1. The Effect of Inter-letter Spacing on Reading Yu-Chi Tai, PhD James E
The Effect of Inter-letter Spacing on Reading Yu-Chi Tai, PhD James E. Sheedy, OD, PhD, FAAO John R. Hayes, PhD Pacific University Vision Performance Institute Vision Ergonomics Laboratory Forest Grove OR 97116

2. Background The Original Ultimate goal:
Through systematic investigation on designing factors (e.g., font type, font size, font smoothing, character spacing) to identify a set of critical objective (display environment) and subjective (user characters, e.g., visual acuity, perceptual span) factors to design individualized optimal displays for maximum onscreen task performance
Previous findings
Threshold word legibility < letter legibility (Sheedy et al, 2005, Human Factor)
Inference
Within-word inter-letter spacing should affects threshold legibility and reading

3. Study Design Target measurements
Effect of inter-letter spacing on word recognition
Effect of inter-letter spacing on text reading
Objective factor:
Inter-letter spacing (default, narrower or wider)
Subjective factors:
Visual acuity
Span of perception / recognition

4. Exp. 1: Inter-letter spacing and legibility
Subjects:
30 subjects, 20/20+ vision
Measurement:
Recognition of invidiaul letters or words
Step-back distance visual acuity measure, transformed to (1/logMAR) for relative legibility
Manipulation:
Inter-letter spacing

5. Examples of condensed, default, & expanded spacings

6. Exp 1. Results Word legibility decreased with condensed spacing & increased with expanded spacing, but not surpass single letter legibility

7. Inter-letter spacing affects threshold word recognition
Threshold word legibility improved as inter-letter spacing increased, until it reaches asymptote at single letter legibility.
 suggesting:
The existence of lateral interference on word recognition
Expanding letter spacing lessens lateral interference and potentially enhances word recognition
Letter recognition as the basis for word identification, at least with single presentation and not precue priming

8. Exp. 2: Inter-letter spacing & reading
Subjects:
41 subjects, 20/20+ vision
Task & Material:
Read a novel presented in 9 inter-letter spacing conditions (-1.75 ~ +2.0 point form the default)
Text:
About 2800~3200 words/condition
10-pt Verdana
Measurement:
Reading speed
Eye movements
Apparatus:
SR Research Inc. EyeLink II eye tracker (250/500 Hz)

9. Examples of default, condensed, & expanded spacings

10. Exp 2. Result:
Fixation duration decreased with spacing
Saccade amplitude increased with spacing

11. The number of fixations and regression rate increased with spacing

12. However, words per fixation does not change…

13. Nor does the reading speed…

14. Results
In reading, visual system adapts to the change of inter-letter spacing efficiently with changes of eye movement patterns.
Visual system adapted to the change by changing eye movement patterns.
Condensed spacing: smaller saccades + longer fixation duration
Vice versa with expanded spacing
Rate of cognitive uptake (e.g., characters per fixation) is mostly unaffected
Reading speed remained unchanged.

15. Exp. 3: Individual characters vs. spacing effect
Subjects:
The same 41 subjects from exp. 2
Tasks:
Peripheral span (span of bilateral letter recognition)
Visual span (span of within-string letter recognition)
Cognitive span (span of word recognition)
Materials:
10-point Courier New font (0.277 degree at 60 cm)
Presentation time: 200 msec
Measurements:
Accuracy RT

16. Peripheral span of visual acuity Correct trial: when both letters are correct

17. Average accuracy across peripheral span (each side) - Peripheral span increased at 3-character steps - Individual peripheral span was calculated separately according to individual trend of 90% accuracy

18. Visual span: Decide whether the testing letter was presented on the exact location earlier

19. Average accuracy across visal span - Visual span increased at 1-character steps - Individual visual span was calculated separately

20. Cognitive span: Decide whether the flashed text is a real word

21. Average cognitive span - Cognitive span increased at 1-character steps - Individual cognitive span was calculated separately

22. Can individual characters predict reading measurements across letter spacing?

23. Reading Progress are correlated to… - Cognitive Span, - but not to peripheral span or visual span

24. High vs. Low cognitive span
Information intake is regulated accorgind to Cognitive Span
Group
Fixation duration
Saccade Amplitude
Regression rate
Word/fixation; Reading speed
High
cognivite span
shorter
Larger
Lower
More/Faster
Low
cognitive span
Longer
Smaller
Higher
Fewer/Slower

25. Conclusion Increasing letter spacing enhances single word recognition.
In reading, spacing effect was tuned by changes in eye movement patterns so Reading speed remained uncahnged.
Reading speed and eye movement adjustments relative to letter spacing differed by cognitive span, but not peripheral span and visual span.
Cognitive processing regulates information intake by adjusting eye movements and ultimately limits reading speed, at least for normally sighted individuals.