The Arlene R. Gordon Research Institute of Lighthouse International has carried out a set of studies examining the role of several factors in reading and text legibility.
Senior Fellow in Vision Science, Dr. Aries Arditi, former Lighthouse researcher, Dr. Kenneth Knoblauch, and former Lighthouse research assistant, Ilana Grunwald began by comparing reading performance (speed) with fixed and variable (proportional) character widths. What they found was that for small characters approaching the acuity limit in central vision, and for somewhat larger characters in peripheral vision, fixed spacing is clearly easier to read. On the other hand, for medium and large characters, where acuity is not the limiting factor, they found that variable character widths (such as those used in proportionally spaced text fonts) are easier to read than variable spacing. Further research has shown that it is the greater horizontal compression, and consequently the reduced eye-movement requirements of variable pitch, that are responsible for its superiority at medium and large character sizes (Knoblauch, Arditi, & Szlyk, 1991).
Reading performance was also measured with text that was defined by differing amounts of color and luminance contrast, in order to determine the influence of color information on reading. Results indicated that when luminance contrast was well above threshold, varying the chromatic (color) contrast had little effect on reading performance. However, when luminance contrast was very low, near - threshold, chromatic contrast sustained reading rates of nearly 300 words per minute, almost as high as those found with high luminance contrasts. On the other hand, for some low vision observers, text defined by color contrast interfered with reading performance. Further investigations are planned to determine why the reading of some low vision observers is adversely affected by color contrast that is not accompanied by sufficient luminance contrast.
Institute studies have also examined the role of other font parameters in legibility, in addition to the studies of inter-letter spacing, font proportionality and color described above. Letter stroke width (Arditi, Cagenello & Jacobs, 1995a), letter aspect ratio (Arditi, Cagenello & Jacobs, 1995b), outline vs. filled letterforms (Arditi, Liu & Lynn, 1997a, 1997b), the presence or absence of letter serifs (Arditi & Cho, 2000a; Arditi & Cho, 2005) and letter case (Arditi & Cho, 2000b; Arditi and Cho, in press) were all found to affect legibility to some degree. The results from the earlier of these studies are summarized in a review article (Arditi, 1996).
Since many different typographic parameters may affect legibility, the Institute also undertook a two-year collaboration, funded by the National Institute on Aging, with Mars Perceptrix Inc., to develop adjustable font software for users with low vision. The software allows adjustment of many font parameters in realtime, so that the user can see the impact of parameter adjustments on legibility as it happens. The prototype software works, but evaluation using participants with low vision suggested that font legibility could not be significantly improved above and beyond that provided by highly evolved and readily available fonts such as TrueType Arial and TrueType Times New Roman (Arditi, 2004).
A second thread of the Institute's reading and text legibility research program to elucidate the factors common to both reading and acuity. Despite the elemental relationship of letters to written words, letter acuity is well known to be a poor predictor of several indices of reading performance. In one paper, Arditi (1994) provided a general discussion of differences between reading and letter acuity stimuli and cognitive demands in the two acuity tasks that may account for the poor power of single letter acuity to predict reading performance. He also outlined a sensor model of text processing that can account for text and optotype crowding phenomena (crowding refers to the observation that closely-spaced contours tend to interfere with letter recognition). Two subsequent reports (Arditi & Cagenello, 1993; Arditi, Cagenello, & Jacobs, 1995) were devoted to examining some of the implications and merits of the proposed sensor model.
In addition, Drs. Liu and Arditi have completed a study of letter confusion under conditions of close letter spacing. This study was intended to reveal the neural mechanisms responsible for the crowding effect. By comparing the letter confusions observed under wide and narrow inter-letter spacing, they were able to determine that a large portion of the deterioration of legibility under narrow spacing condition could be attributed to unique letter confusions which did not occur when inter-letter spacing was wider. The cause of these unique letter confusions was lateral interference (inhibition) from neighboring letters. A preliminary report of these findings appeared in abstract form (Liu & Arditi, 1996) and a full report was published in the journal Optometry and Vision Science (Liu & Arditi, 2001).
Drs. Liu and Arditi have also completed a study that concerned the number of letters read from a closely packed letter string. They found that human observers not only made mistakes in identifying closely packed small letters, but they also misjudged the number of letters in the string. Using interlaced four-letter and five-letter strings, Drs. Arditi and Liu demonstrated that human observers tended to mistake more five-letter strings for four-letter strings when the inter-letter spacing was narrowed. Typically, the observers would either omit one of the three letters in the middle of the five-letter string, or combine two neighboring letters into a new letter. The researchers used a computer simulation to demonstrate that optical blur of the eye might have played an important role in this new aspect of the "crowding effect". A preliminary report of these findings appeared in abstract form (Liu & Arditi, 1997a) and a full report was published in the journal Vision Research (Liu & Arditi, 2000).
