Browsing by Author "Pullela, Mythri"
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Item Fixational Saccades and Their Relation to Fixation Instability in Strabismic Monkeys(Investigative Ophthalmology & Visual Science, 11/1/2017) Upadhyaya, Suraj; Pullela, Mythri; Ramachandran, Santoshi; Adade, Samuel; Joshi, Anand C.; Das, Vallabh E.Purpose: To evaluate the contribution of fixational saccades toward fixation instability in strabismic monkeys. Methods: Binocular eye movements were measured as six experimental monkeys (five strabismic monkeys and one monkey with downbeat nystagmus) and one normal monkey fixated targets of two shapes (Optotype, Disk) and two sizes (0.5°, 2°) during monocular and binocular viewing. Fixational saccades were detected using an unsupervised clustering algorithm. Results: When compared with the normal monkey, amplitude and frequency of fixational saccades in both the viewing and nonviewing eye were greater in 3 of 5 strabismic monkeys (1-way ANOVA on ranks P < 0.001; median amplitude in the normal monkey viewing eye: 0.33°; experimental animals: median amplitude range 0.20–0.82°; median frequency in the normal monkey: 1.35/s; experimental animals: median frequency range 1.3–3.7/s). Increase in frequency of fixational saccades was largely due to quick phases of ongoing nystagmus. Fixational saccade amplitude was increased significantly (3-way ANOVA; P < 0.001) but by small magnitude depending on target shape and size (mean difference between disk and optotype targets = 0.02°; mean difference between 2° and 0.5° targets = 0.1°). Relationship between saccade amplitude and the Bivariate Contour Ellipse Area (BCEA) was nonlinear, showing saturation of saccade amplitude. Fixation instability in depth was significantly greater in strabismic monkeys (vergence BCEA: 0.63 deg2–2.15 deg2) compared with the normal animal (vergence BCEA: 0.15 deg2; P < 0.001). Conclusions: Increased fixational instability in strabismic monkeys is only partially due to increased amplitude and more frequent fixational saccades. Target parameter effects on fixational saccades are similar to previous findings of target effects on BCEA.Item Longitudinal Evaluation of Eye Misalignment and Eye Movements Following Surgical Correction of Strabismus in Monkeys(Investigative Ophthalmology & Visual Science, 11/1/2016) Pullela, Mythri; Degler, Brittany A.; Coats, David K.; Das, Vallabh E.Purpose: Strabismus correction surgery is well documented in both the literature and practice with varying levels of success and permanence. Our goal was to characterize longitudinal changes in eye alignment and eye movements following strabismus correction surgery in a monkey model for developmental strabismus. Methods: We studied two juvenile rhesus monkeys with exotropia previously induced via an optical prism-rearing paradigm in infancy. Eye misalignment was corrected via a resection–recession surgery of the horizontal rectus muscles of one eye. Binocular search coils were used to collect eye movement data during smooth-pursuit, saccades, and fixation tasks before surgical treatment, immediately after surgery, and through 6 months after treatment. Results: Both animals showed an immediate ?70% reduction in misalignment as a consequence of surgery that regressed to a 20%–40% improvement by 6 months after treatment. Significant changes were observed in saccade and smooth-pursuit gain of the nonviewing eye after surgery, which also reverted to presurgical values by 6 months. A temporary improvement in fixation stability of the nonviewing eye was observed after surgery; naso-temporal (N/T) asymmetry of monocular smooth-pursuit remained unchanged. Conclusions: Surgical realignment is followed by plastic changes that often lead to reversal of surgery effects. Immediate improvement in misalignment and changes in eye movement gains are likely a result of contractility changes at the level of the extraocular muscle, whereas longer-term effects are likely a combination of neural and muscle adaptation.Item Longitudinal Investigation of Behavioral and Neuronal Responses before and after Strabismus Correction Surgery in Non-Human Primates(2018-08) Pullela, Mythri; Das, Vallabh E.; Chino, Yuzo M.; Harwerth, Ronald S.; McLoon, Linda K.Purpose: Strabismus is a developmental disorder observed in 2-4% of children. A preferred treatment of strabismus is the surgical manipulation of extraocular muscles (EOM) to alter muscle force-tension relationships to reduce ocular misalignment. Although widely practiced, treatment effects vary in success and permanence. The aims of this dissertation were to longitudinally follow the behavioral and neuronal changes that occur following surgical correction of strabismus in non-human primates with a goal of understanding neuronal plasticity following treatment. Fixation stability is an important aspect of gaining information about the visual scene, which is often compromised in strabismic patients. Another line of research in this dissertation was to examine the relationship between fixational saccades (Fsac) and fixation stability in normal and strabismic animals. Methods: Strabismus treatment studies: Single cell recordings from 530 cells in the oculomotor (OMN) and abducens (Abd) nuclei were obtained from two monkeys (M1 and M2) with divergent strabismus before and after they underwent a strabismus correction surgery. Neuronal drive (ND) to the deviated eye was calculated using the motoneuron responses as the monkeys performed a horizontal smooth pursuit task. Behavioral data such as eye alignment, saccade and smooth pursuit metrics were recorded before and after treatment using scleral search coils in the same animals to help interpret neuronal changes. Fixational saccade study: Eye movement data were recorded using scleral search coils as 5 strabismic and 3 normal monkeys fixated targets that varied in shape (between an optotype ‘%’ and a disk), size (0.5° and 2°) and viewing conditions (Left eye viewing, right eye viewing, binocular). Goal of the analysis was to examine relationship of Fsac direction to target shape and relationship of Fsac amplitude to fixation stability. Results: Strabismus treatment studies: Strabismus correction surgery resulted in a ~70% reduction in strabismus angle in both animals that reverted towards pre-surgical misalignment by about 6 months after treatment. Significant changes in saccade and smooth-pursuit eye movement gains were observed immediately after treatment, but these resolved within 1 month after surgery. Analysis of the population neural responses within the oculomotor and abducens nuclei showed that in the first month after surgery, the ND to the treated MR reduced in one animal and ND to the LR increased in the other animal, both indicating active neural plasticity that reduced the effectiveness of the treatment. Adaptive changes in ND to the untreated eye were also observed. An analytical method used to determine muscle contractility showed that significant peripheral muscle remodeling after surgery also contributed to reversal of strabismus correction. Fixation saccade study: Target parameters had a significant effect on the amplitude of Fsac with the larger targets resulting in larger Fsac. A strong interaction of target size and shape was seen in the direction of Fsac in the normal monkey cohort. This effect was absent in the strabismic monkey group as the direction of Fsac were influenced largely by the direction of nystagmus present in these monkeys. Conclusion: Active neuronal and muscle plasticity following surgical correction of strabismus was identified in both the ND to the treated eye and untreated eye that lead to the failure of treatment in these two monkeys. We suggest that outcome of strabismus correction surgery could be improved by identifying ways to enhance ‘positive’ adaptation and limit ‘negative’ adaptation. Target parameters have an effect on Fsac metrics and suggest that they can be used in scanning salient features of the target. Fixation stability is hindered in strabismic monkeys due to the presence of nystagmus and large drifts.