Minor effect of inaccurate fixation on VEP-based acuity estimates
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TECHNICAL NOTE
Minor effect of inaccurate fixation on VEP-based acuity estimates Amal A. Elgohary . Sven P. Heinrich
Received: 15 May 2020 / Accepted: 25 September 2020 The Author(s) 2020
Abstract Purpose VEP-based estimation of visual acuity may be used in cases of suspected malingering to objectify subjective complaints. In such an application, a lack of cooperation needs to be expected. The same may apply to young children with suspected functional impairments. In the present study, we assessed how inaccurate fixation affects the acuity estimates obtained with a VEP technique. Methods VEP-based acuity estimates were obtained by stimulating with a series of different check sizes using a ‘stepwise sweep’ protocol. Sixteen participants were tested with normal and degraded vision under five different fixation conditions (central fixation and eccentric fixation at top, bottom, right, and left edge of the stimulus area). Results The majority of individual acuity estimates with eccentric fixation differed by less than 0.1 logMAR from central fixation, and almost all
estimates differed by less than 0.3 logMAR. Median estimates with eccentric fixation differed only slightly (up to 0.08 logMAR) and, except for top fixation with normal vision, non-significantly. However, data quality was lower with eccentric fixation, which increased the probability that no acuity estimate could be derived from the recording. Conclusion VEP-based acuity estimates are relatively insensitive to eccentric fixation. Unnoticed deviations from central fixation in routine applications will probably be smaller than in the present study and will have even less impact on the outcome. Keywords Objective acuity testing Visual evoked potentials Eccentric fixation Visual acuity malingering functional impairment
Introduction A. A. Elgohary Department of Vision Science, Research Institute of Ophthalmology, Giza, Cairo, Egypt S. P. Heinrich (&) Eye Center, Medical Center, University of Freiburg, Killianstr. 5, 79106 Freiburg, Germany e-mail: [email protected] S. P. Heinrich Faculty of Medicine, University of Freiburg, Freiburg, Germany
VEP-based estimation of visual acuity is increasingly popular as a technique in cases of suspected malingering or when a patient is unable to cooperate during standard psychophysical acuity testing [1]. Within certain limits, the reliability of the approach has been demonstrated in a number of studies (e.g., [2–4]), except with disorders that are associated with distortions of the visual percept, such as amblyopia [5, 6]
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and probably also when the reason for the impairment has a locus beyond the primary visual cortex. During VEP measurements, patients are normally asked to fixate the center of the screen, where a fixation mark is usually provided. Given that suspected malingering is one of the prime scenarios for the use of VEP-based acuity estimation, it is an important question whether incorrect fixation leads to a sizable e
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