The spatial grain of the perifoveal visual field.

Measurements for normal human photopic vision in the fovea and in 2.5, 5 and 10 degree positions in the periphery reveal that hyperacuity thresholds rise faster with eccentricity than visual resolution thresholds. On the other hand, the retinal distances over which hyperacuity detection remains optimal show a much slower rise. At 10 degrees eccentricity, hyperacuity thresholds are 10 times higher visual resolution thresholds 4-5 times higher, and optimal processing distances for hyperacuity only 2-3 times larger than at the fovea. Psychophysical procedures do not, therefore, provide a single unambiguous measure for the changes of spatial grain across the visual field.