Motivators and deterrents for seeking eye care in a Canadian region.

BACKGROUND: Motivators and deterrents for seeking eye care in a Canadian setting were sought using a qualitative study. Provincial deregulation of eyewear dispensing in 2010 allows consumers to order eyewear without an optical prescription, thus eliminating a potential motivator for obtaining an eye examination. METHODS: Convenience sampling was used to obtain 25 members of the public who contributed to one of seven focus groups that were facilitated, audiotaped, anonymised and transcribed. Participants completed a demographic questionnaire. Focus group data analysis employed grounded theory and theme saturation determined the number of focus groups. RESULTS: Nine men and 16 women participated, ranging in age from 18 to 71 years (mean 41.5; median 40.0). Three main themes were identified as influencers for seeking eye care: priority; advice; and capacity. Priority served as a motivator ('lived experience', 'symptoms', and 'habit') and deterrent ('test distress', 'asymptomatic', 'don't know' and 'other priorities'). Advice was a motivator ('professional' and 'family/friends'), while capacity was a motivator ('insurance') and deterrent ('cost'). CONCLUSION: The motivators and deterrents of seeking eye care in these focus groups were framed by three themes. Key findings not reported previously included the motivators of 'habit', 'advice' and 'insurance' and the potential deterrent of 'test distress'. These factors should be added to other previously reported motivators and deterrents in further exploration of Canadian eye-care seeking behaviours. Such knowledge is needed to develop strategies for improving eye-care literacy in Canada. This is particularly important because eyewear deregulation and/or online eye examinations may encourage members of the public to bypass comprehensive eye care without fully understanding the implications of this decision for their health and wellness.

Refractive error magnitude and variability: Relation to age / Magnitud y variabilidad del error refractivo: relación con la edad

Purpose: To investigate mean ocular refraction (MOR) and astigmatism, over the human age range and compare severity of refractive error to earlier studies from clinical populations having large age ranges. Methods: For this descriptive study patient age, refractive error and history of surgery affecting refraction were abstracted from the Waterloo Eye Study database (WatES). Average MOR, standard deviation of MOR and astigmatism were assessed in relation to age. Refractive distributions for developmental age groups were determined. MOR standard deviation relative to average MOR was evaluated. Data from earlier clinically based studies with similar age ranges were compared to WatES. Results: Right eye refractive errors were available for 5933 patients with no history of surgery affecting refraction. Average MOR varied with age. Children < 1 yr of age were the most hyperopic (+1.79 D) and the highest magnitude of myopia was found at 27 yrs (-2.86 D). MOR distributions were leptokurtic, and negatively skewed. The mode varied with age group. MOR variability increased with increasing myopia. Average astigmatism increased gradually to age 60 after which it increased at a faster rate. By 85+ years it was 1.25 D. J0 power vector became increasingly negative with age. J45 power vector values remained close to zero but variability increased at approximately 70 years. In relation to comparable earlier studies, WatES data were most myopic. Conclusions: Mean ocular refraction and refractive error distribution vary with age. The highest magnitude of myopia is found in young adults. Similar to prevalence, the severity of myopia also appears to have increased since 1931

Objetivo: Estudiar la refracción ocular media (MOR) y el astigmatismo a lo largo del rango de la vida humana, y comparar la magnitud del error refractivo con estudios previos sobre poblaciones clínicas con rangos de edad amplios. Métodos: Para este estudio descriptivo, se extrajeron de la base de datos Waterloo Eye Study (WatES) la edad del paciente, el error refractivo y el historial de cirugía con repercusión en la refracción. Se evaluaron la MOR media, la desviación estándar de MOR y el astigmatismo con relación a la edad. Se calcularon las distribuciones refractivas para los grupos de edad evolutiva. Se evaluó la desviación estándar de MOR con respecto a MOR media. Se compararon los datos de los estudios clínicos previos con los rangos de edad similares de WatES. Resultados: Se dispuso de los errores refractivos del ojo derecho de 5.933 pacientes sin historial de cirugía con repercusión en la refracción. La MOR media sufrió variaciones con la edad. Los niños con edad <1 año reflejaron mayor hipermetropía (+1,79 D), encontrándose el mayor valor de miopía a los 27 años (-2,86 D). Las distribuciones de MOR fueron leptocúrticas, y negativamente sesgadas. La moda varió con el grupo de edad. La variabilidad de MOR se incrementó al aumentar la miopía. El astigmatismo medio aumentó gradualmente hasta los 60 años, pasados los cuales se incrementó a mayor velocidad. A los 85 años, o más, su valor fue de 1,25D. El vector de potencia J0 se modificó hacia valores más negativos con la edad. Los valores del vector de potencia J45 fueron cercanos a 0, aunque su variabilidad se incrementó a los 70 años, aproximadamente. Con relación a los estudios previos comparables, los datos WatES fueron más miópicos. Conclusiones: La refracción ocular media y la distribución del error refractivo varían con la edad. La mayor magnitud de la miopía se encontró en los adultos jóvenes. Al igual que la prevalencia, la gravedad de la miopía parece haberse incrementado desde 1931

Refractive error magnitude and variability: Relation to age.

