High correlation between absolute psychophysical threshold and the scotopic threshold response to the same stimulus

Abstract

The scotopic threshold response (STR) is a negative potential to low intensity light recorded from the dark-adapted retina. It reflects inner retinal function. The STR is now recorded routinely as part of our electroretinography protocol. The projected absolute threshold calculated from the amplitude versus intensity function for the STR has been found to correlate well (r=0.59) with the absolute subjective threshold to the same stimulus. The correlation holds for about 1.5 log units above normal. With further elevation the STR is usually abnormal or absent as b wave threshold is approached. The correlation would have been much greater were it not for this truncated range over which both are recordable. This report includes our findings in 127 patients and examines several disease groups. When the STR had a reduced and abnormal intensity series or was absent, the subjective threshold was elevated in almost all cases (92.2%). The converse relationship also held. When there was a discrepancy, recording problems were usually identified. Since the STR requires difficult, time consuming signal averaging for reliable recording, it may be adequate to record the subjective threshold alone to provide a relatively easily recordable indicator of inner retinal function. Additional STR recording will seldom be warranted.