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Self-consistent estimation of mislocated fixations during reading.

Engbert R, Nuthmann A - PLoS ONE (2008)

Bottom Line: During reading, we generate saccadic eye movements to move words into the center of the visual field for word processing.Our approach is based on iterative computation of the proportions of several types of oculomotor errors, the underlying probabilities for word-targeting, and corrected distributions of landing positions.These results show that fixation probabilities are strongly affected by oculomotor errors.

View Article: PubMed Central - PubMed

Affiliation: Department of Psychology, University of Potsdam, Potsdam, Germany. Ralf.Engbert@uni-potsdam.de

ABSTRACT
During reading, we generate saccadic eye movements to move words into the center of the visual field for word processing. However, due to systematic and random errors in the oculomotor system, distributions of within-word landing positions are rather broad and show overlapping tails, which suggests that a fraction of fixations is mislocated and falls on words to the left or right of the selected target word. Here we propose a new procedure for the self-consistent estimation of the likelihood of mislocated fixations in normal reading. Our approach is based on iterative computation of the proportions of several types of oculomotor errors, the underlying probabilities for word-targeting, and corrected distributions of landing positions. We found that the average fraction of mislocated fixations ranges from about 10% to more than 30% depending on word length. These results show that fixation probabilities are strongly affected by oculomotor errors.

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Random component of saccade errors and probabilities of mislocated fixations.(A) The random component of saccade errors is characterized by the standard deviation of the landing site distribution as a function of launch site distance. Distances are given as values from the center of the word. Best linear fits are also presented for experimental (green) vs. simulated (red) data. The reduced slope for the simulated data indicates the variance reduction achieved by our iterative estimation procedure. (B) Proportion of mislocated fixation as a function of relative fixation position for different word lengths (each curve represents a word length). For iteration 0, within-word distributions of mislocated fixations for word lengths 2 to 12 peak near word edges. (C) The overall probability of mislocated fixations (black) varies between about 10% and 30% for word lengths 2 to 12. For short to medium-sized words (<8 characters), failed skippings represent the most frequently occurring case of mislocated fixations, while for long words (>8 characters) unintended refixations are most important. (D) After 20 iterations, the proportion of mislocated fixations can be approximated by a single U-shaped curved for all words with lengths greater than 5 characters.
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pone-0001534-g004: Random component of saccade errors and probabilities of mislocated fixations.(A) The random component of saccade errors is characterized by the standard deviation of the landing site distribution as a function of launch site distance. Distances are given as values from the center of the word. Best linear fits are also presented for experimental (green) vs. simulated (red) data. The reduced slope for the simulated data indicates the variance reduction achieved by our iterative estimation procedure. (B) Proportion of mislocated fixation as a function of relative fixation position for different word lengths (each curve represents a word length). For iteration 0, within-word distributions of mislocated fixations for word lengths 2 to 12 peak near word edges. (C) The overall probability of mislocated fixations (black) varies between about 10% and 30% for word lengths 2 to 12. For short to medium-sized words (<8 characters), failed skippings represent the most frequently occurring case of mislocated fixations, while for long words (>8 characters) unintended refixations are most important. (D) After 20 iterations, the proportion of mislocated fixations can be approximated by a single U-shaped curved for all words with lengths greater than 5 characters.

Mentions: For refixations, a high rate of unintended refixations (Fig. 3b, green line) induced a higher refixation rate in the simulations than observed in the experimental data. After 20 iterations, however, the simulations suggested that the experimentally observed refixation probability (Fig. 3d, black line) can be decomposed into intended (light blue line) and unintended refixations (green line). Simultaneously to adjusting the fixation probabilities, our iterative procedure corrected the distributions of within-word landing positions by the amount of mislocated fixations (Fig. 1, red). As predicted, the resulting well-located landing position distributions are characterized by smaller variances (Fig. 2b). In the oculomotor model, this reduction of variance leads to a significantly reduced standard deviation of the random error component of saccades (Fig. 4a).


Self-consistent estimation of mislocated fixations during reading.

