Investigating the relationship between human foveal anatomy and fixation behavior across different visual tasks

When looking at fine details, normally sighted observers center stimuli on a specific location in the foveola, the preferred retinal locus. This locus is slightly offset from the point of peak cone density and has been reported to remain consistent between tasks. However, differences in fixational oculomotor behavior are known to occur across tasks, which may lead to fine changes in the average stimulus position on the retina. Here we examined the distribution of the retinal positions of the fixated stimulus in different tasks in relation to cone density across the foveola. Using an adaptive optics scanning laser ophthalmoscope for simultaneous stimulus delivery and retinal imaging, we investigated oculomotor behavior of subjects (N=8) in three different tasks: fixation on a blinking square, a moving Maltese Cross, and a high-acuity Snellen task. Stimulus size was the same across tasks. We then quantified the difference between the average stimulus position on the retina across the tasks with respect to the location where cones are most densely packed. Peak cone densities across subjects varied from 13,847 to 20,897 cones per square degree, and the stimuli remained within the region where cone density was above 70% of the peak density in all conditions. This region spanned on average 0.23 degrees squared. Yet, differences across tasks were present. We found that the stimulus positions in the Snellen task spanned an area that was more than 50% larger on average when compared to either fixation task (p<0.05). Further, we found a small consistent shift (1.5+/-1.1 arcmin, p<0.033) toward the location of peak cone density in the Snellen task compared to the blinking square fixation. Although the mechanism responsible for the observed shift remains unclear, it raises interesting questions, as it does not yield a significant difference in the Nyquist sampling frequency.