PURPOSE: To investigate mean ocular refraction (MOR) and astigmatism, over the human age range and compare severity of refractive error to earlier studies from clinical populations having large age ranges. METHODS: For this descriptive study patient age, refractive error and history of surgery affecting refraction were abstracted from the Waterloo Eye Study database (WatES). Average MOR, standard deviation of MOR and astigmatism were assessed in relation to age. Refractive distributions for developmental age groups were determined. MOR standard deviation relative to average MOR was evaluated. Data from earlier clinically based studies with similar age ranges were compared to WatES. RESULTS: Right eye refractive errors were available for 5933 patients with no history of surgery affecting refraction. Average MOR varied with age. Children <1 yr of age were the most hyperopic (+1.79D) and the highest magnitude of myopia was found at 27yrs (-2.86D). MOR distributions were leptokurtic, and negatively skewed. The mode varied with age group. MOR variability increased with increasing myopia. Average astigmatism increased gradually to age 60 after which it increased at a faster rate. By 85+ years it was 1.25D. J0 power vector became increasingly negative with age. J45 power vector values remained close to zero but variability increased at approximately 70 years. In relation to comparable earlier studies, WatES data were most myopic. CONCLUSIONS: Mean ocular refraction and refractive error distribution vary with age. The highest magnitude of myopia is found in young adults. Similar to prevalence, the severity of myopia also appears to have increased since 1931.

Difference between vertical and horizontal saccades across the human lifespan.

The purpose of the study was to analyze vertical saccade parameters (latency, peak velocity, amplitude gain), and compare them to those of horizontal saccades in a cross-sectional study across the ages of the human lifespan. One hundred and thirty one participants (62 males) between the ages of 3 and 86 years made vertical prosaccades of 2-44° in response to a dot stimulus projected on a screen. A subset of participants also made horizontal prosaccades of 2-60° under the same conditions. The El-Mar (Downsview, Ontario, Canada) eye tracker was used to record binocular eye movements. Measures of saccadic latency, peak velocity and amplitude gain were calculated for each participant. Differences between saccade parameters for upward & downward saccades were calculated. Vertical saccade parameters were evaluated as a function of age and age related differences between vertical and horizontal saccade parameters were determined. There was no significant difference between upward and downward saccades and no effect of age for either latency or peak velocity. Downward saccades had significantly higher gains than upward saccades (p = 0.0001) and this difference increased significantly with age (p = 0.001). Vertical saccadic latency initially decreased from about 400 ms at 4 years of age, remained stable for a period of time and then increases again in later life. The lowest peak velocities were found in participants under 20 and over 70 years of age, while the highest peak velocities were seen in participants between 20 and 60 years of age. The majority of vertical saccades were hypometric. Saccadic amplitude gains varied depending on both the stimulus size (p = 0.0001) and age (p = 0.0001) of participants. Vertical saccades are most accurate for small amplitudes and for participants between 20 and 30 years of age. Vertical saccades had significantly longer latencies than horizontal saccades (p = 0.0001) but there was no significant effect of age. Vertical saccades had lower peak velocities than horizontal saccades in very young children but this difference decreased with age (p = 0.0015). Large vertical saccades were more hypometric than their horizontal counterparts across all ages. The observed differences in saccadic parameters could be related to the different areas in the brain used for saccadic generation, different periods and/or mechanisms of development and senescence within the visual system and brain and/or the effects of differential use.

Proximal Stimulus for Brewster's Phenomenon is Equivalent to That of Wheatstone's Stereogram.

We examined Brewster's perceptual phenomenon that is produced by the same binocularly equivalent proximal stimulus as Wheatstone's stereogram. We then constructed and observed the perceptual effect created by another distal stimulus that also created an equivalent binocular proximal stimulus. All three stimulus situations produced the appearance of a tilted line in the median plane which is consistent with Hering's rule that the average of the two local signs and binocular eye position determine visual direction. The characteristics of the proximal stimulus are also relevant to the question raised by Helmholtz regarding the empirical vertical horopter.

