RESUMEN

Stroke width is an important factor affecting letter legibility. Although there seems to be a critical or optimal value for stroke width, it has been difficult to estimate quantitatively because shape deterioration and the increase of stroke width are covariates. This study attempted to predict the optimal value of stroke width considering it as a duty ratio. The duty ratio is related to the amplitude of the fundamental frequency, with the maximum amplitude of the fundamental frequency occurring at a duty ratio of 0.5. The fundamental frequency decreases when the duty ratio is set either lower or higher than 0.5. Based on previous research indicating that letters also have a critical band (CB), or the frequency band which contributes most to letter recognition, which corresponds to their fundamental frequency, we hypothesized that the amplitude of CB is related to the legibility of letters. We measured the contrast thresholds using square-wave gratings and letters of differing duty ratios. In both the grating and letter stimuli, the amplitude of CB reached its maximum when the duty ratio was around 0.5. Contrast thresholds for stimuli with various duty ratios were accurately predicted by the amplitudes of CB and the contrast thresholds in the individual's sine-wave grating. Our results suggest that the amplitude of a single critical component contributes significantly to the legibility of letters, even though they contain diverse frequency components.

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RESUMEN

Both luminance contrast and character size are critical factors affecting reading performance. Previous studies reported on the effect of luminance contrast on the reading-speed function, that is, the relationship between reading speed and character size. In particular, when contrast was reduced, the critical print size (CPS) was found to shift to a larger character size even though the maximum reading speed and function shape did not change [Japanese Journal of Ophthalmology 52(1) (2008) 44-47]. In the present study, the effect of luminance contrast on the reading function was quantitatively examined. Japanese phrases with a luminance contrast of 0.03-0.99 were prepared as stimuli. Observers with normal vision were asked to read aloud phrases with several character sizes. Then, the reading functions were obtained for each luminance contrast. CPS was found to increase as the luminance contrast decreased. The relationship between contrast and CPS was linear in log-log coordinates, that is, log-CPS increased as the log-contrast of the characters decreased. It was found that the contrast of the stimulus systematically affects the location of the reading function.

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RESUMEN

This study examined the effect of character sample density on legibility. As the spatial frequency component important for character recognition is said to be 1 to 3 cycles/letter (cpl), six dots in each direction should be sufficient to represent a character; however, some studies have reported that high-density characters are more legible. Considering that these seemingly contradictory findings could be compatible, we analyzed the frequency component of the character stimulus with adjusted sample density and found that the component content of 1 to 3 cpl increased in the high-density character. In the following three psychophysical experiments, high sample density characters tended to have lower contrast thresholds, both for normal and low vision. Furthermore, the contrast threshold with characters of each sample density was predicted from the amplitude of the 1 to 3 cpl component. Thus, while increasing the sample density improves legibility, adding a high frequency is not important in itself. The findings suggest that enhancing the frequency components important for recognizing characters by adding the high-frequency component contributes to making characters more legible.

RESUMEN

The visual system cannot recognize an object (target) in peripheral vision when presented with neighboring similar stimuli (flanker). This object recognition disability is known as crowding. Studies have shown that various types of proximity, such as spatial distance or semantic category, affect the degree of crowding. However, thus far, these effects have mostly been studied separately. Hence, their underlying similarities and differences are still unknown. In this study, we developed a novel gap detection task and tested whether the effect of three different types of proximity in crowding (the relative position between target gap and nearest flanker edge, the flanker location compared with the target location, and the semantic category of the target) can be measured within a single task. A psychometric function analysis revealed that two of the assumed types of proximity affected the degree of crowding within a single task.