The human photopic ERG luminance-response function: Analysis, interpretation and application

Abstract

Purpose: With progressively brighter flashes, the amplitude of the photopic b-wave first increases, briefly saturates and then diminishes gradually; a phenomenon previously presented as the Photopic Hill. We present reproducible key descriptors of this function. Methods: Full field photopic ERGs were recorded from normal subjects to flashes of white light (intensity: −0.8 to 2.84 log cd.sec. m−2; DTL electrodes) presented against a background of 17(N=18) or 32 (N=30) cd.m−2. Results were compared to luminance-response functions obtained using colored flashes (red, green and blue) or collected at different moment within a biological cycle (circadian or menstrual). Results: Analysis of Photopic Hills generated against the 32 cd.m−2 background yielded the following 7 function descriptors: Vmax: maximal b-wave (98±17 μvolts); K (.35±.21 log cd.sec. m−2): the flash intensity generating Vmax; Ka (−.47±.12 log cd.sec. m−2) and Kd (1.59±.48 log cd.sec. m−2): the flash intensities generating a b-wave half of Vmax amplitude on the ascending (Ka) and descending (Kd) limb of the Photopic Hill; Ka=b (1.45±.22 log cd.sec. m−2): the flash intensity where the a- and b-wave amplitudes are equal; amax(27±6 μvolts): the amplitude of the a-wave in the Vmax response, and; b/ amax (3.49±.70): the b-to a-wave ratio in the Vmax response. Modifying the recording conditions (such as: change in background luminance, use of colored flashes, circadian or other cyclical modulation or pathology) produced a significant and reproducible change in one or more of the above-mentioned parameters. Conclusion: Results indicate that we have identified yield highly reproducible features of the human Photopic Hill. Their use should raise the diagnostic sensitivity of the photopic responses to a level comparable to that reached when the scotopic responses are analyzed with the Naka-Rushton equation. Funded CIHR, Réseau Vision, GRENE.