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The educational value of Saccades
Damian T Roland, Ben McNaughten
Published on: 30 January 2019
The educational value of Saccades
Tayyib Mubashar
Published on: 30 January 2019

Published on: 30 January 2019
The educational value of Saccades
- Damian T Roland, Consultant and Honorary Associate Professor Leicester Hospitals and University
- Other Contributors:
  Ben McNaughten, Paediatrician
Many thanks for your response to the editorial ‘What are you looking at?’, which highlights some important principles for this extensively studied research area (despite being a relatively new field in healthcare) [1]
Despite the emergence of new methods to analyse gaze behaviour terminology has not been revised to reflect scientific advances. A recent article by Hessels et al. outlined significant inconsistencies in the definitions of fixations and saccades held by eye movement researchers and highlighted the conceptual confusion surrounding these terms.[2]

The term saccade is derived from the French for ‘jerk’. The phrase appears to have been coined by Emile Javal, a French ophthalmologist, in the 1800’s.[3] By 1916 it had been accepted into the English literature.[4]

Saccades are frequently defined in the literature as rapid, ballistic movements of the eyes that abruptly change the point of fixation.5 Definitions have included;

‘Rapid eye movements used to voluntarily move gaze from one target of interest to another.’[6]

‘Ballistic movements, 20-150ms long, reaching a velocity up to 800°/s. They direct the eye so that external visual objects are projected onto the fovea.’[7]

‘Rapid eye movements used in repositioning the fovea to a new location in the visual environment.’[8]

The term ballistic refers to the fact that the saccade-generating system cannot respond to subsequent changes in the position of a target during th...
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Many thanks for your response to the editorial ‘What are you looking at?’, which highlights some important principles for this extensively studied research area (despite being a relatively new field in healthcare) [1]
Despite the emergence of new methods to analyse gaze behaviour terminology has not been revised to reflect scientific advances. A recent article by Hessels et al. outlined significant inconsistencies in the definitions of fixations and saccades held by eye movement researchers and highlighted the conceptual confusion surrounding these terms.[2]

The term saccade is derived from the French for ‘jerk’. The phrase appears to have been coined by Emile Javal, a French ophthalmologist, in the 1800’s.[3] By 1916 it had been accepted into the English literature.[4]

Saccades are frequently defined in the literature as rapid, ballistic movements of the eyes that abruptly change the point of fixation.5 Definitions have included;

‘Rapid eye movements used to voluntarily move gaze from one target of interest to another.’[6]

‘Ballistic movements, 20-150ms long, reaching a velocity up to 800°/s. They direct the eye so that external visual objects are projected onto the fovea.’[7]

‘Rapid eye movements used in repositioning the fovea to a new location in the visual environment.’[8]

The term ballistic refers to the fact that the saccade-generating system cannot respond to subsequent changes in the position of a target during the course of an eye movement. If the target was to move a second saccade would be required to correct the error. Acceptance of such definitions suggests that the primary function of saccades is to bring objects of interest onto or near the fovea. The fovea operates at high resolution and, although it only provides 1-2 degrees of vision, it plays a central role in resolving objects. Whilst saccades assist vision by moving the eyes rapidly to various objects of interest so that parts of a scene can be seen in greater resolution, there is probably little or no meaningful information absorbed during the saccadic movements.

However, other papers have been more liberal in their definitions defining a saccade as the inter-fixation interval.[9,10] These definitions imply that a saccade is any movement outside of periods of fixation. This definition will therefore also capture smooth pursuit movements (slower tracking movements), vestibulo-ocular movements (stabilise eyes relative to external world) and visual scanning movements. Consequentially during saccades defined in this manner visual information may be absorbed both consciously and/or subconsciously including, for example, the presence or absence of pathology, as outlined in your letter.

As the field of eye tracking research in healthcare expands it is imperative that authors explicitly define their key measurements, including fixations, dwell time and saccades, to allow accurate interpretation and comparison of results and to minimise ambiguity. Without this it will be difficult to examine the links between visual scanning patterns and decision making. For example it may be hypothesised that subconscious visual scanning behaviours may be a clue to understanding the concept of gut feeling, whereby a particular clinician sees things perhaps others do not.

We certainly agree greater understanding of saccade pathways should undoubtedly be an area for future research in eye tracking studies in healthcare.

References:
1. Roland D. What are you looking at? Arch Dis Child 2018; 103: 1098-1099.

2. Hessels RS, Niehorster DC, Nyström M, Andersson R, Hooge IT. Is the eye-movement field confused about fixations and saccades? A survey among 124 researchers. Royal Society open science. 2018 Aug 29;5(8):180502.