Engbert R, Nuthmann A - PLoS ONE (2008)

Random component of saccade errors and probabilities of mislocated fixations.(A) The random component of saccade errors is characterized by the standard deviation of the landing site distribution as a function of launch site distance. Distances are given as values from the center of the word. Best linear fits are also presented for experimental (green) vs. simulated (red) data. The reduced slope for the simulated data indicates the variance reduction achieved by our iterative estimation procedure. (B) Proportion of mislocated fixation as a function of relative fixation position for different word lengths (each curve represents a word length). For iteration 0, within-word distributions of mislocated fixations for word lengths 2 to 12 peak near word edges. (C) The overall probability of mislocated fixations (black) varies between about 10% and 30% for word lengths 2 to 12. For short to medium-sized words (<8 characters), failed skippings represent the most frequently occurring case of mislocated fixations, while for long words (>8 characters) unintended refixations are most important. (D) After 20 iterations, the proportion of mislocated fixations can be approximated by a single U-shaped curved for all words with lengths greater than 5 characters.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2211408&req=5

pone-0001534-g004: Random component of saccade errors and probabilities of mislocated fixations.(A) The random component of saccade errors is characterized by the standard deviation of the landing site distribution as a function of launch site distance. Distances are given as values from the center of the word. Best linear fits are also presented for experimental (green) vs. simulated (red) data. The reduced slope for the simulated data indicates the variance reduction achieved by our iterative estimation procedure. (B) Proportion of mislocated fixation as a function of relative fixation position for different word lengths (each curve represents a word length). For iteration 0, within-word distributions of mislocated fixations for word lengths 2 to 12 peak near word edges. (C) The overall probability of mislocated fixations (black) varies between about 10% and 30% for word lengths 2 to 12. For short to medium-sized words (<8 characters), failed skippings represent the most frequently occurring case of mislocated fixations, while for long words (>8 characters) unintended refixations are most important. (D) After 20 iterations, the proportion of mislocated fixations can be approximated by a single U-shaped curved for all words with lengths greater than 5 characters.
Mentions: For refixations, a high rate of unintended refixations (Fig. 3b, green line) induced a higher refixation rate in the simulations than observed in the experimental data. After 20 iterations, however, the simulations suggested that the experimentally observed refixation probability (Fig. 3d, black line) can be decomposed into intended (light blue line) and unintended refixations (green line). Simultaneously to adjusting the fixation probabilities, our iterative procedure corrected the distributions of within-word landing positions by the amount of mislocated fixations (Fig. 1, red). As predicted, the resulting well-located landing position distributions are characterized by smaller variances (Fig. 2b). In the oculomotor model, this reduction of variance leads to a significantly reduced standard deviation of the random error component of saccades (Fig. 4a).

Bottom Line: During reading, we generate saccadic eye movements to move words into the center of the visual field for word processing.Our approach is based on iterative computation of the proportions of several types of oculomotor errors, the underlying probabilities for word-targeting, and corrected distributions of landing positions.These results show that fixation probabilities are strongly affected by oculomotor errors.

View Article: PubMed Central - PubMed

Affiliation: Department of Psychology, University of Potsdam, Potsdam, Germany. Ralf.Engbert@uni-potsdam.de

ABSTRACT
During reading, we generate saccadic eye movements to move words into the center of the visual field for word processing. However, due to systematic and random errors in the oculomotor system, distributions of within-word landing positions are rather broad and show overlapping tails, which suggests that a fraction of fixations is mislocated and falls on words to the left or right of the selected target word. Here we propose a new procedure for the self-consistent estimation of the likelihood of mislocated fixations in normal reading. Our approach is based on iterative computation of the proportions of several types of oculomotor errors, the underlying probabilities for word-targeting, and corrected distributions of landing positions. We found that the average fraction of mislocated fixations ranges from about 10% to more than 30% depending on word length. These results show that fixation probabilities are strongly affected by oculomotor errors.

Show MeSH
Related in: MedlinePlus