Value of Routine Eye Examinations in Asymptomatic Patients.

PURPOSE: To determine if routine eye examinations in asymptomatic patients result in spectacle prescription change, new critical diagnosis, or new management of existing conditions. We also investigate whether age and time between assessments (assessment interval) impact detection rates. METHODS: The Waterloo Eye Study (WatES) database was created from a retrospective file review of 6397 patients seen at the University of Waterloo Optometry Clinic. Significant changes since the previous assessment (significant change) were defined as a change in spectacle prescription, presence of a new critical diagnosis, or a new management. Significant change, assessment interval, and age were extracted from the database for all asymptomatic patients presenting for a routine eye examination. The frequency of patients with significant change and the median assessment interval were determined for different age groups. RESULTS: Of 2656 asymptomatic patients, 1078 (41%) patients had spectacle prescription changes, 434 (16%) patients had new critical diagnoses, 809 (31%) patients had new managements, and 1535 (58%) patients had at least one of these (significant change). Median assessment intervals were 2.9 and 2.8 years for age groups 40 to <65 years and 20 to <40 years, respectively, approximately 1.5 years for patients 7 to <20, and between 1 and 1.5 years for patients <7 or >64. Controlling for assessment interval and sex, increasing age was associated with having a significant change (OR = 1.03, 95% CI 1.029-1.037). Similarly, controlling for age and sex, increased assessment interval was associated with having a significant change (OR = 1.06, 95% CI 1.02-1.11). CONCLUSIONS: In asymptomatic patients, comprehensive routine optometric eye examinations detect a significant number of new eye conditions and/or result in management changes. The number detected increases with age and assessment interval.

Unaffected smooth pursuit but impaired motion perception in monocularly enucleated observers.

The objective of this paper was to study the characteristics of closed-loop smooth pursuit eye movements of 15 unilaterally eye enucleated individuals and 18 age-matched controls and to compare them to their performance in two tests of motion perception: relative motion and motion coherence. The relative motion test used a brief (150 ms) small stimulus with a continuously present fixation target to preclude pursuit eye movements. The duration of the motion coherence trials was 1s, which allowed a brief pursuit of the stimuli. Smooth pursuit data were obtained with a step-ramp procedure. Controls were tested both monocularly and binocularly. The data showed worse performance by the enucleated observers in the relative motion task but no statistically significant differences in motion coherence between the two groups. On the other hand, the smooth pursuit gain of the enucleated participants was as good as that of controls for whom we found no binocular advantage. The data show that enucleated observers do not exhibit deficits in the afferent or sensory pathways or in the efferent or motor pathways of the steady-state smooth pursuit system even though their visual processing of motion is impaired.

Psychophysical point: a disc tends to become a point when Weber's law fails.

In three experiments, we showed that the visual system treats a dot somewhat like a geometrical point, which has a location but no area. We represented a "point" or "dot" with a small disc (diameter of 0.08º of visual angle), and a "disc" with a larger disc (diameter of 1.5º). The Weber fraction of the dot was larger than that of the disc. In Experiment 1, the relative retinal image size cues for depth for the dot and the disc were placed in conflict with the motion parallax cue. We found that the dot indicated the positions defined by the motion parallax cue better than the disc did. In Experiment 2, we placed a constant retinal image size in conflict with convergence eye movements. We found that a binocularly fused dot appeared to move in depth with convergence eye movement, whereas a fused disc appeared to move less. In Experiment 3, we examined the apparent sizes of the afterimages of a dot and a disc and found that Emmert's law failed for the dot afterimage; the apparent size of the dot afterimage changed very little for different distances-as though it had no area-whereas the apparent size of the disc afterimage changed by an extent predicted by Emmert's law. The differences in the dot and disc conditions could not be explained by the differences in the Weber fractions alone.

Eye examinations improve visual acuity across ages.

