The effect of inter-letter spacing on reading
Yu-Chi Tai, John Hayes , James Sheedy
College of Optometry Pacific University, Forest Grove, Oregon
In Experiment 1, the threshold legibility of lower case
letters and words was measured using a distance visual
acuity method derived from standardized clinical testing of
visual acuity (ISO, 1994) and as used in previous legibility
studies. Thirty subjects (age 18 to 35 years) were recruited
and screened to meet the following inclusion criteria:
best-corrected visual acuity of 20/20 or better, no ocular
pathology and no binocular vision anomalies.
Experiment 2 explored the effects of spacing and legibility
upon reading by measuring eye movement patterns and
reading speed when reading text with different inter-letter
spacings. Forty-one subjects (11 male and 30 female), 1835 years of age, were screened to have corrected or
uncorrected visual acuity of 20/20 or better in both eyes.
Sample
Thequickbrownfox
1.0 condensed
The quick brown fox
Fixation duration (ms)
325
Saccade amplitude (visual degree)
The quick brown fox
0.0 default
Average Fixation Duration (ms)
0.5 condensed
The quick brown fox
0.5 expanded
The quick brown fox
1.0 expanded
Th e qu i ck b r o wn f ox
1.5 expanded
The quick brown fox
2.0 expanded
4
The quick brown fox
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)
Word legibility improves with spacing,
but does not surpass individual letter
legibility – the “letter superiority effect.”
Average Saccade Amplitude
(degree of visual angle)
1.75 condensed
CONCLUSIONS
The visual system “adapts” to spacing
changes by altering fixation duration and
eye movement size
2.00
However, words/fixation does not change
1.3
1.3
*
#
1.1
*
#
1
#
0.9
*
#
*
#
#
*
#
*
*
*
*
*
*
*
Words per fixation
1.2
#
#
#
#
0.8
1.2
1.1
1
Expanded 10.0
Expanded 9.0
Expanded 8.0
Expanded 7.0
Expanded 6.0
Expanded 5.0
Expanded 4.0
Expanded 3.0
Expanded 2.0
Expanded 1.0
Expanded 0.8
Expanded 0.6
Expanded 0.4
Expanded 0.2
0.8
Deafult spacing
0.6
Condensed 0.2
0.9
Condensed 0.4
0.7
-2
-1
0
1
2
Character spacing (points from default)
Character Spacing (points from default)
…nor does the reading speed
Figure 2. Relative legibility of individual letters and words with
various spacing levels. Error bars are SEM. “#” indicates significant
difference (p < 0.05) from the letter legibility; “*” indicates
significant difference (p < 0.05) from the legibility in the setting of
default character spacing.
Measuring eye movements
As inter-letter spacing increases,
threshold word legibility
significantly improves until it
reaches asymptote at single letter
legibility, suggesting the existence of
crowding effect on word recognition
and letter recognition as the
fundamention for word
identification. However, inter-letter
spacing does not affect reading
speed; rather, the effect is
compensated by changes in eye
movement patterns. Although text
legibility decreases with condensed
spacing, the visual system adapts
with smaller saccade amplitudes
and increased fixation duration; and
vice versa with expanded spacing.
The number of words processed per
fixation is largely unchanged,
indicating that cognitive processing
regulates eye movement and
ultimately limits reading speed for
normally sighted individuals.
260
Reading speed (words/min)
MATERIALS & METHODS
Spacing
Condensed 0.6
We investigated the effects of inter-letter
spacing upon threshold word legibility as
well as eye movements and speed during
reading.
RESULTS (cont’d)
Spacing Samples
Single letter
Reading is a complex task where linguistic
codes are picked up through the visual
system for cognitive processing. The
typical reading pattern includes a
repetitive pattern of saccadic eye
movements and intervening fixations on
the text. The saccadic eye movements
typically span 7-9 characters and have
duration of approximately 35 ms during
which no visual information is processed
but cognitive processing may continue.
During the fixations of approximately
200-250 ms, text information is acquired
during the first 100 ms and cognitively
interpreted during the remainder of the
fixation. Efficient reading requires the
ability to quickly extract information
during the short pause of an eye fixation;
presumably more legible text enhances
visual acquisition.
RESULTS
Relative Legibility
ABSTRACT
ACKNOWLEDGEMENTS
240
220
This study was supported by a grant from Microsoft
Corporation to James E. Sheedy.
200
180
160
-2
-1
0
1
Character spacing (points from default)
2
CONTACT INFORMATION
Principal Investigator
Yu-Chi Tai
T: 503.352.2289
E: [email protected]