Abstract
Introduction. The primary rod pathway in human retina has a univariant rhodopsin spectral sensitivity peaking at ~500 nm; slower dynamics than cones with a b-wave time-to-peak of ~80 ms; no Off-bipolar cell contribution to decreases in intensity since the rod pathway lacks Off bipolars; and a response limit at ~10 cd/m2 due to some form of saturation mechanism. Beyond this limit the ERG switches to a cone response with a b-wave peak at ~40 ms. Methods. Full-field spectral ERGs were recorded with a RETeval device for 6 narrowband chromatic stimuli (magenta, red, yellow, green, cyan, blue) plus white, using 250 ms light pulses at 2Hz for 30 sec at 5 intensities from 6 to 600 cd/m2. Results. At the lowest (scotopic) intensity, the responses showed a surprising diversity, violating univariance. They were slow, with no Off-responses, but manifested two time courses. Blue, cyan and magenta responses had narrow b-waves peaking at ~60 ms, while those for green, yellow and white pulses had significantly slower b-waves peaking at ~90 ms. Negligible responses to red pulses confirmed the lack of a significant cone contribution to these dim (photopically-equated) stimuli. Conclusions. Paradoxically, the faster component was consistent with a mechanism maximal at ~500 nm (cyan) - a rod-like spectral sensitivity – despite being much faster than typical rod responses. Conversely, the spectral sensitivity of the slower component was maximal in the green (~540 nm), approximating that of the M-cones, even though they are expected to be inactive at this dim adaptation level. This spectral tuning for the slower component seems to imply cone intrusion at these low intensity levels, despite its slow, rod-like time course, suggesting the existence of an ancillary pathway with a slower response time but enhanced sensitivity, such as a cone-to-rod gap junction network specific to the M-cone pathway.