One Preferred Retinal Locus to rule them all: A fine dissection of the PRL in space and time

The preferred retinal locus (PRL) is defined functionally as the retinal position used to see or guide gaze. A refined definition of the PRL satisfying either the best performance (acuity) or stability (motor) will help inform about human foveal vision. A high-speed, sub-arcmin, retinal-image-based eye-tracker (Adaptive Optics Scanning Laser Ophthalmoscope:AOSLO) was used to measure PRL properties for a cadenced, fine-discrimination task. The AOSLO projects and unambiguously records the stimulus location over the constantly moving retinal image. Six subjects reported offsets between two tiny 2x1 arcmin horizontal Vernier bars separated by 1 arcmin along 7 offsets of 6 arcsecs. The bars were decrements presented within a 0.9° red AOSLO 30 frame-per-second display containing four fixation guides. Stimuli were flashed for two frames every 2sec in a cadence, so subjects could adopt a constant fixation strategy. 2100 trials were presented in pseudorandom order for each participant. Spatially-defined PRLs and fixation stabilities (ISOA) were computed for either all fixation points (PRL), fixation points during stimulus onset, saccade starting positions, saccade landing positions and the subset of positions leading to correct responses for the smallest Vernier offset (cPRL). Temporally defined PRLs were isolated at mid-drift duration between saccades or by the stable eye position plateau observed 200-400ms after stimulus onset (itPRL). Saccade rate, amplitude and landing distance to cPRL, corresponding fixation stability all work to reduce and converge 200-400ms after Vernier appearance. Interestingly this most stable, congruent epoch appears driven by finely programmed saccades in timing and landing position. Accurate image placement on the retina in both space and time is important for fine visual tasks. The observation of fixation ISOA decreasing after the brief stimulus may indicate a strategy to integrate visual information from normally more stationary targets. Overall, cPRL and itPRL offer the most reliable and meaningful functional definitions of the PRL.