Variation of Response times to letter strings with increasing viewing eccentricity

Background: Some individuals with severe central scotomas employ eccentric viewing for reading but at a slower rate compared to normal foveal fixation, despite utilizing adequate magnification. It is conceivable both pre-lexical and lexical processing delays may contribute to reduced reading speeds with increasing viewing eccentricity. Therefore, the purpose of this experiment is to measure the response times (inferred as processing speed) and recognition accuracy of random letter strings with increasing viewing eccentricity to explore pre-lexical processing delay trends. Methods: Letter recognition accuracy and keyed-entry response times were measured for trigrams and pentagrams in 5 adult subjects under partial reporting conditions in which a single precued letter within a letter string was reported. Letters were presented for 100ms, rendered in high contrast (0.8), utilizing black lowercase Courier font, at a fixation distance of 57cm. Results: Under partial reporting conditions, response accuracy and response times depended on string length, serial position of a letter within a string and viewing eccentricity. In trigrams and pentagrams, a trend of increasing response times as response accuracy decreased was observed. Additionally, when response accuracy was high (for example at fixation) response times were higher when flanked by more targets. Conclusions: The results suggests that crowded targets are not only associated with decreased recognition accuracy but also delayed perceptual processing as inferred from longer measured response times. This interaction increases with viewing eccentricity. Additionally flanking targets impose additional processing delays even when recognition accuracy is affected minimally.