Interface Design and Optimization of Reading of Continuous Text

Paul Muter
University of Toronto
muter@psych.utoronto.ca

In van Oostendorp, H., and de Mul, S. (Eds.) (1996), Cognitive aspects of electronic text processing. Norwood, N.J.: Ablex.
(c) Copyright 1996 Ablex Publishing Corp.


"Reading is the means by which the world does a large part of its work.... The slightest improvement either in the page or in the method of reading means a great service to the human race" (Huey, 1908).

Introduction

At present, we do not know how to optimize reading via electronic equipment. In this chapter, some considerations that may help us do this in the future will be raised, and some of the relevant evidence and theory that does exist will be cited and briefly highlighted. The focus of this paper is on reading of continuous text, whether in linear form or hypertext form, and with or without the presence of graphics or other types of information.

Paradoxically, computer technology may lead to an increase in the use of text and an increase in literacy, because we are in a window of time in which it is easy and efficient to produce, store, manipulate, and transmit computerized text files, and comparatively difficult to process graphics, sound, or video. The amount of reading of text from electronic displays is increasing substantially every year.

Computerized presentation of text has some clear advantages over paper media (Egan, Remde, Gomez, Landauer, Eberhardt, & Lochbaum, 1989; Yankelovich, 1985):
In past research on optimization of reading, the main two dependent variables have been reading time and comprehension, as measured by recall, questionnaires, or error detection. Some other dependent measures of interest are:effective reading rate:
The issue of visual fatigue will not be emphasized in the present paper, but following are some of the dependent measures that have been used:

Paper vs. CRTs (cathode ray tubes)

Much of the published research on optimization of reading has been done with paper media. Research on reading from paper media has yielded the following results (Frenckner, 1990):
Bever, Jandreau, Burwell, Kaplan, and Zaenen (1990) found that comprehension was facilitated by adding spaces between major phrases, as determined by a simple automatic parser.

It is unknown to what extent findings from paper media can be extended to electronic media. Certainly at least some of the results do not generalize. For example, Wright and Lickorish (1988) found that color cues were effective as location aids for paper texts but not for computerized texts (though in the former case the backgrounds were colored and in the latter case the text itself). Prominent researchers in the field have expressed the opinion that it is risky to generalize from research on paper to electronic media (e.g., Kolers, Duchnicky, & Ferguson, 1981).

Muter, Latremouille, Treurniet, and Beam (1982) compared speed and comprehension in reading from a videotex terminal and a book. Results over two hours of reading indicated that, though extended reading from videotex was feasible, it was 28% slower than reading from paper. There was no significant difference in comprehension. Several other researchers have also found decreased efficiency from CRTs of the 1980s (e.g., Gould & Grischkowsky, 1984; Wilkinson & Robinshaw, 1987), some with reading and some with proofreading. Proofreading and reading share some component processes, but other processes are unique to each skill.

Dillon (1992) complained of the "great disparity in procedures" in studies demonstrating slower reading with CRTs than with paper, but this disparity could be interpreted as a virtue: Seeking "robustness in variation" can be a useful strategy in the face of complex interactions (see Interactions, below).

There are many typical differences between book and computer reading that conceivably could account for the observed slower reading from computer screens of the 1980s (adapted from Muter & Maurutto, 1991):
It is quite clear that no single variable accounts for the obtained differences in performance between CRTs and paper. Several of the above variables, including resolution, interline spacing, polarity, and edge sharpness contribute to the effect (Gould, Alfaro, Barnes, Finn, Grischkowsky, & Minuto, 1987; Kruk & Muter, 1984; Muter & Maurutto, 1991). With a more modern system, including a large, higher-resolution screen with dark characters on a light background, reading from a computer can be as efficient as reading from a book (Muter & Maurutto, 1991).

Of course, the efficiency of books notwithstanding, electronic text presentation should not simply mimic a book. The strengths and potential of computerized presentation should be pursued and exploited.

Selected Independent Variables

It is beyond the scope of the present chapter to comprehensively review the empirical evidence and theory on the effects of all of the independent variables that might affect reading via computers. (For reviews, see Dillon, 1992; Frenckner, 1990: and Mills & Weldon, 1988.) In this section, I will present brief comments on a number of independent variables, and more extensive treatment of one variable: color. In my opinion, the worst sins in the computerized presentation of text are committed with the use of color.

Of course, some of the following variables are inextricably intertwined, and some of them interact with each other.

