In
Figure 3 , some acuity differences between Chinese characters and Sloan letters remained even when stroke width was equalized. Such differences may be explained by the structural similarity among the eight Chinese characters in the CC
thick group. To demonstrate, raw data from the Sloan and CC
thick experimental sessions that yielded correct rates between 45% and 85% were organized into letter confusion matrices
(Fig. 5) , in which prominent confusions are highlighted by boxes. In a typical letter acuity experiment, the correct rate of a letter stimulus depends on how confusable it is with its fellow members in the stimulus group.
20Among the eight Sloan letters used in our experiment, the three most legible letters were V, H, and Z (91%, 88%, and 76% correct, respectively). The chalice, the two vertical bars, and the two horizontal bars were unique within the group. These letters were not confused consistently with other letters
(Fig. 5a) . In CC
thick, however, the chalice of V was replaced by a blade of

, and the two vertical bars of H were replaced by two horizontal bars of

. Although these replacements did not change the overall complexity of the group, they introduced more confusion
(Fig. 5b) . Now, the horizontal bars of

could be confused with those of

. The blade of

was now shared by part of

. Indeed, stimulus

was confused with

12% of the time, and stimulus

was confused with

11% of the time
(Fig. 5b) . As a consequence, the correct rates of stimuli

,

, and

were 76%, 65%, and 62%, respectively—lower than the correct rates of V, H, and Z. The increased confusions in CC
thick could account for a large portion of the difference between acuity size of this group and that of the Sloan letters. Therefore, in addition to spatial complexity measures such as stroke frequency, the structural similarity among optotypes may influence acuity outcomes.