Browsing by Author "Kramm, Alexei"

The sensitivity of our conscious visual system comes remarkably close to the sensitivity limits imposed by the quantal nature of light. This exquisite sensitivity is made possible by the rod bipolar pathway (RBP), the most sensitive neural circuit studied to date, which allows us to consciously perceive light stimuli producing, in total, fewer than a dozen single-photon absorptions in rod photoreceptors. One of the central features of the RBP is the pooling of signals arising in thousands of rod photoreceptors scattered over the surface of the retina (spatial integration) into individual retinal ganglion cells (RGC), which subsequently encode visual scene as a train of action potentials and transfer these signals to the brain. However, the ultimate limits of sensitivity and the retinal circuitry underlying non-conscious vision at the absolute threshold of visual sensitivity are poorly understood. Here, we utilized the pupillary light reflex (PLR) as a functional readout of the non-conscious visual system to simultaneously measure and compare the threshold sensitivities of the conscious and non-conscious visual systems across different spatial scales in dark-adapted human observers. For this purpose, we designed, built, and calibrated an apparatus capable of producing precisely calibrated stimuli across five orders of magnitude in intensity, and four orders of magnitude in size. We find that the PLR and conscious vision express stimulus size-dependent differences in their threshold sensitivities, where when utilizing stimuli covering the whole visual field the PLR matches the sensitivity of conscious vision, by responding to stimuli producing, fewer than three photon absorptions spread over a pool of ten thousand rod photoreceptors, but when utilizing small stimuli the threshold sensitivity of the PLR falls short by an order of magnitude as compared to conscious visual system. Additionally, we find that the PLR produces a constant response to a constant number of photons (complete spatial summation), for stimulus sizes of up to 570µm in diameter. Thus, the PLR is capable of complete spatial summation over a retinal area 9-fold larger than conscious vision. Our results are consistent with RBP input into both visual systems, with each visual system providing a readout to the brain through separate RGCs.