Color

Many disadvantages and potential problems with the use of color in text presentation have been pointed out (Rubin, 1988; Shneiderman, 1987): In addition: Many factors affect the ability to distinguish colors (Silverstein, 1987), including: Of course, people often prefer color, and color can be useful to emphasize format, to highlight, to categorize, and to improve aesthetics (Nes, 1986; Rubin, 1988). Color can also aid in visual search (Smith, 1962). To maximize discriminability of colors, evidence suggests that differences in hue and lightness should be maximized and differences in saturation should be minimized (Laar & Flavell, 1988).

Polarity

Evidence suggests (Radl, 1983) that a large majority of users prefer positive polarity (dark characters on a light background). In theory, positive polarity reduces optical distortion, and increases visual acuity, contrast sensitivity, speed of accommodation, and depth of field (Bauer & Cavonius, 1983). It also decreases the problem of interfering reflections of external light (Bauer, 1987). However, the effects of polarity are controversial (Pawlak, 1986; Taylor & Rupp, 1987). A definite disadvantage of positive polarity is an increase in the risk of perceived flicker, though this problem can be overcome with a sufficiently high refresh rate.

Variables Affecting Perception of Flicker

The probability of perceiving flicker increases (Nylen & Bergqvist, 1987; Pawlak, 1986):

Pixel Attributes

Two goals in display design were suggested by Murch and Beaton (1987):

Resolution

One measure of resolution is the resolution/addressability ratio, which is the width of pixels divided by the peak-to-peak distance between pixels (Harpster, Freivalds, Shulman, & Liebowitz, 1989). Harpster et al. found that a high ratio resulted in better visual search performance.

Interline Spacing

Evidence of Wilkins and Nimmo-Smith (1987) suggests that increasing spacing between lines and proportionately decreasing horizontal spacing between letters may improve the clarity and comfort of text without affecting the density of the text. Close inter-line spacing may impair reading because of vertical masking, and because return sweeps are more difficult (Kruk & Muter, 1984). Evidence of Lunn and Banks (1986) suggests that interline spacing should be variable to prevent fatigue resulting from adaptation to spatial frequency.

Words per Screen

With respect to words per screen, Muter, Latremouille, Treurniet, and Beam (1982) suggested that reading speed tends to decrease as words per page decreases. Findings consistent with this idea have been reported several times (Creed, Dennis, & Newstead, 1988; de Bruijn, de Mul, & van Oostendorp, 1992; Reisel & Shneiderman, 1987).

Screen Size

There is both theory (Lansdale, 1988) and data (de Bruijn, de Mul, & van Oostendorp, 1992; Dillon, Richardson, & McKnight, 1990) to support the idea that large screens enhance the processing of text, perhaps partly because the number of words per screen can be larger (see above).

Multiple Windows

An experiment on reading of lengthy texts indicated that, after practice with the system, a multi-window display helped readers to relocate information (Tombaugh, Lickorish, & Wright, 1987).

Scrolling vs. Paging

Paging is apparently superior to scrolling in terms of both performance and user preference (Kolers, Duchnicky, & Ferguson, 1981; Schwarz, Beldie, & Pastoor, 1983). One advantage of paging is that incidental memory for location within a page (Rothkopf, 1971) may facilitate processing.

Distance

Within reasonable limits, the distance between the reader and the reading material has no effect on perceptual span (Morrison & Rayner, 1981) or reading efficiency (Kruk & Muter, 1984). With increasing distance, retinal image size decreases linearly, but so does retinal eccentricity (distance of the image from the fovea), and these two effects offset each other exactly. Acuity is a decreasing linear function of eccentricity (Anstis, 1974).

Size of Characters


Within reasonable limits, size of the characters has no effect on proofreading speed (Gould & Grischkowsky, 1986). The probable reason is analogous to the reason that distance has no effect (see above).

Proportional Spacing

Variable letter width (proportional spacing) led to faster reading of lists of isolated words (Beldie, Pastoor, and Schwartz, 1983).

Indentation

Huey (1908) recommended three-space indentation of every other line to facilitate return sweeps, but to my knowledge this idea has never been tested directly.

Hyphenation

Reading is slower if words are divided (hyphenated) at the ends of lines (Nas, 1988).

Highlighting

Techniques for highlighting include (Nes, 1986; Shneiderman, 1987; Tullis, 1988): The evidence on highlighting suggests that sometimes it helps and sometimes it has a negative effect (Fisher & Tan, 1989). A key variable seems to be highlighting validity: the percentage of time that a target, as opposed to a distractor, is highlighted.