OBJECTIVE: To investigate changes in distance best corrected visual acuity (BCVA) relative to presenting visual acuity (PVA) as a function of age and the number of years between eye examinations. DESIGN: Retrospective cross-sectional study. PARTICIPANTS: 6397 patient files at a large Canadian Optometric Clinic. METHODS: A retrospective file review of patients aged 4 to 93 years from the University of Waterloo, Optometry Clinic was conducted. Mean decimal PVA values were calculated for all patients grouped by year of age, and a function was fit to these data using nonlinear regression. The same was done for BCVA and the functions were compared. In addition, the mean logMAR difference between PVA and BCVA was determined for patients grouped by the length of time between the study visual assessment and their previous visual assessment independent of patient age. RESULTS: Right and left eye functions were not significantly different from each other for either PVA or BCVA. BCVA functions were significantly greater than PVA functions for both right (F[3174] = 194.7, p < 0.0001) and left (F[3174] = 206.0, p < 0.0001) eyes. PVA differed (≥1 line) from BCVA in at least 1 eye in 70% (n = 4054) of patients. The difference between BCVA and PVA increased as the length of time between assessments increased from 0.075 logMAR for <1 year to 0.107 logMAR for ≥5 years. CONCLUSIONS: Refractive testing resulted in an improvement in BCVA over PVA in the majority of patients, over a wide range of ages.

Horizontal saccade dynamics after childhood monocular enucleation.

PURPOSE: We investigated the effects of monocularity on oculomotor control by examining the characteristics of the horizontal saccades of people with one eye, and comparing them to those of a group of age-matched controls who viewed the stimuli monocularly and binocularly. METHODS: Participants were tested in a no-gap, no-overlap saccadic task using a video-based remote eye tracker. One group consisted of unilaterally eye enucleated participants (N = 15; mean age, 31.27 years), the other of age-matched people with normal binocular vision (N = 18; mean age, 30.17 years). RESULTS: The horizontal saccade dynamics of enucleated people are similar to those of people with normal binocularity when they view monocularly and, with the exception of latency, when they view binocularly. The data show that the monocular saccades of control and enucleated observers have longer latencies than the binocular saccades of the control group, the saccades of the enucleated observers are as accurate as those of the controls viewing monocularly or binocularly, smaller saccades are more accurate than the larger ones, and abducting saccades are faster than adducting saccades. CONCLUSIONS: Our data suggest that the true monocularity produced by early enucleation does not result in slower visual processing in the afferent (sensory) pathway, or in deficits in the efferent (motor) pathways of the saccadic system. Possible mechanisms to account for the effects of monocular vision on saccades are discussed.

Replicating and extending Bourdon's (1902) experiment on motion parallax.

Bourdon conducted the first laboratory experiment on observer-produced motion parallax as a cue to depth. In three experiments, we replicated and extended Bourdon's experiment. In experiment 1, we reproduced his finding: when the two cues, motion parallax and relative height, were combined, accuracy of depth perception was high, and when the two cues were in conflict, accuracy was lower. In experiment 2, the relative height cue was replaced with relative retinal image size. As in experiment 1, when the two cues (motion parallax and relative retinal image size) were combined, accuracy was high, but when they were in conflict, it was lower. In experiment 3, the stimuli from experiments 1 and 2 were viewed monocularly with head movement and binocularly without head movement. In the binocular conditions, accuracy, certainty, and the extent of perceived depth were higher than in the monocular condition. In the conflict conditions, accuracy, certainty, and the extent of perceived depth were lower than in the no-conflict condition, but the extent of perceived motion was larger. These results are discussed in terms of recent findings about the effectiveness of motion parallax as a cue for depth.

The singular vision of William Charles Wells (1757-1817).

William Charles Wells retained an interest in vision throughout his life. His first book was on single vision with two eyes; he integrated vision and eye movements to determine principles of visual direction. On the basis of experiments and observations he formulated three principles of visual direction, which can readily be demonstrated. In the course of these studies, he also examined visual acuity, accommodation and convergence, visual persistence, and visual vertigo. Insights into visual processing were mainly derived from observations of afterimages that were used to provide an index of how the eyes moved. His experiments enabled him to distinguish between the consequences of active and passive eye movements (later called outflow and inflow) as well as describing nystagmus following body rotation. After providing a brief account of Wells's life, his neglected research on vision is described and assessed.

Effect of stimulus type on the eye movements of children.

