Scotopic Threshold Response (STR) of the Human Electroretinogram

We recorded the human ERG using full-field stimuli at light intensities near absolute threshold to examine characteristics of the scotopic threshold response (STR). The human STR was seen below PII threshold and was the only component of the ERG evident near absolute rod psychophysical threshold. The human STR was detectable in the corneal ERG at stimulus intensities 0.6-1.0 log units above psychophysical threshold and had about a 2 log unit range to apparent saturation. Maximum STR amplitude was 12-20 microV. The STR latency ranged from 100-185 msec, depending on stimulus duration and intensity. The STR returned to baseline by 300 msec after onset, for very brief flashes, but it was prolonged with longer flashes. The spectral characteristics of the human STR matched rods and not cones. The STR followed Bloch's law and exhibited temporal integration for at least 80 msec. At 2.5-3 log units above visual threshold, corneal positive PII (b-wave and d.c. component) progressively obscured the negative STR. We propose that the human STR reflects post receptor processing in the retina. This is based on the similarity of the human STR to the STR of the cat and monkey, both of which originate at postreceptoral sites. Thus the human STR may find clinical use to evaluate the rod pathway in the proximal retina.