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Visual Fixation Development in Children

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Visual Fixation Development in Children Graefe’s Arch Clin Exp Ophthalmol (2007) 245:1659–1665 DOI 10.1007/s00417-007-0585-6 CLINICAL INVESTIGATION Visual fixation development in children Eva Aring & Marita Andersson Grönlund & Ann Hellström & Jan Ygge Received: 12 December 2006 /Revised: 2 March 2007 /Accepted: 31 March 2007 / Published online: 24 April 2007 # Springer-Verlag 2007 Abstract there were no significant differences with regard to gender Background The ability to keep steady fixation on a target or laterality in any of the investigated variables. No is one of several aspects of good visual function. However, nystagmus was observed. there are few reports on visual fixation during childhood in Conclusion This study establishes values for visual fixation healthy children. behaviour in a non-clinical population aged 4–15 years, Methods An infrared eye-tracking device (Orbit) was used which can be used for identifying children with fixation to analyse binocular fixation behaviour in 135 non-clinical abnormalities. participants aged 4–15 years. The children wore goggles and their heads were restrained using a chin and forehead Keywords Blinks . Drifts . Intruding saccades . rest, while binocularly fixating a stationary target for 20 s. Centre of gravity Results The density of fixations around the centre of gravity increased with increasing age (p<0.01), and the time of fixation without intruding movements increased Introduction with increasing age (p=0.02), while intruding saccades decreased with increasing age (p<0.01). The number of The ability to visually fixate a target is one of several blinks and drifts did not differ between 4 and 15 years, and aspects of good visual function [1]. The ability to steadily fixate is not yet developed at birth, but is usually acquired E. Aring : M. A. Grönlund : A. Hellström during the first 6 months of life [2], parallelling the Institute of Neuroscience and Physiology/Ophthalmology, maturation of the fovea and the central nervous system Sahlgrenska Academy at Göteborg University, (CNS). Since humans are equipped with foveal vision, Göteborg, Sweden where the highest visual acuity is limited to the central 1–2° A. Hellström of the visual field, the eye must be kept within this area. International Paediatric Growth Research Centre, Furthermore, any eye movement with a velocity of >5°/s Department of Paediatrics, will degrade foveal vision during fixation [3]. It is well Sahlgrenska Academy at Göteborg University, known that children with CNS lesions, and or CNS Göteborg, Sweden haemorrhage [4, 5] or neurodevelopmental disorders may J. Ygge have defective ocular motor control [6, 7], which could Section of Ophthalmology and Vision, manifest as unstable fixation. A high frequency of intruding Department of Clinical Neuroscience, Karolinska Institutet, saccades during fixation has also been described in adult Stockholm, Sweden patients with different neurological disorders, such as E. Aring (*) progressive supranuclear palsy, Friedreich’s ataxia and Department of Paediatric Ophthalmology, focal cerebral lesions [8, 9]. Monocular visual loss may ’ Queen Silvia Children s Hospital, result in fixation being disrupted by low frequency and low Sahlgrenska University Hospital/Östra, SE 416 85 Göteborg, Sweden amplitude drifts, and bilateral visual loss can present with e-mail: [email protected] nystagmus [10]. Patients with morphological changes in the 1660 Graefe’s Arch Clin Exp Ophthalmol (2007) 245:1659–1665 macula may have stable fixation despite reduced visual 35); and IV, 13–15 years (n=30). A detailed description of acuity (20/100); thus, eyes with functional impairment the population and the ophthalmologic data [27], as well as mostly show unstable fixation (deviation of >3°), even in the orthoptic evaluation [28] has been presented elsewhere. patients with only slightly reduced visual acuity (20/25) Informed consent to participate was obtained according [11]. Some ocular and visual defects, for example congen- to the Declaration of Helsinki from all children and their ital cataract, strabismus and amblyopia, can also directly parents. The study was approved by the Ethics Committee produce unstable fixation [12], and fixation behaviour can at the Medical Faculty of the University of Göteborg, therefore be used as a marker for both CNS lesions and Sweden. ocular dysfunction. Many mechanisms are involved in keeping steady Eye movement recordings fixation on a target [13]. One such mechanism is the vestibulo-ocular reflex, which depends on the ability of the The right and left eye positions (simultaneous horizontal labyrinthine mechanoreceptors to sense head acceleration and vertical) were recorded with an infrared (IR) system [14]. Other mechanisms are the visually mediated reflexes (Orbit; (IOTA, Timrå, Sweden) [29, 30]. This IR device (optokinetic and smooth pursuit tracking), which depend on works with pulsed