PURPOSE: The authors investigated whether pictures elicit superior response rates and eye movement dynamics on saccade and pursuit tasks than do dots or spots of light and whether the need for more interesting stimuli is age dependent. METHODS: Using video eye tracking, horizontal eye movements were investigated in children and adults using dots and small colored pictures as stimuli. Saccade data were obtained from 61 people and pursuit data from 53 people, ages 3 to 30 years, with no known ocular, ocular motor, neurologic, or systemic disease. Saccadic stimuli were randomly presented in steps ranging in size from 5° to 30°. Pursuits at four velocities (5°/s, 10°/s, 20°/s, and 30°/s) were tested using step ramp stimuli. RESULTS: Picture targets result in age-dependent improvements in ocular motor responses compared with dots. With the exception of saccadic accuracy, the youngest children are most affected by the type of target. Adults are affected very little. For pictures, saccadic response rates (t((60)) = 4.30, P < 0.001), saccadic peak velocities (t((60)) = 2.24, P = 0.03), saccadic accuracy (t((59)) = 2.34, P = 0.02), and closed-loop pursuit gains (F((3,50)) = 2.86, P = 0.046) are higher. Saccadic error rates (t((60)) = 3.91, P < 0.001) and saccadic latencies (t((59)) = 9.5, P < 0.001) are lower with pictures. CONCLUSIONS: Stimulus characteristics can affect response rates and eye movement dynamics, particularly in young children. To avoid underestimation of eye movement performance in young children, it is important to use meaningful targets. Furthermore, when comparing the ocular motor performance of children across studies one must consider the type of stimuli used.

Binocular vision: defining the historical directions.

Ever since Kepler described the image-forming properties of the eye (400 years ago) there has been a widespread belief, which remains to this day, that an object seen with one eye is always seen where it is. Predictions made by Ptolemy in the first century, Alhazen in the eleventh, and Wells in the eighteenth, and supported by Towne, Hering, and LeConte in the nineteenth century, however, are contrary to this claimed veridicality. We discuss how among eighteenth- and nineteenth-century British researchers, particularly Porterfield, Brewster, and Wheatstone, the erroneous idea continued and also why observations made by Wells were neither understood nor appreciated. Finally, we discuss recent data, obtained with a new method, that further support Wells's predictions and which show that a distinction between headcentric and relative direction tasks is needed to appreciate the predictions.

Visual fixation in Chiari type II malformation.

Chiari type II malformation is a congenital deformity of the hindbrain. Square wave jerks are horizontal involuntary saccades that interrupt fixation. Cerebellar disorders may be associated with frequent square wave jerks or saccadic oscillations such as ocular flutter. The effects of Chiari type II malformation on visual fixation are unknown. We recorded eye movements using an eye tracker in 21 participants with Chiari type II malformation, aged 8 to 19 years while they fixated a target for 1 minute. Thirty-eight age-matched healthy participants served as controls. Square wave jerks' parameters were similar in the 2 groups. Saccadic oscillations were not seen. Chiari type II malformation is not associated with pathological square wave jerks or abnormal saccadic oscillations. The congenital nature of this deformity may permit compensation that preserves stable visual fixation. Alternatively, the deformity of Chiari type II malformation may spare parts of the cerebellum that usually cause fixation instability when damaged.

Mixed pro and antisaccade performance in children and adults.

Pro and antisaccades are usually presented in blocks of similar type but they can also be presented such that prosaccade and antisaccade eye movements are mixed and a cue, usually the shape/colour of the fixation target or the peripheral target, determines which type of eye movement is required in a particular trial. A mixed-saccade task theoretically equalizes the inhibitory requirements for pro and antisaccades. Using a mixed-saccade task paradigm the aims of the study were to: 1) compare pro and antisaccades of children, 2) compare performance of children and adults and 3) explore the effect of increased working memory load in adults. The eye movements of 22 children (5-12 years) and 22 adults (20-51 years) were examined using a video-based eye tracking system (El-Mar Series 2020 Eye Tracker, Toronto, Canada). The task was a mixed-saccade task of pro and antisaccades and the colour of the peripheral target was the cue for whether the required saccade was to be a pro or an antisaccade. The children performed the mixed-saccade task and 11 adults performed the same mixed-saccade task alone and in a dual-task paradigm (together with mental subtraction or number repetition). A second group of 11 adults performed the mixed-saccade task alone. Children made mainly antisaccade errors. The adults' error rates increased in the mental subtraction dual-task condition but both antisaccade and prosaccade errors were made. It was concluded that the increased error rates of these two groups are reflective of different processing dynamics.

Fixation stability, fixation location, and visual acuity after successful macular hole surgery.

