test patch experiments are not exclusively measure- 

 ments of dye spectra. 



Concerning abnormal vision, Figure 2 has arrows or numbers marked 

 for the structures suggested missing in (1) protanopia (the i cones), 

 (2) deuteranopia (the p cell fiber to the optic nerve), (3) tritanopia (the 

 rods), and (4) tetartanopia (the yellow connections from e and /to d). 

 The detailed account will not be reviewed here. Suffice it to point out 

 that, with a complex system of this nature, plus duplicate mechanisms 

 within the central nervous system, there is almost no end of types of 

 color blindness possible. The theory of vision outlined in Figure 2 can 

 account for any known or conceivable type of color blindness. 



In the present section, the results of subjective experiments on color 



