6i6 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



the essentials of physiological optics were beginning 

 to be clear. In the first few chapters of his book, The 

 Retina, Polyak (ii) summarized in scholarly and 

 interesting style the early history of this subject, from 

 the Greeks and Arabs through Medieval to Modern 

 times. 



As the science of optics developed, it took two 

 paths. On the one hand, the physics of light emerged. 

 Optics through most of its history depended ulti- 

 mately on visual observations made by the human 

 eye as the final detecting and measuring instrument. 

 Only relatively recently have physicists been able to 

 replace human vision to ad\antage by the photo- 

 graphic plate and by elaborate photoelectric detecting 

 and recording devices. The laws of reflection and re- 

 fraction were first derived by simple visual operations, 

 conducted in a scientific manner. This history is 

 discussed in some detail in a recent paper by Ratliff 

 (13). Combined with the lens maker's art, the physics 

 of lenses and mirrors developed into our present day 

 geometrical optics. Physical optics is based on the 

 observation of the maxima and minima detected by 

 the eye in interference and diffraction patterns, and 

 by brightness changes produced by polarization op- 

 tics. Photometry was, and still is to some extent, 

 dependent on the ability of a human observer acting 

 as a null device to detect very small inequalities in 

 brightness in an illuminated field. Even color vision, 

 properly a subject belonging to physiology, has 

 fascinated physicists from the time of Newton, when 

 it formed the basis for the emerging science of spec- 

 troscopy. As these physical sciences developed, they 

 in turn were applied to the eye itself, and physiologi- 

 cal optics resulted. 



Physiological optics had its great flowering in the 

 last century, with the epochal work of v'on Helm- 

 holtz. In its essentials and in many of its details, the 

 physics of the dioptric system of the human eye was 

 put into satisfactory shape by Helmholtz, and is em- 

 bodied as part of a broad study of visual physiology 

 in his three monumental volumes Handbuch der 

 Physiologischen Oplik (16). 



Physiological optics is by no means a finished sub- 

 ject, as shown plentifully in Fry's chapter. Even the 

 physics of the eye, narrowly defined, invites creative 

 effort today. In a broad sense, physiological optics is 

 often taken to include most of visual physiology. 

 Perhaps this is too broad a definition, but it is wise 

 to avoid drawing arbitrary boundaries to this field. 



Photosensitivity, that essential property that makes 

 a visual organ possible, is conferred upon the special- 



ized receptor cells of an eye by their possession of 

 certain chemical substances that can absorb light 

 (and therefore are pigments) and undergo photo- 

 chemical change. This reaction must be such as to 

 initiate a change of events in the irritable mechanism 

 of the receptor, leading to the transmission of nervous 

 influences along the optic pathwav. 



The \isual pigment of the retinal rods of the verte- 

 brate eye was discovered by Boll and carefully in- 

 \estigated by Kiihne nearly a hundred years ago. 

 The essential importance of ' visual purple' or " rho- 

 dopsin' in \ision was questioned for many years be- 

 cause of two misconceptions. First, it was argued 

 that since no such pigment could be observed in the 

 cones, none was there. True, the pigment of the cones 

 is different from, though closely related to, that of 

 the rods, and it is more diflicult to detect; but modern 

 methods are adequate for its detection in the cones 

 and its extraction and study in vitro. The second, and 

 less obviously fallacious argument was that the visual 

 purple in a retina bleached on exposure to light, and 

 yet photosensitivity remained. It was not realized 

 that the restorative processes (already described by 

 Kijhne) would operate in light as well as darkness, 

 and would lead to a'stationary state' in which a small 

 but significant amount of visual pigment would be 

 present in the receptor for indefinitely long periods. 

 Even in bright light, an active turnover of visual pig- 

 ment, with photolysis and regeneration, takes place 

 continually, and photosensitivity, while reduced, is 

 still present. The clear, quantitative formulation of 

 these ideas by Hecht in his classic studies of the 

 photosensory mechanism of the clam, Mya, opened 

 a new era of visual physiology. Before the advent of 

 modern biochemistry, Hecht applied these ideas of 

 photolysis, regeneration and the stationary state to 

 ijasic \'isual phenomena such as light and dark adap- 

 tation, inten.sity discrimination and flicker. The 

 experiments that he and his colleagues performed 

 using animals and with human observers, and the 

 theories they devised to explain their results, still 

 play a fruitful role in the field of visual physiology (9). 

 But by now it has become clear that Hecht's ideas, 

 while basically sound, were oversimplified, and need 

 to be reworked in the light of more recent biochemical 

 developments. 



At the present time, the significance of visual purple 

 and the photosensitive substances related to it is 

 firmly established. The biochemistry of these visual 

 pigments is one of the most actively pursued and 

 most exciting topics of receptor physiology, as amply 



