April 1959 LETTERS TO THE EDITOR 411 Peculiar Form of Interaction between Cones and Rods Revealed by the Electroretinogram of a Human Dark-Adapted Eye LUCIA RONCHI Instituto Nazionale di Ottica, Arcetri, Florence, Italy (Received October 6, 1958) I N a recent research 1 on the behavior of the electroretinogram recorded from a dark-adapted human retina with increasing intensity, we found an unexpected increase in the size of the b wave at very high levels. We recall briefly that the angular aperture of the test field, centrally fixated, is about 3°. This behavior is not influenced in a relevant manner by an increase of the duration of the stimulus and holds for blue, green, or white light. When the height of the b wave is less than 500 μv, its nature is scotopic, the cone response being strongly inhibited 2 ; at this level, where the b wave seems to be "saturated," a further increase of luminance seems to release the inhibition, and the response is now due to the combined action of cones and rods. 1 In this region, a peculiar and unexpected increase of size is tested, as is shown in Fig. 1, until finally the size decreases abruptly, and the response seems to consist only of a purely photopic x wave.
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April 1959 L E T T E R S TO T H E E D I T O R 411
Peculiar Form of Interaction between Cones and Rods Revealed by the Electroretinogram of a
Human Dark-Adapted Eye LUCIA RONCHI
Insti tuto Nazionale di Ottica, Arcetri, Florence, Italy (Received October 6, 1958)
IN a recent research1 on the behavior of the electroretinogram recorded from a dark-adapted human retina with increasing
intensity, we found an unexpected increase in the size of the b wave at very high levels. We recall briefly that the angular aperture of the test field, centrally fixated, is about 3°. This behavior is not influenced in a relevant manner by an increase of the duration of the stimulus and holds for blue, green, or white light.
When the height of the b wave is less than 500 μv, its nature is scotopic, the cone response being strongly inhibited2; at this level, where the b wave seems to be "saturated," a further increase of luminance seems to release the inhibition, and the response is now due to the combined action of cones and rods.1 In this region, a peculiar and unexpected increase of size is tested, as is shown in Fig. 1, until finally the size decreases abruptly, and the response seems to consist only of a purely photopic x wave.
412 L E T T E R S T O T H E E D I T O R Vol.49
FIG. 1. The height of the b wave (expressed in μv) is plotted against the log of luminance (in arbitrary units).
Thus a peculiar interaction, which paradoxically looks like an "adding up" process between cone and rod responses occurs just before the final inhibition of the rod response.
We would like to emphasize that a sort of analogous interaction probably occurs when the eye is presented with a spatial gradient of luminance, provided its slope is sufficiently high; the bright Mach band3 seems to occur just at the point where the rivalry of two different types of inhibitions occur. We cannot exclude the fact that the spatial distribution of luminance results from more complex form of interaction between adjacent elements than that shown in Fig. 1. In a provisional explanation we might suggest that "it is as if" the occurrence of the spatial interactions modifies, but does not radically alter the behavior represented in Fig. 1.
A recent theory due to Fry4 seems to confirm partly at least our suggestion.
1 Lucia Ronchi, "On the behavior of the human electroretinogram with increasing intensity" Atti Fond. G. Ronchi (to be published). 2 R. Granit, Sensory Mechanisms of the Retina (Oxford University Press, London, 1947). 3 Adriana Fiorentini, Atti Fond. G. Ronchi, 371 (1955).