3. Javal E. Essai sur la physiologie de la lecture. Annales d'Ocilistique. 1878;80:61-73.

4. Wade NJ. Scanning the seen: Vision and the origins of eye-movement research. In Eye Movements 2007 (pp. 31-63).

5. Purves D, Augustine GJ, Fitzpatrick D, Katz LC, LaMantia AS, McNamara JO, Williams SM. Neuroscience 2nd Edition. Sunderland (MA) Sinauer Associates.

6. Ramat S, Leigh RJ, Zee DS, Optican LM. What clinical disorders tell us about the neural control of saccadic eye movements. Brain. 2006 Nov 21;130(1):10-35.

7. Falkmer T, Dahlman J, Dukic T, Bjällmark A, Larsson M. Fixation identification in centroid versus start-point modes using eye-tracking data. Perceptual and motor skills. 2008 Jun;106(3):710-24.

8. Duchowski AT. Eye tracking methodology. Theory and practice. 2007;328.

9. Holmqvist K, Nyström M, Andersson R, Dewhurst R, Jarodzka H, Van de Weijer J. Eye tracking: A comprehensive guide to methods and measures. OUP Oxford; 2011 Sep 22.

10. Larsson L, Nyström M, Andersson R, Stridh M. Detection of fixations and smooth pursuit movements in high-speed eye-tracking data. Biomedical Signal Processing and Control. 2015 Apr 1;18:145-52.
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Conflict of Interest:
None declared.
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Published on: 30 January 2019
The educational value of Saccades
- Tayyib Mubashar, Medical Student University of Exeter Medical School
Dear Sir,

As a final year medical student, I read with great interest this article highlighting the pedagogical value of gaze study. Whilst the article raises many poignant features of this new medium of data collection, I believe the definition of saccade in your article may be slightly incomplete.

The article states that “Saccades...are movements of the eyes between fixations in which information is not meaningfully acquired or absorbed.” However, I would suggest that whilst the primary purpose of saccades may be to move between fixation points, there is subconscious information acquisition completed by these movements-- the subconscious identification of a lack of pathology.

If one does not take certain regions of an image as fixation points, it suggests that they are not looking for pathology in those regions. This is reinforced by one study analysing gaze study in mammogram analysis which reported that “long saccades often missed the target but were followed by corrective saccades”. (1-Kundel H, Nodine C, Conant E et al. Holistic component of image perception in mammogram interpretation: gaze-tracking study. Radiology 2007;242(2):396-402.) This highlights that within a saccade is also inert analysis of a lack of pathology rather than just transformational eye movement.
Another study analysing a gaze study of virtual histopathology slides between trained pathologists, residents and medical students reported that “fully trained pathologists h...
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Dear Sir,

As a final year medical student, I read with great interest this article highlighting the pedagogical value of gaze study. Whilst the article raises many poignant features of this new medium of data collection, I believe the definition of saccade in your article may be slightly incomplete.

The article states that “Saccades...are movements of the eyes between fixations in which information is not meaningfully acquired or absorbed.” However, I would suggest that whilst the primary purpose of saccades may be to move between fixation points, there is subconscious information acquisition completed by these movements-- the subconscious identification of a lack of pathology.

If one does not take certain regions of an image as fixation points, it suggests that they are not looking for pathology in those regions. This is reinforced by one study analysing gaze study in mammogram analysis which reported that “long saccades often missed the target but were followed by corrective saccades”. (1-Kundel H, Nodine C, Conant E et al. Holistic component of image perception in mammogram interpretation: gaze-tracking study. Radiology 2007;242(2):396-402.) This highlights that within a saccade is also inert analysis of a lack of pathology rather than just transformational eye movement.
Another study analysing a gaze study of virtual histopathology slides between trained pathologists, residents and medical students reported that “fully trained pathologists had prolonged saccadic movement and lower saccadic velocity [compared to residents and medical students]”. (2-Krupinski E, Tillack A, Richter L et al. Eye-movement study and human performance using telepathology virtual slides. Implications for medical education and differences with experience. Hum Pathol 2006;37(12):1543-56.) This suggests that experts do not need to have greater fixation points as they were aware of a lack of pathology through subconscious processing which led to a lower saccade velocity.

Through greater understanding about saccade pathways, we may be able to both add to our knowledge of how participants are analysing the data they are being given as well as position ourselves to better analyse where participants are also missing pathology.
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Conflict of Interest:
None declared.
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