Does size-threshold text legibility relate to performance with suprathreshold-sized text? Yu-Chi Tai, PhD James E. Sheedy, OD, PhD, FAAO John R. Hayes, PhD Pacific University College of Optometry Forest Grove OR 97116, Tai Y, Sheedy J, Hayes J. Can threshold legibility predict suprathreshold performance? 2007;84:E-abstract 070058.

Issue Threshold legibility measures “Back-up” method used by Tinker Visual acuity method Similar Follows standardized rules of visual acuity measurement Do threshold measures of relative legibility apply to normally sized characters that are suprathreshold?

Methods 45 healthy adult individuals (18 men and 27 women) Relative text legibility threshold visual acuity letters and words Reaction time (RT) to suprathreshold letters and words

Font stimulus conditions Five stimulus conditions for letters and words, used to evaluate threshold legibility and suprathreshold recognition time (TNR = Times New Roman).

Relative threshold legibility of letters and words across 5 conditions (n=45) Significant differences across all condition pairs for letters and for words, except for letter legibility between TNR and Consolas (p = .081), TNR and ClearType (p = .182), and Consolas and ClearType (p = 1.000)

This validates the selection of the font stimuli as providing a legibility range Note that words have more clearly defined steps than letters

Response time measurements Voice response latency with voice key Same font stimuli conditions Letters and words Each of the following sizes: 20/20, 20/25, 20/32, 20/40, 20/50, 20/62, and 20/80

Response time by font condition and size

Letter superiority appears to function only for small characters The letter superiority effect, demonstrated for threshold legibility, is also shown for RT, but only smaller angular sizes show a faster RT for letters. At larger angular sizes, i.e. those commonly used for typical reading, the RT to recognize a word is about the same as for a letter. The likely reason: lateral inhibition or lateral interference

Effects of spacing on word legibility (another study)

To note RT increases with smaller characters Curves flatten for larger sizes “Acuity reserve” Longer RT for words compared to letters - with small text only Effect disappears with larger characters Potential explanation: Contour effect is greater for words with smaller characters The “letter superiority effect” applies only to small characters Possiblities A large benefit of greater size is defeating the contour interaction effect Also, character spacing should not be proportional across character sizes – spacing should be a greater percentage of character size for smaller font sizes

To note Does TNR perform poorly for words because of the serif contribution to lateral interference? If lateral inhibition is responsible for letter superiority effect at smaller character sizes – then word legibility should be improved with greater spacing for small font sizes

Threshold legibility and suprathreshold reaction time Letters Two-way randomized block ANOVA significant effects on reaction time in recognizing suprathreshold-size letters threshold legibility (F(1,1517)=1033.3, p<.0001), angular size (F(6,264)=225.03, p<.0001) their interaction (F(6,1517)=25.80, p<.0001).

Letters Displayed conditions represent the following threshold legibility measures: Verdana, -0.203; Consolas -0.125; ClearType -0.124; TNR -.105; Low contrast 0.031.

To note Larger RT with poorer legibility Larger RT with smaller characters Diminishing improvements with larger letter steps (“Acuity Reserve”) Effect of letter size on RT is diminished for fonts with better legibility

Threshold legibility and suprathreshold reaction time Words Two-way randomized block ANOVA significant effects on reaction time in recognizing suprathreshold-size words threshold legibility (F(1,1510)=781.97, p<.0001) angular size (F(6,264)=489.81, p<.0001) their interaction (F(6,1510)=29.31, p<.0001)

Words Displayed conditions represent the following threshold legibility measures: Verdana, -0.138; Consolas -0.101; ClearType -0.047; TNR -0.013; Low contrast 0.094

Letters Words

To note Similar findings as for letters, except…. Improvement with letter size continues into larger letter sizes i.e. greater acuity reserve Longer RT for words with small text compared to letters Effect disappears with larger characters Potential explanation: Contour effect is greater for words with smaller characters The “letter superiority effect” applies only to small characters Consequences of the above A large benefit of greater size is defeating the contour interaction effect Also, character spacing should not be proportional across character sizes – spacing should be a greater percentage of character size for smaller font sizes

Acknowledgment These studies are supported by Microsoft Corporation