PURPOSE: This study examined whether changes in fixation stability and fixation location are good predictors of visual acuity after successful macular hole surgery. METHODS: Ten patients with macular hole were tested before surgery and at 1 and 3 months after surgery. Visual acuity was measured with the ETDRS; fixation stability and fixation location were assessed with the MP-1 Microperimeter (Nidek Technologies Srl., Vigonza, PD, Italy). The quantitative measure of fixation stability was calculated with a bivariate contour ellipse area (BCEA). Fixation location shift was evaluated using the differential map analysis feature of the MP-1 Microperimeter. RESULTS: There was a significant improvement in visual acuity after macular hole closure. Fixation location shifted an average of 0.55 deg and 0.87 deg at 1 month and 3 months after surgery, respectively. The fixation shift was not a good predictor of visual outcome. Fixation stability improved from an average of 0.35 deg(2) before surgery to 0.29 deg(2) at 3 months after surgery. The change in fixation stability (DeltaBCEA = BCEA before - BCEA after surgery) correlated highly with visual outcome. The regression model showed that DeltaBCEA accounted for a significant proportion of the variance in visual acuity both 1 and 3 months after surgery. CONCLUSIONS: Some changes in ocular motor function explain the visual outcome after the anatomic success of macular hole surgery. Fixation location shift has no influence on visual acuity post-operatively; however, change in fixation stability is a strong predictor of visual outcome after successful closure of the macular hole.

The vestibulo-ocular reflex during active head motion in Chiari II malformation.

BACKGROUND: Chiari type II malformation (CII) is a developmental anomaly of the cerebellum and brainstem, which are important structures for processing the vestibulo-ocular reflex (VOR). We investigated the effects of the deformity of CII on the angular VOR during active head motion. METHODS: Eye and head movements were recorded using an infrared eye tracker and magnetic head tracker in 20 participants with CII [11 males, age range 8-19 years, mean (SD) 14.4 (3.2) years]. Thirty-eight age-matched healthy children and adolescents (21 males) constituted the control group. Participants were instructed to 'look' in darkness at the position of their thumb, placed 25 cm away, while they made horizontal and vertical sinusoidal head rotations at frequencies of about 0.5 Hz and 2 Hz. Parametric and non-parametric tests were used to compare the two groups. RESULTS: The VOR gains, the ratio of eye to head velocities, were abnormally low in two participants with CII and abnormally high in one participant with CII. CONCLUSION: The majority of participants with CII had normal VOR performance in this investigation. However, the deformity of CII can impair the active angular VOR in some patients with CII. Low gain is attributed to brainstem damage and high gain to cerebellar dysfunction.

Increased role of peripheral vision in self-induced motion in patients with age-related macular degeneration.

PURPOSE: The contribution of peripheral vision in inducing self-motion (vection) was investigated in people with bilateral age-related macular degeneration (AMD). METHODS: Eleven patients with bilateral AMD and dense central scotomas with no islands of functional central retina and 12 age-matched control subjects were exposed to random-dot patterns projected on a large screen. The dots either moved from left to right, inducing linear vection, or rotated about the roll axis, inducing roll vection. Latency, total vection time, and objective and subjective measures of tilt were recorded. RESULTS: The patients with AMD experienced shorter latencies than did the age-matched control participants, but the total vection time in both conditions and tilt during roll vection were the same in both groups. There was a positive correlation between objective tilt and subjective measures of tilt in the AMD, but not in the age-matched control group. There was a negative relationship between absolute scotoma size and latency. CONCLUSIONS: Two main conclusions were drawn. First, the role of peripheral vision in inducing vection is enhanced in people with bilateral central vision loss. Second, people with bilateral AMD adapt successfully to a moving environment (they do not experience vection longer, nor do they tilt more) and are more aware of their postural position than are age-matched control subjects.

Square wave jerks in children and adolescents.

Square wave jerks are involuntary, horizontal, saccadic intrusions that interrupt fixation. Each square wave jerk consists of an initial saccade that moves the fovea away from the intended position of fixation, followed by a second saccade in the opposite direction, which refoveates the fixation position. Square wave jerks reportedly occur in 24-60% of healthy adults. No previous study of square wave jerks in children and adolescents is available. We recorded eye movements using an infrared eye tracker in 38 participants aged 8-19 years while they fixated on a visual target for 1 minute. The frequency of square wave jerks, and the durations, amplitudes, and peak velocities of their saccades, were calculated and correlated with age. Ninety percent of participants had square wave jerks. Their median frequency was 3 per minute (range, 1-18), median duration was 249 milliseconds, the median amplitude of their saccades was 0.81 degrees, and the median peak velocity was 60 degrees/second. No parameter of square wave jerks correlated with age. The prevalence of square wave jerks is high in children and adolescents. This finding may be a feature of the less mature brain, and may reflect an inability to suppress unwanted supranuclear triggers for saccades.