In two recent studies, Dr. Liu further explored the contour interactions exhibited in a now classic experiment paradigm. In experiments that follow this paradigm, a visual acuity target, usually a Landolt C, is presented with four flanking bars. Visual acuity is measured as a function of the separation between the Landolt C and the flanking bars. The inhibitory effect of the flanking bars is demonstrated as reductions of visual acuity at certain separations. Drs. Liu and Arditi studied the effects of contrast polarity by measuring contour interaction between a black C with four white bars, and between a white C with four black bars. They found that features of different contrast polarities were still engaged in inhibitory interaction, although the interaction appeared to be weaker than that observed with features of the same contrast polarity. Therefore, a simple linear receptive field model was not applicable to suprathreshold contour interaction. A preliminary report of these findings appeared in abstract form (Liu & Arditi, 1997b).
Dr. Liu and research assistant Katherine Lee also investigated the contour interactions between features of different orientations. They measured contour interaction curves for pairs of flanking bars that were either parallel or orthogonal to the gap of the Landolt C. They found that flanking bars that were parallel exerted the strongest inhibition at the narrowest separation between the Landolt C and the bars. At separations narrower than two gap widths, the inhibition caused by a pair of parallel bars was stronger than that observed when all four bars were present. The orthogonal bars only exerted moderate inhibition at wider separations. It appeared that when four flanking bars were present, the orthogonal bars alleviated the inhibitory effect of the parallel bars at narrower separations and enhanced it at wider separations. The preliminary results of this study appeared in abstract form (Liu & Lee, 1999). A full report was published in the journal Vision Research (Liu, 2001b). In a theoretical study of the Landolt C/flanking bar stimulus configuration, Dr. Liu analyzed spatial frequency components at various ring/bar separations, and concluded that the foveal crowding effect could not be accounted for by the physical characteristics of the stimulus. These findings appeared in abstract form (Liu, 2001a) and a full report appeared in the journal Vision Research (Liu, 2001c).
In a third thread of this research, Drs. Kent Higgins, Aries Arditi, and former Lighthouse researcher, Dr. Kenneth Knoblauch, completed an initial investigation which is part of a larger program aimed at understanding why reading performance is poorer in the peripheral retina than in the central retina. The initial study was prompted by the work of other researchers which suggested that the peripheral retina was, in comparison to the central retina, deficient with respect to spatial phase discrimination. Such a deficit could affect reading performance by interfering with the discriminability of letters having the same spatial frequency content but different spatial phase spectra (i.e., mirror image letters like "b" vs. "d"). However, they found that when letters were size-scaled to compensate for differences in contrast sensitivity, the relationship between detection and identification performance was the same in both the central and peripheral retinas (Higgins, Arditi, & Knoblauch, 1992, 1996a, 1996b). These results thus argue against the hypothesis that the poorer reading performance outside the fovea is, somehow, due to reduced letter discriminability that might occur secondarily to a loss of peripheral-retina phase sensitivity. These results were summarized in paper that appeared in Vision Research (1996a). This paper was also selected for inclusion in Human Symmetry Perception and Its Computational Analysis (1996b), which was edited by Christopher Tyler of the Smith-Kettlewell Institute.
The next phase of this project will involve an evaluation of the effect of contour interaction on the mechanisms that mediate detection and discrimination of mirror-image letters in the central and peripheral retina. Several studies have suggested that the magnitude of the crowding effect may be greater in peripheral vision than in central vision. If this is true, it might imply that the optimal letter spacing for legibility is different for central vision as opposed to peripheral vision, a finding that would have important implications for the type of magnification device prescribed for low vision reading (optical vs. electronic).
A major goal of our legibility work is the dissemination of simple, comprehensible guidelines for selecting typography that works effectively for all, including those with low vision. We have developed a brochure (Arditi, 1999/2002), that we hope constitutes one important step toward accomplishing this goal.
A new line of research concerns how contrast affects letter legibility. It is well known that it takes more contrast to recognize a letter than to detect it. There is, however, no study on why detectable letters may not be recognizable. Dr. Liu and research assistant Hillary Gauthier created 12 letter-like figures, and measured normal subjects' detection and recognition performance at different contrast and different stimulus durations. They found that low contrast letters elicit less stable percepts than high contrast letters. When one looks at a low contrast letters, the appearance of letter spontaneously changes to a different one. The lower the letter contrast is, the more likely that the letter appearance changes. If the task is not just to see the letter but to correctly name the letter, the subject is more likely to make an error with a low contrast letter, because she may see several letters over time, and cannot decide which one is the correct one. Such perceptual instability occurs when letters become detectable. At two or three time detection contrast threshold, the letter appearance may change every 2 or 3 seconds. Increasing letter contrast reduces the perceptual instability and thus reduces the chance to make recognition errors. Therefore, to recognize a letter, one not only has to have enough contrast to see it, but also has to have additional contrast to see it stable. The results have been presented in two conferences, and a manuscript has been submitted to the journal of Vision Research.
Arditi, A. (1994). On the relationship of letter acuity to reading acuity. In A. C. Kooijman, P. L. Looijestijn, J. A. Welling, & G. J. van der Wildt (Eds.), Low vision: Research and new developments in rehabilitation (pp. 38-45). Amsterdam, The Netherlands: IOS Press.