Case

Searching for words is faster with uppercase characters, but reading of continuous text is slower (Vartabedian, 1971), perhaps because interline masking is greater with uppercase (Nes, 1986). In addition, lowercase enhances reading efficiency because word shape is helpful in word recognition (Rudnicky & Kolers, 1984).

Integration

Ideally, reading from a computer should be easily integrated with other tasks such as decision making, annotating (including unofficial comments), and report writing (Erickson & Salomon, 1991; Wright & Lickorish, 1984). Van Oostendorp (in press) found that performance in annotating text was as good in several computer conditions as in a paper and pencil condition.

Access Devices

Following are some potential enhancements of conventional text that can mimic hypertext and enable selective access, and which may affect efficiency of reading (Jones, 1987):

Dynamic Text Presentation

Until this point in the present chapter, it has been assumed that presentation of text is static. In dynamic text presentation, an attempt is made to optimize reading by utilizing some of the special capabilities of the computer. Two methods of dynamic text presentation that have been tested are rapid serial visual presentation (RSVP) and the Times Square format.

RSVP


With RSVP, text is presented at a fixed location on the screen, one word at a time or a few words at a time. Several researchers have demonstrated that readers can perform approximately as efficiently with RSVP as with normal page-format reading (e.g., Juola, Ward, & McNamara, 1982). There are several potential uses of RSVP: The optimal conditions for RSVP seem to be the following (Juola, Haugh, Trast, Ferraro, &
Liebhaber, 1987): Giving the user control over RSVP presentation, e.g., over regressions and rate of presentation, sometimes has adverse effects on performance (Chen and Chan, 1990; Muter, Kruk, Buttigieg, & Kang, 1988). But, of course, under many circumstances, people will prefer to have this control.

Times Square Format

Kang & Muter (1989) found that smooth (pixel-by-pixel) horizontal scrolling with a small window (Times Square format) produced performance at least as good as RSVP, contrary to earlier studies which did not use pixel-by-pixel scrolling. In addition, subjects preferred the Times Square format, which is often used in electronic billboards.

Interactions

A major difficulty in research in text presentation, whether static or dynamic, is that the various independent variables often interact, sometimes in extremely complex ways: The effect of one variable depends on the level of other variables. For example, this has been a problem with to respect to typographic variables such as type size, line length, and interline spacing (Frenckner, 1990). Following are some possible approaches for handling the problem of intractable high-order interactions:

Individual Differences


A second source of problems is individual differences. The use of computers entails huge individual differences, but it also permits extensive individualization (Rich, 1983). It is particularly important to take individual differences into account in human-computer interaction, for several reasons (Bailey, 1982; Egan, 1988): The effects of age are an important source of individual differences in reading. For example, the following visual functions decline with age (Czaja, 1988): Various ways of accommodating user differences have been developed:

Concluding Comments

Despite the large number of published experiments on reading continuous text from computers, to my knowledge no circumstances have been found in which reading in normal subjects is more efficient, with respect to speed and comprehension, than from a book. Perhaps performance superior to that achieved with the book has not been demonstrated because the new techniques of presentation require that the user have extended practice, and the experiments in the literature last no more than several hours per reader. Bigger experiments with dozens or even hundreds of hours of testing per subject may be necessary.

A more economical strategy is suggested by some work by Coleman and Kim (1961). They found that several formats, in particular a center-justified format with one word per line, had a positive effect on the processing of tachistoscopically presented text, but no effect or a negative effect on reading, probably because of entrenched habits. With extended practice, the effect might emerge in reading. If it could be established that tachistoscopic studies like this are good predictors of results of reading studies with extensive practice, then studies using tachistoscopic presentation - or analogous studies using computers to flash stimuli - could be used as short cuts to determine which presentation techniques optimize performance in reading.

On the other hand, perhaps the reason that no computer condition superior to a book has been found is that the bottleneck is in the central processing in the human brain, rather than in the input channels. Carver (1982) found that the optimal rate of reading and listening tended to be constant under a wide range of conditions. It is possible, though unlikely in my opinion, that tinkering with modes of presentation will do little or no good past a certain point, a point which has been reached by the technology of the book. The book has evolved over several centuries to its present highly efficient form. Of course, the evolution of the human brain has not kept pace with the evolution of technology. However, perhaps co-ordinated developments in computer technology and cognitive science can pave the way toward more efficient reading, and therefore toward the facilitation of work and problem-solving in many areas of endeavor.


