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

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

The number of fixations and regression rate increased with spacing

However, words per fixation does not change…

Nor does the reading speed…

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

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

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

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 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

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.