Alignment conditions of the human eye for few-photon vision experiments

In experiments probing human vision at the few-photon level, precise alignment of the eye is necessary such that stimuli reach the highest-density rod region of the retina. However, in literature there seems to be no consensus on the optimal eye alignment for such experiments. Typically, experiments are performed by presenting stimuli nasally or temporally, but the angle under which the few-photon pulses are presented varies between 7 deg and 23 deg. Here we combine a $3$-dimensional eye model with retinal rod density measurements from literature in a ray tracing simulation to study the optimal eye alignment conditions and necessary alignment precision. We find that stimuli, directed at the eye's nodal point, may be best presented under an inferior angle of 13.1 deg with respect to the visual axis. Defining a target area on the retina with a radius of 0.5 mm around the optimum location, we find the horizontal and vertical angular precision should be better than 0.85 deg given a horizontal and vertical translational precision of 1 mm and a depth translational precision of 5 mm.