Author Note

Dept. of Psychology, University of Toronto, Toronto, Ont., Canada, M5S 1A1.,
muter@psych.utoronto.ca.
I thank Valerie Temple for general assistance, and Boyd Blackburn, Pavel Muresan, Oren Satov, and Herre van Oostendorp for helpful comments.

References


Anstis, S.M. (1974). A chart demonstrating variations in acuity with retinal position.
Vision Research, 14, 589-592.

Bailey, R.W. (1982). Human performance engineering. Englewood Cliffs, N.J.: Prentice Hall.

Bauer, D. (1987). Improving the VDU workplace by introducing a physiologically optimized
bright-background screen with dark characters: advantages and requirements. In B. Knave, & P.G.
Wideback (Eds.), Work with display units 86. Amsterdam: North Holland.

Bauer, D., & Cavonius, C.R. (1983). Improving the legibility of visual display units through
contrast reversal. In E. Grandjean, & E. Vigliani (Eds.), Ergonomic aspects of visual display
terminals. London: Taylor & Francis.

Beldie, I.P., Pastoor, S., & Schwarz, E. (1983). Fixed versus variable letter width for televised text. Human Factors, 25, 273-277.

Bever, T.G., Jandreau, S., Burwell, R., Kaplan, R., & Zaenen, A. (1990). Spacing printed text
to isolate major phrases improves readability. Visible Language, 25, 75-87.

Bournique, R., & Treu, S. (1985). Specification and generation of variable, personalized
graphical interfaces. International Journal of Man-Machine Studies, 22, 663-684.

Carver, R.P. (1982). Optimal rate of reading prose. Reading Research Quarterly, 18, 56-88.

Chen, H., & Chan, K. (1990). Reading computer-displayed moving text with and without self-
control over the display rate. Behaviour & Information Technology, 9, 467-477.

Coleman, E.B., & Kim, I. (1961). Comparison of several styles of typography in English.
Journal of Applied Psychology, 45, 262-267.

Creed, A., Dennis, I., & Newstead, S. (1988). Effects of display format on proof-reading with
VDUs. Behaviour & Information Technology, 7, 467-478.

Czaja, S.J. (1988). Microcomputers and the elderly. In M. Helander (Ed.), Handbook of human-
computer interaction. Amsterdam: Elsevier.

de Bruijn, D., de Mul, S., & van Oostendorp, H. (1992). The influence of screen size and text
layout on the study of text. Behaviour & Information Technology, 11, 71-78.

Dillon, A. (1992). Reading from paper versus screens: a critical review of the empirical
literature. Ergonomics, 35, 1297-1326.

Dillon, A., Richardson, J., & McKnight, C. (1990). The effect of display size and paragraph
splitting on reading lengthy text from screens. Behaviour & Information Technology, 9, 215-227.

Egan, D.E. (1988). Individual differences in human-computer interaction. In M. Helander (Ed.),
Handbook of human-computer interaction. Amsterdam: Elsevier.

Egan, D.E., & Gomez, L.M. (1985). Assaying, isolating and accommodating individual
differences in learning a complex skill. In R. Dillon (Ed.), Individual differences in cognition.
New York: Academic Press.

Egan, D.E., Remde, J.R., Gomez, L.M., Landauer, T.K., Eberhardt, J., & Lochbaum, C.C. (1989).
Formative design-evaluation of SuperBook. ACM Transactions on Information Systems, 7, 30-57.

Erickson, T., & Salomon, G. (1991). Designing a desktop information system: Observations and
issues. Human Factors in computing systems: CHI'91, New York: ACM.

Fisher, D.L., & Tan, K.C. (1989). Visual displays: The highlighting paradox.
Human Factors, 31, 17-30.

Frenckner, K. (1990). Legibility of continuous text on computer screens -- a guide to the
literature (TRITA-NA-P9010). Stockholm: Royal Institute of Technology.

Gould, J.D., Alfaro, L., Barnes, V., Finn, R., Grischkowsky, N., & Minuto, A. (1987). Reading
is slower from CRT displays than from paper: attempts to isolate a single-variable explanation.
Human Factors, 29, 269-299.

Gould, J.D., & Grischkowsky, N. (1984). Doing the same work with hard copy and with
cathode-ray tube (CRT) computer terminals. Human Factors, 26, 323-337.

Gould, J.D., & Grischkowsky, N. (1986). Does visual angle of a line of characters affect
reading speed? Human Factors, 28, 165-173.

