The issue of reading on electronic devices is getting important as the popularity of mobile devices, such as cell phones or
PDAs, increases. In this study, we used the spatial summation paradigm to measure the spatial constraints for text detection.
Four types of stimuli (real characters, non-characters, Jiagu and scrambled lines) were used in the experiments. All
characters we used had two components in a left-right configuration. A non-character was constructed by swapping the left
and right components of a real character in position to render it unpronounceable. The Jiagu characters were ancient texts
and have the same left-right configuration as the modern Chinese characters, but contain no familiar components. Thus,
the non-characters keep the components while destroy the spatial configuration between them and the Jaigu characters
have no familiar component while keep the spatial configuration intact. The detection thresholds for the same stimulus size
and the same eccentricity were the same for all types of stimuli. When the text-size is small, the detection threshold of a
character decreased with the increase in its size, with a slope of -1/2 on log-log coordinates, up to a critical size at all
eccentricities and for all stimulus types. The sensitivity for all types of stimuli was increased from peripheral to central
vision. In conclusion, the detectability is based on local feature analysis regardless of character types. The cortical
magnification, E2, is 0.82 degree visual angle. With this information, we can estimate the detectability of a character by its
size and eccentricity.