Arditi, A. (1996). Typography, print legibility, and low vision. In R. Cole & B. Rosenthal (Eds.), Remediation and management of low vision (pp. 237-248). St. Louis: Mosby.
Arditi, A. (1999, 2002). Making text legible: Designing for people with partial sight. New York: Lighthouse International.
Arditi, A. (2003). Selecting fonts for low vision. In A. Lueck (Ed.) From Assessment to Training: A Guide for Working with Individuals with Low Vision in Education and Rehabilitation, Amanda Lueck (Ed.) American Foundation for the Blind Press.
Arditi, A. (2004). Adjustable typography: An approach to enhancing low vision text accessibility. Ergonomics 47(5), 469-482.
Arditi, A., & Cagenello, R. (1993). Why reading acuity is worse than and is poorly predicted by letter acuity. Investigative Ophthalmology and Visual Science, 34 (4), 1417.
Arditi, A., Cagenello, R., & Jacobs, B. (1995a). Letter stroke width, spacing, and legibility. Vision science and its applications, OSA technical digest series (pp. 324-327). Washington, D.C.: Optical Society of America.
Arditi, A., Cagenello, R., & Jacobs, B. (1995b). Effects of aspect ratio and spacing on legibility of small letters. Supplement to Investigative Ophthalmology and Visual Science, 36 (4), S671.
Arditi, A., & Cho, J. (2000a). Do serifs enhance or diminish text legibility? Investigative Ophthalmology and Visual Science (Supplement) 41(4), S437.
Arditi, A., & Cho, J. (2000b). Letter case and text legibility. Supplement to Perception, 29, 45.Arditi, A., Knoblauch, K., & Grunwald, I. (1990a). Reading with fixed and variable character pitch. Journal of the Optical Society of America A, 7, 2011-2015.
Arditi, A. and Cho, J. (2005). Serifs and font legibility. Vision Research, 45(23), 2926-2933.
Arditi, A. and Cho, J. (in press). Letter case and legibility in normal and low vision. Vision Research.
Arditi, A., Knoblauch, K., & Grunwald, I. (1990b). Text density, eye movements, and reading. Human Vision, Visual Processing, and Digital Display, Proceedings of the SPIE, 1249, 38-44.
Arditi, A., Liu, L., & Lynn, W. (1997a). Legibility of outline fonts. In Vision science and its applications. OSA technical digest series Vol. 1, (pp. 204-209). Washington, DC: Optical Society of America.
Arditi, A., Liu, L., & Lynn, W. (1997b). Legibility of outline and solid fonts with wide and narrow spacing. In D. Yager (Series Ed.), Trends in Optics and Photonics (Vol. 11). Washington, D.C.: Optical Society of America.
Higgins, K. E., Arditi, A., & Knoblauch, K. (1992). Detection and discrimination of mirror-image letter pairs in central and peripheral vision. Noninvasive Assessment of the Visual System, Technical Digest Series, Vol. 1 (pp.6-9). Washington, D.C.: Optical Society of America.
Higgins, K. E., Arditi, A., & Knoblauch, K. (1996a). Detection and discrimination of mirror-image letter pairs in central and peripheral vision. Vision Research, 36, 331-337.
Higgins, K. E., Arditi, A., & Knoblauch K. (1996a). Detection and discrimination of mirror-image letter pairs in central and peripheral vision. In C. Tyler (Ed), Human symmetry perception and its computational analysis. Zeist, The Netherlands: VSP Press.
Knoblauch, K., Arditi, A., & Szlyk, J. (1991). Effects of chromatic and luminance contrast on reading. Journal of the Optical Society of America A, 8, 428-439.
Liu, L. (2001a). A theoretical analysis of the spatial frequency explanation for the foveal crowding effect. Investigative Ophthalmology & Visual Science, (Suppl.) 42, S849.
Liu, L. (2001b). Dual oriented mechanisms in contour interaction. Vision Research, 41, 853-858.
Liu, L. (2001c). Can the amplitude difference spectrum peak frequency explain the foveal crowding effect? Vision Research, 41, 3693-3704.
Liu, L., & Arditi, A. (1996). Letter confusion observed under small interletter spacings. Investigative Ophthalmology & Visual Science (Suppl.), 37, S167.
Liu, L., & Arditi, A. (1997a). Read as many letters as you can. Investigative Ophthalmology & Visual Science (Suppl.), 38, S647.
Liu, L., & Arditi, A. (1997b). Contour interaction between features of different contrast polarities. Optometry and Vision Science, 74, 109.
Liu, L. & Gauthier, H. (2005). Unstable percept of low contrast letter-like patterns. Vision Research, submitted.
Liu, L. & Gauthier, H. (2004). Why low contrast letters are difficult to recognize. Optometry and Vision Science, 81, 128S
Liu, L. & Gauthier, H. (2004). Perceptual instability of low contrast letters, Presentation at the Vision Science Society Annual Meeting, May 2004, Sarasota, FL.