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.