ELECTRICAL PHENOMENA IN VISION 611 



PHI in Granit's analysis) were enhanced by cathodal stimulation of the 

 interior of the eye, but that both were depressed by currents of opposite 

 polarity. This they took to mean that PII and PHI are components 

 whose negative and positive signs are inherent within the underlying 

 structures. Their antagonistic properties are thus not to be considered 

 as resulting from opposite orientations with respect to the electrodes. 



POLARIZATION OF THE EYEBALL 



An interesting development is experimentation in which prolonged 

 polarizing currents have been applied to the eye. One application of 

 this technique has been to study the effects of such polarizing currents 

 upon human visual sensitivity. Kravkov and Galochkina (1947) have 

 performed experiments in which anodal stimulation of the eyelids (with 

 the cathode in the subject's hand) appears to lower the absolute threshold 

 for blue-green light while raising the threshold for orange and red regions 

 of the spectrum. Cathodal stimulation has opposite effects. Dark adap- 

 tation is also said to be significantly influenced both during and after 

 electrical polarization of the eyeball, and again opposite effects are 

 obtained with opposite polarities. The authors attribute these effects 

 to changes in the relative concentrations of potassium and calcium ions 

 within the retina, supporting this conclusion with experiments in which 

 these concentrations were altered by means of iontophoresis. Sensitivity 

 to green light was raised by calcium and lowered by potassium. 



POLARIZATION OF INDIVIDUAL RETINAL UNITS 



A second application of polarizing currents has been to study their dif- 

 ferential effects upon the responses of individual retinal units. Gernandt 

 and Granit (1947) applied microelectrodes to the nasal retina of the 

 cat and listened, with the aid of an amplifier and loud-speaker, to the 

 responses of small units of the retina (presumably from ganglion cells or 

 their fibers). Electrical polarization was through electrodes on the nasal 

 and temporal surfaces of the eyeball. When the nasal electrode is the 

 cathode, the pure on-elements respond to the onset of the electrical cur- 

 rent ; the pure off-elements respond to the cessation of the current ; and 

 the on-off-elements are mixed, some responding to the onset and some to 

 the cessation of the current. When the nasal electrode is the anode, 

 however, the normal functions are upset in that on-elements respond to 

 the cessation of the current and off-elements to its onset. On-off-ele- 

 ments again respond to either onset or cessation, but in their case the 

 situation is greatly complicated by wide variations from one element to 

 another in relative sensitivity to cessation or onset of light. Some of 

 the on-off-elements are highly off-sensitive, and these particular elements 

 behave like off-elements in responding readily to cessation of cathodal 

 stimulation. The on-sensitive elements (low off/on ratios for stimulation 



