Extrinsic cone-mediated post-receptoral noise inhibits the rod temporal impulse response function

We determined how extrinsic white noise correlating with cone inputs to the three primary visual pathways affects both rod-pathway temporal contrast sensitivity and the impulse response function. A four-primary photostimulator provided independent control of rod and cone photoreceptor excitations under mesopic illumination (20 photopic Td). We show that rod-pathway temporal contrast sensitivity uniformly decreases across all temporal frequencies in the presence of cone noise correlating with the inferred magnocellular, parvocellular, or koniocellular pathways. The rod-pathway temporal impulse response functions derived using the Stork-Falk procedure (with a minimum phase assumption) had lower amplitudes in the pathway-specific cone noise. Therefore, cone noise impairs rod-pathway temporal contrast sensitivity without delaying rod-pathway signal transmission.