Harpster, J.L., Freivalds, A., Shulman, G.L., & Liebowitz, H.W. (1989). Visual performance on
CRT screens and hard-copy displays. Human Factors, 31, 247-257.

Harwood, K., & Foley, P. (1987). Temporal resolution: An insight into the video display
terminal (VDT) "problem". Human Factors, 29, 447-452.

Huey, E.B. (1908/68). The psychology and pedagogy of reading. Massachusetts: MIT Press.

Jackson, M.D., & McClelland, J.L. (1979). Processing determinants of reading speed.
Journal of Verbal Learning and Verbal Behavior, 108, 151-181.

Jameson, D., & Hurvich, L.M. (1964). Theory of brightness and color contrast in human vision.
Vision Research, 4, 135-154.

Jones, W.P. (1987). How do we distinguish the hyper from the hype in non-linear text? In H.-J.
Bullinger, & B. Shackel (Eds.), Human-computer interaction - INTERACT '87. Amsterdam: Elsevier.

Juola, J.F., Haugh, D., Trast, S., Ferraro, F.R., & Liebhaber, M. (1987). Reading with and
without eye movements. In J.K. O'Regan, & A. Levy-Schoen (Eds.), Eye movements:
From physiology to cognition. Amsterdam: Elsevier.

Juola, J.F., Ward, N.J., & McNamara, T. (1982). Visual search and reading of rapid serial
presentations of letter strings, words, and text. Journal of Experimental Psychology: General,
111, 208-227.

Kang, T.J., & Muter, P. (1989). Reading dynamically displayed text.
Behaviour & Information Technology, 8, 33-42.

Kolers, P.A., Duchnicky, R.L., & Ferguson, D.C. (1981). Eye movement measurement of readability of CRT displays. Human Factors, 23, 517-527.

Kruk, R.S., & Muter, P. (1984). Reading of continuous text on video screens.
Human Factors, 26, 339-345.

Laar, D.V., & Flavell, R. (1988). Towards the construction of a maximally-contrasting set of
colors. In D. M. Jones, & R. Winder (Eds.), People and computers IV.
Cambridge: Cambridge University Press.

Landauer, T.K. (1988). Research methods in human-computer interaction.
In M. Helander (Ed.), Handbook of human-computer interaction. Amsterdam: Elsevier.

Lansdale, M.W. (1988). The psychology of personal information management.
Applied Ergonomics, 19, 55-66.

Lunn, R., & Banks, W.P. (1986). Visual fatigue and spatial frequency adaptation to video
displays of text. Human Factors, 28, 457-464.

Mills, C.B., & Weldon, L.J. (1988). Reading text from computer screens.
ACM Computing Surveys, 19, 329-358.

Morris, A. (1987). Expert systems. In D. Diaper, & R. Winder (Eds.), People and computers III.
Cambridge: Cambridge University Press.

Morrison, R.E., & Rayner, K. (1981). Saccade size in reading depends upon character spaces and
not visual angle. Perception & Psychophysics, 30, 395-396.

Murch, G.M., & Beaton, R.J. (1987). Matching display characteristics to human visual capacity.
In B. Knave, & P.G. Wideback (Eds.), Work with display units 86. Amsterdam: North Holland.

Muter, P., Kruk, R., Buttigieg, M.A., & Kang, T.J. (1988). Reader-controlled computerized
presentation of text. Human Factors, 30, 473-486.

Muter, P., Latremouille, S.A., Treurniet, W.C., & Beam, P. (1982). Extended reading of
continuous text on television screens. Human Factors, 24, 501-508.

Muter, P., & Maurutto, P. (1991). Reading and skimming from computer screens and books: The
paperless office revisited? Behaviour & Information Technology, 10, 257-266.

Nas, G.L.J. (1988). The effect on reading speed of word divisions at the end of a line. In G.C.
van der Veer, & G. Mulder (Eds.), Human-computer interaction: Psychonomic aspects. Berlin:
Springer-Verlag.

Nelder, J.A., & Mead, R. (1965). A SIMPLEX method for function minimization. Computer Journal, 7, 308-313.

Nes, F.L. van (1986). Space, colour and typography on visual display terminals.
Behaviour & Information Technology, 5, 99-118.

Nylen, P., & Bergqvist, U. (1987). Visual phenomena and their relation to top luminance,
phosphor persistence time and contrast polarity. In B. Knave, & P.G. Wideback (Eds.), Work with
display units 86. Amsterdam: North Holland.

