Visual Acuity and Eye Movements

Several longstanding theories and some recently published experimental evidence support the hypothesis that eye movements serve to improve acuity. By measuring eye movements during a simple acuity task, and during a control non-acuity task, we have shown that certain patterns of eye movement are characteristic of acuity tasks. Similarly, specific patterns of eye movement are generated during spatial localization tasks. These observations provide circumstantial evidence for the existence of mechanisms by which eye movements mediate acuity and spatial localization information. Through a comparison of acuity for stabilized retinal images with acuity for normal retinal images we have found that eye movements improve acuity very slightly at most, and that even this small improvement may be adequately accounted for by the residual fade ­ out effects commonly observed during prolonged viewing of stabilized images. Measurement of distance and angle estimation ability in both normal and stabilized vision reveals much the same result. Stabilization diminishes the accuracy of these estimates only slightly, as might be expected from the persistent fade effects observed during the stabilized trials. Residual retinal image movement in the stabilized trials was less than approximately 3 min arc. If such acuity improving mechanisms exist, they either operate on very small retinal image movements (less than 3 min arc), or they improve acuity only slightly (e.g., by less than 0.1 log unit in sine wave grating contrast sensitivity). Thus eye movements serve to sustain all sensory visual inflow by countering the slow process of fading of a stabilized image. They do not, however, play a vital role in the much more rapid processes which determine visual acuity as well as distance and angle estimation ability.