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. 2017 Sep 21;12(9):e0185180.
doi: 10.1371/journal.pone.0185180. eCollection 2017.

Microsaccades During Reading

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Free PMC article

Microsaccades During Reading

Norick R Bowers et al. PLoS One. .
Free PMC article

Abstract

Recent research has shown that microsaccades contribute to high acuity vision. However, little is known about whether microsaccades also play a role in daily activities, such as reading, that do not involve stimuli at the limit of spatial resolution. While the functions of larger saccades in reading have been extensively examined, microsaccades are commonly regarded as oculomotor noise in this context. We used high-resolution eyetracking and precise gaze localization to investigate fine oculomotor behavior during reading. Our findings show that microsaccade characteristics differ from those measured during sustained fixation: microsaccades are larger in size and primarily leftwards during reading, i.e. they move the line of sight backward on the text. Analysis of how microsaccades shift gaze relative to the text suggests that these movements serve two important functions: (1) a corrective function, by moving the gaze regressively within longer words when the preceding saccade lands too far toward the end of these words, and (2) an exploratory function, by shifting the gaze on adjacent words to gain additional information before the execution of the next saccade. Thus, microsaccades may benefit reading by enhancing the visibility of nearby words. This study highlights the importance of examining fine oculomotor behavior in reading, and calls for further research to investigate the possible roles of microsaccades in reading difficulties.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Methods and stimuli.
(A) An example of eye movements during reading. The lines of text are an excerpt from one of the paragraphs used in the study. The size of the foveola (the high-acuity region of the retina) is also shown for comparison. Different types of eye movements are color-coded. (B) Accuracy in gaze localization. Error in gaze localization for one subject, each dot represents the offset between the gaze position and the fixated marker in an individual repetition of the calibration procedure (see Methods for details). The 99% confidence ellipse and the marginal distributions with their Gaussian fits are also shown. The numbers on the graph indicate horizontal and vertical standard deviations of the distribution. (C) Number of characters spanned as a function of saccade size. The shaded area in the graph represents the microsaccade range. Data points are averages across observers, and error bars represent s.e.m.
Fig 2
Fig 2. Microsaccades rates and amplitudes.
(A) Rates of microsaccades during sustained fixation and during reading. The asterisk marks a statistically significant difference (p<0.01, two-tailed paired t-test). (B) Correlation between microsaccade rates in the two conditions. Each point represents the average rates for one subject. Error bars represent s.e.m. Probability distribution of saccade amplitudes during reading (C) and sustained fixation (D) across all the tested subjects. The vertical lines in the graphs mark the microsaccade threshold. The insets show microsaccade amplitude distributions.
Fig 3
Fig 3. Microsaccades and saccades direction.
Average 2D polar histograms of microsaccades direction and amplitude during reading (A) and sustained fixation (B). (C) Average probability of microsaccade direction during reading and during sustained fixation. Microsaccade direction was classified into three different categories; progressive (rightward), regressive (leftward) and vertical (up and downward). (D) Average probability of regressive and progressive saccades during reading as a function of saccade amplitude. Error bars represent s.e.m.
Fig 4
Fig 4. Microsaccades and fixation duration.
Average probability of microsaccade occurrence as a function of the preceding fixation duration. Error bars represent 95% C.I.
Fig 5
Fig 5. The pattern of microsaccades on the text.
Microsaccades were divided into different categories depending on their starting and landing positions. (A) Average probabilities of task-relevant and task-irrelevant microsaccades obtained in three conditions: (1) the actual data, (2) by randomly placing the microsaccades performed during sustained fixation over the text (Random placement), and (3) by substituting the microsaccades performed during reading with those performed during sustained fixation (Original placement). Error bars represent s.e.m. Asterisks indicate statistically significant differences (p<0.01, two-tailed t-test). (B) Task-relevant microsaccades were mostly characterized by microsaccades branching two nearby words (Inter-word), and by microsaccades shifting the gaze within one word (Intra-word).
Fig 6
Fig 6. Intra-word microsaccadic gaze shifts.
(A) Start position of intra-word microsaccades. The x-axis marks the number of characters from the last in a word. (B) Number of characters spanned by intra-word microsaccades. Positive and negative numbers indicate progressive and regressive shifts, respectively. (C) Landing positions of saccades followed by different types of eye movements (saccades, intra-word microsaccade and other microsaccades). Data in C refer to saccades landing on words longer than 7 characters. Error bars represent s.e.m.

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