700 HANDBOOK OF PHYSIOLOGY ^' NEUROPHYSIOLOGY 



III 



FIG. 6. Components of the cat ERG: PI, PII and PHI. The two alternatives for PII drawn 

 on the basis of experimental results. [From Granit (64).] 



workers to assume that the ERG is an algebraic sum 

 of component processes of opposite signs. The methods 

 used for analysis have been based on variations of 

 stimulus intensity, duration and state of adaptation 

 as well as on direct interference with the ERG by 

 chemical agents and asph\xia. For an orientation in 

 this field the reader is referred to the detailed discus- 

 sion by Granit (69) supplemented by more recent 

 work (7, III, 114). The three components of the 

 author's analysis (fig. 6) are based on many observa- 

 tions in the classical literature (quoted in the intro- 

 duction) and certain of his own experiments (64, 80) 

 and have served for some time now as a summary and 

 a starting point for further work. 



There is general agreement about the existence of a 

 slow cornea-positive component such as PI which is 

 responsiiile for the secondary rise or (-wave (see also 

 114, 144). This requires fairly high intensity and not 

 too short exposures. There is much evidence to show 

 that the cornea-negative a-wave is the first .sign of 

 illumination (cf. 3, 34, 69) and that it passes over into 

 a slower negative phase which is often submerged 

 below a mainly cornea-positive rcspon.se but is some- 

 times visible. The component PHI appears to survive 

 damage to the retina better than the other ones. Noell 

 (114) uses poisoning with iodate to produce it in the 

 rabbit's eye. It has been suggested (69, 73) that the 

 negative PHI consists of two components, one fast 

 and the other slow. This view has been elaisorated in 

 consideraijle detail by Noell (114), particularly with 

 regard to the slow phase. 



A con\cnient way of making the retina respond 

 quickly to illumination by a fairly pure negative ERG 

 is to drop potassium chloride solution into the opened 

 bulb (83, 139). This is a well-known depolarizing 

 agent and accordingly the remaining cornea-negative 

 response to light cannot itself be a depolarization of 

 already depolarized structures. The cornea-positive 

 PII, however, is likely to represent depolarization by 

 light. Both components are increased by running a 

 polarizing current across the bulb, inside negative, 

 and are decreased by reversal of this current (18, 28, 



76). A negative ERG can be made positive Ijy drop- 

 ping alcohol into the bulb (19). 



At cessation of illumination PHI returns towards 

 the base line of the record (the .so-called resting po- 

 tential discussed below), first rapidly, then more 

 slowly. At least in the isolated state the slow returning 

 phase may appear as a kind of 'remnant negativity.' 

 [There are apparently still slower changes of poten- 

 tial, both negative and po-sitive (see 114), than the 

 ones generally counted as belonging to the ERG 

 proper.] At the same time the cornea-positive PII 

 ends at cessation of illumination, either by returning 

 to the base line or even going below it or else contrib- 

 uting to the rf-wave that otherwise would have been 

 due merely to interference between PII and PHI. 

 There is evidence for both alternatives in the litera- 

 ture according to the view of Granit (73). Further 

 experimentation with different eyes seems necessary 

 to establish the dominant event in difierent types of 

 eye (see 6, 82, 113, 141). 



In considering questions of this kind it is necessary 

 never to forget that the ERG is a mass response re- 

 corded at a distance from the sources generating its 

 potential. VVirth & Zcttcrstrom (150) illuminated the 

 cat's eye through perspex cones applied directly onto 

 the retina and found that illumination of an area of 

 20 mm- was necessary for maximal responses. Con- 

 sidering that the diameter of the rods is 0.D02 mm, 

 there is ample margin for a large variety of elementary 

 component responses to complicate the issue. A gen- 

 eral analysis can merely aim at describing dominant 

 features. Localized leads and localized light projec- 

 tions on the retina are necessary for a study of details. 

 If one illuminates through a glass electrode applied 

 directly onto the retina (20), the individual retino- 

 grams are very different in different places. 



There are a number of interesting features by which 

 the cornea-positive PII and the cornea-negative PHI 

 of the general analysis differ from one another. Figure 

 7.-I, which illustrates for the frog retina the effect of 

 reilluminating at different times after cessation of 

 illumination (46, 80, 112), shows that the cornea- 



