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
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
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
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
Examples of condensed, default, & expanded spacings
Exp 1. Results
Word legibility decreased with condensed spacing & increased with
expanded spacing, but not surpass single letter legibility
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
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)
Examples of default, condensed, & expanded spacings
Exp 2. Result:
 Fixation duration decreased with spacing
 Saccade amplitude increased with spacing
325
Fixation duration (ms)
4
300
3.5
275
3
250
2.5
225
2
200
1.5
175
1
-1.75 -1.50 -1.00 -0.50
0
0.50 1.00 1.50
Character Spacing (points from default)
2.00
Average Saccade Amplitude
(degree of visual angle)
Average Fixation Duration (ms)
Saccade amplitude (visual degree)
The number of fixations and regression rate
increased with spacing
However, words per fixation does not
change…
Words per fixation
1.3
1.2
1.1
1
0.9
0.8
-2
-1
0
1
Character spacing (points from default)
2
Nor does the reading speed…
Reading speed (words/min)
260
240
220
200
180
160
-2
-1
0
1
Character spacing (points from default)
2
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.
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
Peripheral span of visual acuity
Correct trial: when both letters are correct
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
accuracy
RT
1
1100
0.9
1000
0.8
Accuracy
0.6
800
0.5
700
0.4
600
0.3
500
0.2
0.1
400
0
300
3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 57 60
Peripheral span (characters)
RT (ms)
900
0.7
Visual span:
Decide whether the testing letter was presented on the
exact location earlier
Average accuracy across visal span
- Visual span increased at 1-character steps
- Individual visual span was calculated separately
accuracy
RT
1
1200
0.95
1150
0.9
1100
0.8
0.75
1050
0.7
1000
0.65
0.6
950
0.55
0.5
900
3
4
5
6
7
8
9
10
11
Visual span (characters)
12
13
14
RT (ms)
Accuracy
0.85
Cognitive span:
Decide whether the flashed text is a real word
Average cognitive span
- Cognitive span increased at 1-character steps
- Individual cognitive span was calculated separately
accuracy
RT
1
1050
1000
950
900
0.9
850
800
0.85
750
0.8
700
3
4
5
6
7
8
9
10
11
Cognitive span (characters)
12
13
14
RT (ms)
Accuracy
0.95
Can individual characters
predict reading measurements
across letter spacing?
Reading Progress are correlated to…
- Cognitive Span,
- but not to peripheral span or visual span
High vs. Low cognitive span
Information intake is regulated accorgind to Cognitive Span
Group
High
cognivite span
Low
cognitive span
Fixation
duration
Saccade
Amplitude
Regression rate
Word/fixation;
Reading speed
shorter
Larger
Lower
More/Faster
Longer
Smaller
Higher
Fewer/Slower
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.