Pawlak, V. (1986). Ergonomic aspects of image polarity.
Behaviour & Information Technology, 5, 335-348.

Radl, G.W. (1983). Experimental investigations for optimal presentation mode and colours of
symbols on the CRT screen. In E. Grandjean, & E. Vigliani (Eds.), Ergonomic aspects of visual
display terminals. London: Taylor & Francis.

Reisel, J.F., & Shneiderman, B. (1987). Is bigger better? The effects of display size on
program reading. In G. Salvendy, S.L. Sauter, & J.J. Hurrell, Jr. (Eds.), Social, ergonomic and
stress aspects of work with computers. Amsterdam: Elsevier.

Rich, E. (1983). Users are individuals: individualizing user models. International Journal of
Man-Machine Studies, 18, 199-214.

Rothkopf, E.Z. (1971). Incidental memory for location of information in text. Journal of Verbal
Learning and Verbal Behavior, 10, 608-613.

Rubin, T. (1988). User interface design for computer systems. Chichester: Ellis Horwood.

Rudnicky, A.L., & Kolers, P.A. (1984). Size and case of type as stimuli in reading. Journal of
Experimental Psychology: Human Perception and Performance, 10, 231-249.

Schwarz, E., Beldie, I.P., & Pastoor, S.A. (1983). A comparison of paging and scrolling for
changing screen contents by inexperienced users. Human Factors, 25, 279-282.

Shepard, R.N. (1987). Toward a universal law of generalization for psychological science.
Science, 237, 1317-1323.

Shneiderman, B. (1987). Designing the user interface: Strategies for effective human-computer
interaction. Reading, Mass.: Addison-Wesley.

Silverstein, L.D. (1987). Human factors for color display systems: Concepts, methods, and
research. In Durrett, H.J. (Ed.), Color and the computer. London: Academic Press.

Smith, S.L. (1962). Color coding and visual search.
Journal of Experimental Psychology, 64, 434-440.

Stewart, T.F.M. (1979). Eyestrain and visual display units: a review. Displays, April, 25-32.

Taylor, S.E., & Rupp, B.A. (1987). Display image characteristics and visual response. In B.
Knave, & P.G. Wideback (Eds.), Work with display units 86. Amsterdam: North Holland.

Tombaugh, J., Lickorish, A., & Wright, P. (1987). Multi-window displays for readers of lengthy
texts. International Journal of Man-Machine Studies, 26, 597-615.

Travis, D.S. (1990). Applying visual psychophysics to user interface design.
Behaviour & Information Technology, 9, 425-438.

Tullis, T.S. (1988). Screen design. In M. Helander (Ed.), Handbook of human-computer
interaction. Amsterdam: Elsevier.

van Oostendorp, H. (In press). Studying and annotating electronic text. In J.F. Rouet, & J.
Levonen (Eds.), Hypertext and cognition. Hillsdale, N.J.: Erlbaum.

Vartabedian, A.G. (1971). The effects of letter size, case, and generation method on CRT
display search time. Human Factors, 13, 363-368.

Wilkins, A.J., & Nimmo-Smith, M.I. (1987). The clarity and comfort of printed text.
Ergonomics, 30, 1705-1720.

Wilkinson, R.T., & Robinshaw, H.M. (1987). Proof-reading: VDU and paper text compared for
speed, accuracy and fatigue. Behaviour & Information Technology, 6, 125-133.

Williams, D.R., MacLeod, D.I.A., & Hayhoe, M. (1981). Foveal tritanopia.
Vision Research, 21, 1341-1356.

Williamson, N.L., Muter, P., & Kruk, R.S. (1986). Computerized presentation of text for the
visually handicapped. In E. Hjelmquist, & L.G. Nilsson (Eds.), Communication and handicap:
Aspects of psychological compensation and technical aids. North-Holland: Elsevier.

Wright, P., & Lickorish, A. (1984). Investigating referees' requirements in an electronic
medium. Visible Language, 18, 186-205.

Wright, P., & Lickorish, A. (1988). Colour cues as location aids in lengthy texts on screen and
paper. Behaviour & Information Technology, 7, 11-30.

Yaginuma, Y., Yamada, H., & Nagai, H. (1990). Study of the relationship between lacrimation and
blink in VDT work. Ergonomics, 33, 799-810.

Yankelovich, N., Meyrowitz, N., & van Dam, A. (1985). Reading and writing the electronic book.
IEEE Computer, 18, 15-30.