Abstract
Abstract: :
Purpose: It is not known whether acuity for lower case words varies as a function of the number of letters in a word in normal vision, as it has been observed to do in eccentric vision (Déruaz et al., ARVO abstract 2004). We investigated the relationships between acuities for lower case words of differing length and between lower case words and single letter visual acuity in normal subjects as well as the way in which response time changed during these tests. Methods: A computer–based system of stimulus presentation and stimulus randomisation was used to present either single uppercase Sloan letters or lower case words, in Courier bold font, of 4, 7 and 10 letters to one eye of 10 healthy subjects with normal vision. Stimuli were presented one at a time in an ETDRS style format (5 stimuli at each character size with 0.1 logMAR steps between sizes) and acuity was measured using letter–by–letter or word–by–word scoring. Both acuity and response times were recorded for each test performed by each subject. Results: Mean acuity values (± SE) were: single letters, –0.09 ± 0.029; 4 letter words, –0.15 ± 0.019; 7 letter words, –0.16 ± 0.017 and 10 letter words, –0.17 ± 0.021. Acuities for 4, 7 and 10 letters words were not significantly different (repeated measures ANOVA, p = 0.10). Lower case word acuity was significantly better than acuity for single letters (paired t–tests, p < 0.05 for each word length), although this was < 0.1 logMAR in each case. Response time slowed as stimuli approached threshold (repeated measures ANOVA, p < 0.001), with a critical character size of 0.3 to 0.5 logMAR above threshold observed in each test, above which response time was a minimum. The separation between acuity threshold and critical character size was equivalent for each text stimulus (repeated measures ANOVA, p = 0.85). Conclusions: In normal vision: 1) Acuity for lower case words is equivalent for words from 4 to 10 letters. 2) Acuity for uppercase Sloan letters is practically equivalent to that for lower case words in Courier bold font. 3) Minimum acuity reserve values for optimal processing are similar for single letters and lower case words of differing length. These results provide a basis for comparison to patients with eccentric fixation, where acuity appears to be a function of word length.
Keywords: reading • visual acuity • low vision