1967 Volume 38 Issue 1 Pages 1-13
Threshold value of a small continuous light increases as it approaches in distance to an inducing figure. CFF also becomes larger as a small flickering stimulus near the figure. However, these two results are opposing each other in terms of light sensitivity; since the raise of light threshold is the decrement of but the increase of CFF means the increment of light sensitivity. This discrepancy has not yet been explained.
First of all, the light stimuli employed here are not identical in these procedures. Light threshold is determined by gradual darkening of a small continuous light; thus the stimulus has neither clear onset nor offset. On the other hand, CFF is measured by alternating a lighting period and a dark period; thus, the stimulus has its clear onset and offset.
Recent work of the electroretinogrames of rods and cones indicates that a discrete light stimulus yields at least three different unit potentials: The on-potential at the beginning of illumination, the ordinary potential during illumination, and the offpotential immediately after the termination of illumination. And CFF is believed to be mainly depended upon the on- and off-potentials, while light threshold of continuous stimulus is related to the amount of the ordinary potential. It is also known that the off-potential tends to be larger in the light-adapted cone retina and to be sharper than the on-potential. Thus the obtained relationship between CFF and the distance between the flickering light and the inducing figure may be explained whether the on-reaction or off-reaction is more dominant in that test situation.
It has been shown in the previous paper that Type A stimulus facilitates the on-reaction while Type B stimulus reinforces the off-reaction. (The former reaction is dominant under dark adaptation and the latter under light adaptation.) The Type A stimulus has a clear onset but it disappears gradually, and the Type B stimulus appears gradually, but it has a sharp offset. Evidently, when the inducing light stimulus is placed near the test figure, the retinal cells on which the test figure is projected are adapted by the light. When it is placed away from the figure, the stimulated cells are adapted to dark since the test figure is always visually fixated.
Therefore, the purpose of this present study was to test whether or not the off-reaction becomes more dominant when the inducing figure approaches the test figure. The results obtained are shown in Tables 1, 2 and Fig. 6. Responces found in the centre of the human fovea changed from on-type to off-type as the inducing figure approached the fovea. This finding would explain why CFF is higher near the inducing figure and, consequently, why the sensitivity measured in terms of light threshold decreases while that of CFF increases as the inducing figure approaches the test figure.
Additional experiments were carried out in order to explain the other two contradictions in sensitivity on the bases of the same principle. One is the initial rise in CFF or in apparent brightness as a function of luminance of the inducing figure, the other is the negative effect of the inducing figure. As shown in Figs. 8 and 9, these two contradictions were also explained on the basis of the very same principle.
