Two signals in the human rod visual system: A model based on electrophysiological data

Abstract

In the human rod visual system, self-cancellation of flicker signals is observed at high rod intensity levels near 15 Hz, both perceptually and in the electroretinogram (ERG). This and other evidence suggests that two rod signals are transmitted through the human retina with different speeds of transmission. Here we report a series of flicker ERG recordings from a normal observer and an observer who lacks cone vision. From these results, we propose a quantitative model of the two rod signals, which assumes (1) that the amplitude of the slow signal grows linearly with log intensity but then saturates at ~1 scot, td; (2) that the amplitude of the fast signal grows linearly with intensity; (3) that there is a difference in time delay of ~33 ms between two rod signals of the same polarity (or of ~67 ms if the signals are of inverted polarity); and (4) that the time delay of both signals declines linearly with log intensity (by ~10 ms per log scot. td). These simple assumptions provide a remarkably good account of the experimental data. Our results and model are relevant to current anatomical theories of the mammalian rod visual system. We speculate that the slower signal in the human ERG may reflect the transmission of the rod response via the rod bipolars and the An amacrine cells, while the faster signal may reflect its transmission via the rod-cone gap junctions and the cone bipolars. There are, however, several objections to this simple correspondence.

An extended 15 Hz ERG protocol (1): the contributions of primary and secondary rod pathways and the cone pathway

“…To summarize, we used the differences in both spectral and temporal frequency sensitivity of rods and cones in order to distinguish the rod and cone pathway contributions to 15 Hz ERG responses. In accordance with earlier studies [13][14][15]21], we found a minimum in the 15 Hz amplitude curves of the ERG responses to blue and green stimuli. The results provide evidence that only the rod circuit contributes to the ERG responses for flash strengths close to this minimum.…”

“…This is in contrast to the descriptive model of the primary and secondary rod pathway as developed by Stockman et al [14], who used a fixed time difference of 33.3 ms (180°at 15 Hz) between the two rod pathways for the entire range of flash strengths. This corresponds to the original hypothesis that the 15 Hz minimum is caused by destructive interference of rod pathway responses because they are in opposite phase [7,13,23].…”

“…Stockman et al [14] recorded ERGs to flicker stimuli of 8-21 Hz and found minima in several amplitude curves. Because these minima could not be explained by a fixed time difference between the two rod pathways, they suggested that the primary rod pathway responses decrease at higher flash strengths.…”

“…A decrease in the amplitude curve belonging to the primary rod pathway improved the fit of the model compared to an amplitude curve showing no decrease. If correct, this means that the first 15 Hz minimum is caused not only by destructive interference because of a 180°difference in phase, but also by a decrease in the primary rod pathway responses [14]. Furthermore, the secondary rod pathway may saturate at higher flash strengths.…”

“…At higher flash strengths, the perception of flicker reappears and becomes more pronounced [8][9][10][11][12]. In later studies, this disappearance of flicker perception was found to correlate with a minimum in the amplitude versus flash strength curve of 15 Hz flicker electroretinograms (ERGs) [13,14]. The minimum is accompanied by a phase shift of approximately 180°.…”