Visual acuity for optotypes made visible by relative motion.

There are several visual mechanisms for analyzing spatial information additional to the much researched mechanism sensitive to luminance contrast. We describe a Snellen-type acuity test for motion-defined (MD) letters. Acuity for these MD letters collapsed at dot speeds slower than 0.05 deg/s, but acuity for contrast-defined (CD) letters was unaffected by speed over the entire 0 to 0.3 deg/s range used. Acuity was a power function of presentation duration for both kinds of letter, but the exponent was higher for MD than for CD letters. Acuity for MD letters was comparatively unaffected by dot density from 50 to 0.05%, below which it suddenly collapsed to zero. On the other hand, acuity for CD letters progressively fell as dot density was reduced from 50%, and below about 0.5% approximated acuity for MD letters.