Blink Is Not A Random Event In Reading Yu-Chi Tai, James Sheedy, & John Hayes Pacific University, College of Optometry

Blink Definition –Momentary visual suppression caused by rapid cocontraction of eye muscles. –Resulted in mini blackouts, usually unconscious, as in saccade suppression. Function –To moisten and oxygenate the retina Frequency –Resting : about 17/min. –Conversation: about 26/min. –Reading: about 4.5/min.  Blinks tend to be suppressed during intensive visual concentration.

Blinks in Reading Keith Rayner: “ Blinks are seen as a random event in reading and hence are excluded from data analysis in eye-movement reading studies.” (Response to audience’s questions in a seminar at OSU Cognitive Science Center, 2004).

Questions –Are blinks random events in reading? –When and where do blinks occur in reading? –How are Blinks related to Reading Performance?

The Study Subjects –42 subjects (age 18~35 yr) –Binocular visual acuity ≥ 20/20 Task –15~20 min. onscreen reading of normal format text (12-pt Verdana in ClearType, normal character spacing, triple line spacing) –Answering post-reading comprehension questions to attune attention Eye movement recording –SR EyeLink II eyetracker (250 or 500 Hz) Data analysis –Overall pattern analysis of 42 subjects –Frequency Distribution of critical events of a single subject’s data. (so-far)

Categories of Eye Movements Saccades –Period with velocity > threshold for 2+ continuous samples –4 types Line Change saccade Regressive saccade Forward saccade Corrective saccade Blinks –Period of saccade with the pupil data missing for 3+ continuous samples –3 phases Eye closing (pupil size ↓) Eye closed (pupil size missing) Eye opening (pupil size ↑) Fixation: Any period that is neither a blink nor a saccade

Samples of Critical Eye Movement Events Fixation -2 Fixation -1 Saccade -1 Fixation 1 Fixation 2 Saccade 1 Saccade 2 Start closing Start closed End closed End opening Blink Movement

 While most (66.7%) eye movements were Forward saccades… only close to half (49.9%) blinks occurred in them. Repeated ANOVA of Z scores [(BlinkRate–SaccadeRate)/ SaccadeRate**2] (p<.0001); Corrective(z=.723)>Regressive(.201)≈LineChange(.066)>Forward(-.155) When Do We Blink? P<.0001* P=.01* P=.143

Blink Occurring Rate at different types of eye movements… Corrective (61%) > Regressive (8%) > Line Change (5%) > Forward (3%)

Finding (1) Different from saccade frequency, blinks occurred more often in Corrective, Regressive, and Line Change saccades than in Forward saccades. –In reading, Regressive, Corrective, and Line Change saccades are interruptions for : Cognitive difficulty Interruption in Visual Acquisition  Blinks occur more frequently during these breakdowns in the reading process.

When A Blink Occurs… -- Blink Duration Compare to Normal saccades: Blink duration > Normal saccade duration (p <.0001). Similar duration across blink phases (p=.123), though eyelid seemed to close longer (p=.052) in Line Change blink movement. Regressive ≈ Corrective < Line-Change (p=.019) & Forward (p=.042)

When A Blink Occurs… -- Direction of Critical Saccades Compare to Normal saccades:  Blink-1: no difference.  Blink: ↑ Regressive (p< ), ↑ Line-Change (p<. 001 ), ↓ Forward (p<. 001 ), ↑ Corrective (p< )  Blink+1 : ↑ Regressive (p=.004), ↓ Forward (p=.0008), ↑ Corrective (p<.0001)  Blink+2 : ↓ Line-Change (p=.0002)

When A Blink Occurs… -- Critical Fixation Duration Compare to Normal saccades: Fixation duration was significantly (p<.01) shorter for fixations before (133ms) and after (104ms) a blink. The effect was carried over to the 2nd fixation after a blink (189ms).

Frequency Distribution of Fixation Duration in Critical Fixations Bi-modal Prior fixation duration (peak: 20, 120 ms) Short Next Fixation (peak 40ms)

Frequency Distribution of Horizontal Movement in Critical Saccades All bimodal distributions (peaked -0.6º ~ -0.3º, 0.9º~1.1º), except blink movement (peaked at 0.1º) Word -1 Word 1 Word 2 x

Distribution of Vertical Movement  Blink movements seem to have greater vertical components. L -4 L -3 L -2 L -1 L 0 L 1

Finding (2) Blink duration was longer than normal saccade duration.  Opportunity to rest the eyes? Eyelid Opening took longer than Eyelid Closing  Mechanically reasonable given muscle action Fixation duration before and after a blink was shorter than normal fixations  Problems in reading Bi-modal Fixation -1 Duration  Different functions of blink Uni-modal horizontal and vertical Blink movement  Dwelling in the current text; problems encountered? More regressive and corrective saccades followed a blink  Still in trouble

Blink Behavior in Reading Same trend between groups, Frequent Blinkers are just blink more at all types of saccades. P=.004* P<.001* P=.49

Blink Behavior in Reading 28 Rare Blinkers (4.2/min), more bllinks in Line Change Saccades 14 Frequent Blinkers (16.7/min), more blinks in Regressive saccades P=.004* P<.001*

Blink Behavior & Reading Performance Reading speed is Negatively related to Blink Rate.  For Rare Blinkers, greater mental effort is needed, hence blinks are suppressed.

Conclusions Blinks do not occur randomly in reading but –more associated with the interruptions in reading: Corrective saccades Regressive saccades Line change saccades –Lower rate in forward reading progress When a blink occurs: –The eye dwells around the same place before and after a blink –Follows with higher regressive rate Both Rare Blinkers and Frequent Blinkers tended to blink during the breakdowns in reading. –Rare Blinkers (slower readers) suppress blinks for Greater Mental demand?

Rather than a pure random event, blinks seem to be modulated by some central mechanism, to make blink occurrence less interfere with the normal progressive reading process.

Acknowledgement This project was sponsored by the Reading Advance Group in Microsoft Corporation.

 Most were Forward saccades… Forward (67%) > Regressive (16.7%) > Line-Changing (16%) > Corrective (0.3%) Of All Saccades…

Horizontal Movement in Blink Forward or Regressive, both around 0 degree

Where Are the Blinks? -- Sample page of eye movement sequence during reading

Blink occurrence across a page by saccade category