infrared light, which is emitted from the the ability of the brain to determine the target velocity on inside of goggles and then reflected against the ocular the retina [15] and match it with eye velocity. The surface and detected by eight detectors also situated within attentiveness and reflexes required to turn the fovea towards the goggles. The eye position signals are conducted via a the object of significance, and the ability to suppress sound card to a computer, where they are recorded. The inadequate saccades, also affect the ability to fixate [16– maximum temporal resolution of our system is 100 Hz; 18]. Furthermore, the fusional vergence in response to a the spatial resolution under optimal conditions is 0.01° and retinal disparity and the accommodative vergence affects the linearity 10% (manufacturer’s data). the fixation when there is a loss of focus [2]. Previously, During recording, the child sat comfortably in a chair. fixation stability has been studied during reading [19–21], The head was restrained using a chin and a forehead rest at when studying intruding saccades and nystagmus [8, 22, 53 cm from a computer screen, and the goggles were 23], and in studies using magnetic resonance imaging secured with a strap around the back of the head. The (MRI) [24] and position emission tomography (PET) [25]. child’s head was adjusted so that the eyes were level with The purpose of this study was to characterise fixation the centre of the computer screen. No eyeglasses were used behaviour in a non-clinical population aged 4–15 years in during the recordings, as they interfered with the function order to obtain reference values to be used in children with of the recorder, and in all cases the uncorrected visual fixation abnormalities. A preliminary report on the present acuity was adequate for seeing the stimulus. Ambient study has already been published [26]. illumination was mesopic. Before the IR recordings were performed a pre-test paradigm of two vertical and two horizontal 10° saccades Materials and methods was run. This was done since it is known that the placement of the IR goggles is important for adequate recordings and Participants to avoid crosstalk between the horizontal and vertical channels. Considerable crosstalk in the recordings from One hundred and thirty-five non-clinical participants were the pre-test paradigm led to readjustment of the goggles and studied as part of an ophthalmologic and orthoptic this procedure was repeated until a satisfactory result was evaluation (n=143) in the Department of Paediatric obtained. After the goggles were in a satisfactory position Ophthalmology at the Queen Silvia Children’s Hospital, the position of the goggles was not changed. After the Göteborg, Sweden [27, 28]. Inclusion criteria were age calibration paradigm, a stable fixation stimulus (a yellow between 4 and 15 years, birth in Sweden, and being dot subtending a visual angle of 0.3°) was shown centrally resident in Västra Götaland Region, Sweden. The children on the otherwise black computer screen for 20 s, and the (72 boys and 63 girls) were recruited from different schools child was requested and encouraged to fixate the stimulus and pre-schools within the area of Göteborg. They were and keep the eyes as steady as possible. born between 1986 and 1997 and had a mean age of 9.8 years at the time of evaluation. The aim was to recruit at Analysis least five girls and five boys from each year of age. Thus, we continued to enrol children until that number had been Eight children were rejected from the study because of reached. The children were divided into four age groups: I, 4– insufficient participation (n=3) or technical problems with 6years(n=31); II, 7–9years(n=39); III, 10–12 years (n= the recording procedure (n=5); thus, recordings from 135 Graefe’s Arch Clin Exp Ophthalmol (2007) 245:1659–1665 1661 children were included in the analysis. About 7% of the Table 1 Showing the distance of fixations around the centre of total data points were deleted due to insufficient quality, or gravity (mean and SD) “bad recordings”. These 7% were the same in the different Age group Mean distance Mean distance year groups and included large drifts of >10° and crosstalking. Degrees SD An off-line data calibration of all four channels (right – and left eye, horizontal and vertical) of the recordings was 4 6 6.8 5.6 7–9 4.9 3.9 performed using the JR program [29], and the calibrated 10–12 3.0 3.1 data were then transferred into a computer program (Origin 13–15 2.2 2.8 7.0; Microcal, Northampton, MA, USA) for further analysis and plotting. During the total 20 s of fixation time, note was taken of the longest fixation time without interruption of Fixation time saccades (defined as rapid change of eye position in any channel of more than 3° amplitude) or drifts (defined as a Analysis of the entire 20 s of fixation recordings showed slow change in eye position in any channel of more than that the mean of the longest fixation time without 3°).
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