Influence of Target Parameters on Fixation Stability in Normal and Strabismic Monkeys

Abstract

PurposeThe purpose of this study was to assess the effect of fixation target parameters on fixation instability in strabismic monkeys.MethodsOne normal and three exotropic monkeys were presented with four differently shaped fixation targets, with three diameters, during monocular or binocular viewing. Fixation targets were white on a black background or vice versa. Binocular eye movements were recorded using the magnetic search coil technique and fixation stability quantified by calculating the bivariate contour ellipse area (BCEA).ResultsFixation instability was greater in all the strabismic monkeys compared with the normal monkey. During monocular viewing, strabismic monkeys showed significantly greater instability in the covered eye compared to the fixating eye. Multifactorial ANOVA suggested statistically significant target parameter influences, although effect sizes were small. Thus, a disk-shaped target resulted in greater instability than other target shapes in the viewing eyes of the normal monkey and two of three strabismic monkeys. A similar target-shape effect was also observed in the covered eye. Least instability was elicited with a 0.5° target in the normal monkey and a 1.0° target in the strabismic monkeys, both in the viewing and the covered eye. Target/background polarity effects were idiosyncratic. In strabismic monkeys, stability of the fixating eye during binocular viewing was not different from the stability of the same eye during monocular viewing.ConclusionsAbnormal drifts and nystagmus contribute to increased fixation instability in strabismic monkeys. Target parameters (shape and size) that influence fixation stability in a normal animal also affected fixation stability in our sample of strabismic monkeys.

Longitudinal Evaluation of Eye Misalignment and Eye Movements Following Surgical Correction of Strabismus in Monkeys

“…In addition, fixation stability may be influenced by several visual factors such as target size, shape, contrast, and luminance. 8 We measured fixation stability in these animals at the different time points and found a temporary improvement in fixation stability of the treated eye when the untreated eye was viewing but no consistent changes when the treated eye was viewing. Dell'Osso et al suggested that tenotomy surgery reduces nystagmus intensity 47,48 in humans with infantile nystagmus syndrome due to manipulation of proprioceptive receptors in the EOM.…”

“…Fixation stability of the nonviewing eye was worse at all time points for both the monkeys, consistent with previous studies of fixation stability in strabismic monkeys. 8 …”

“…4–6 These animal models have been very effective in replicating a variety of strabismus oculomotor features beyond just horizontal misalignment, such as dissociated deviations, latent nystagmus, fixation instability, fixation switch behavior, naso-temporal asymmetry of horizontal smooth-pursuit and optokinetic nystagmus, and disconjugate eye movements. 6–16 Studies in animal models have also led to several insights into neural strabismus mechanisms including identifying neural substrates for misalignment, A/V patterns, and saccade disconjugacies. For example, previous studies from our laboratory and others have shown that, in monkeys with a sensory strabismus, an innervational drive from the motor nuclei (oculomotor and abducens nucleus) plays a critical role in setting the state of ocular misalignment on a moment-to-moment basis.…”

Strabismus and the Oculomotor System: Insights from Macaque Models

“…Fixation instability in strabismus and amblyopia is not only due to fusion maldevelopment nystagmus. Increased drifts and other forms of nystagmus also contribute to fixation instability (Pirdankar & Das 2016, Tusa et al 2001). A recent study in humans has suggested that increased fixation instability in strabismus could result in increased errors in determining the strabismus angle when using clinical testing methods, such as prisms and alternate cover testing (Economides et al 2016).…”

Fixational Saccades and Their Relation to Fixation Instability in Strabismic Monkeys

“…We also showed that fixation instability was influenced by target parameters such as size and shape of the target, in both normal and strabismic monkeys. 23 The goal of the current study was to further investigate the components that contribute to larger fixation instability in strabismus. Specifically, we were interested in investigating the contribution of fixational saccades toward fixation instability in strabismic monkeys, while also investigating the influence of target parameters, such as target shape and size on fixational saccades.…”