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Background: Three-month-old infants can discriminate motion-defined (MD) form, but we do not know the age at which this ability reaches adult levels. Previous psychophysical evidence suggests that different neural mechanisms are involved in the processing of luminance-defined (LD) and MD spatial form in adults. This difference may be reflected in the development of LD versus MD form identification in children.

Methods: We measured speed threshold for identifying MD letters, letter-chart (ie, Snellen) acuity for high-contrast LD letters and single-letter acuity for highand low-contrast LD letters. Forty-seven children between 3 and 12 years of age and 20 adult subjects were tested.

Results: Development to the adult level was observed as follows: low-contrast single-letter acuity before 3 years; high-contrast single-letter acuity by 5 to 6 years; the ability to identify MD letters by 7 to 8 years; letter-chart acuity by 9 to TO …