published between 2019 and 2022
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) 
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.
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. By 1916 it had been accepted into the English literature.
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.’
‘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.’
‘Rapid eye movements used in repositioning the fovea to a new location in the visual environment.’
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...
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.
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.
Hall and Sowdon regret that the Newborn Infant Physical Examination (NIPE)/child health surveillance (CHS) programme fails to deliver improved outcomes for developmental dysplasia of the hip (DDH), contrasting with the success of other screening programmes. I would like to make some proposals for improvement.
Current NIPE standards are focused on timeliness of the screening pathway and explicitly exclude treatment outcomes as ‘outside the screening pathway’1. Yet potential outcome measures are routinely available for three of the four NIPE screening programmes and shown to be measurable for two of these. McAllister et have demonstrated that records of surgical intervention for DDH can be used to show variation in outcomes2. Similarly, the NHS Atlas of Variation has demonstrated that age at orchidopexy can be used for undescended testis (UDT)3. Surgery for congenital cataract could be used in the same way. While I accept that definitions and actual measures might need some discussion to reach a national consensus, measuring these outcomes is possible from routine data.
McAllister et al conclude that dedicated leadership of the DDH screening programme is associated with improved outcomes. This has also been shown for UDT4. Unfortunately, clinical leadership of the Healthy Child Programme (HCP) has been dismantled in recent years and the RCPCH recorded a community paediatric HCP lead in only 16% of services in 2015.
Lastly poor outcomes may indicate...
Lastly poor outcomes may indicate a lack of training. The DDH and UDT examples describe multifaceted interventions including training and support of practitioners. Health visitor training now concentrates on health promotion and safeguarding with a reduced emphasis on child development and clinical skills. There are no longer any specific training requirements for general practitioners (GPs) who perform CHS. This is concerning as we know that only a proportion of GPs have any dedicated training in paediatrics before entering general practice.
There are, therefore, a series of proven measures that, if implemented, could improve the programmes’ performance.
1. Public Health England. Our approach to newborn and infant physical examination screening standards. 2018 https://www.gov.uk/government/publications/newborn-and-infant-physical-e... (accessed 11.01.19)
2. McAllister D A, Morling JR, Fischbacher C M et al. Enhanced detection services for developmental dysplasia of the hip in Scottish children 1997 – 2013. Arch Dis Child 2018; 103: 1021 – 1026.
3. Child and Maternal Health Observatory. NHS Atlas of Variation in Healthcare of Children and Young People. 2012. Ch Map 25 Proportion (%) of all elective orchidopexy procedures performed before the age of 2 years by PCT 2007/08–2009/10 p.68-69. Available at https://fingertips.phe.org.uk/profile/atlas-of-variation
4. Brown JJ, Wacogne I, Fleckney S, et al. Achieving early surgery for undescended testes: quality improvement through a multi-faceted approach to guideline implementation. Child: care, health and development 2004; 30: 97–102.
5. Royal College of Paediatrics and Child Health. RCPCH Medical Workforce Census 2015. London 2017. Available at https://www.rcpch.ac.uk/resources/workforce-census-2015 .