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Parametric Covariability in the Standard Model of the Saccadic Main Sequence

Busettini, Claudio, PhD1*; Christy, Jennifer Braswell, PT, PhD2

doi: 10.1097/OPX.0000000000001291
ORIGINAL INVESTIGATIONS

SIGNIFICANCE Saccades present a direct relationship between the size of the movement (SACSIZE) and its peak velocity (SACPEAK), the main sequence, which is traditionally quantified using the model SACPEAK = V max × (1 − e −SACSIZE/SAT). This study shows that V max and SAT are not veridical indicators of saccadic dynamics.

PURPOSE Alterations in saccadic dynamics are used as a diagnostic tool. Are the 95% reference ranges (RRs) of V max and SAT correctly quantifying the variability in saccadic dynamics of a population?

METHODS Visually driven horizontal and vertical saccades were acquired from 116 normal subjects using the Neuro Kinetics Inc. Concussion Protocol with a 100-Hz I-Portal NOTC Vestibular System, and the main sequence models were computed.

RESULTS The 95% RRs of V max, the asymptotic peak velocity, and SAT, the speed of the exponential rise toward V max, were quite large. The finding of a strong correlation between V max and SAT suggests that their variability might be, in part, a computational interaction. In fact, the interplay between the two parameters greatly reduced the actual peak velocity variability for saccades less than 15°. This correlation was not strong enough to support the adoption of a one-parameter model, where V max is estimated from SAT using the regression parameters. We also evaluated the effects of interpolating the position data to a simulated acquisition rate of 1 kHz. Interpolation had no effect on the population average of V max and brought a decrease of the average SAT by roughly 8%.

CONCLUSIONS The 95% RRs of V max and SAT, treated as independent entities, are not a veridical representation of the variability in saccadic dynamics inside a population, especially for small saccades. We introduce a novel three-step method to determine if a data set is inside or outside a reference population that takes into account the correlation between V max and SAT.

1Department of Optometry and Vision Science and Vision Science Research Center, University of Alabama at Birmingham, Birmingham, Alabama

2Department of Physical Therapy, University of Alabama at Birmingham, Birmingham, Alabama *cbus@uab.edu

Supplemental Digital Content: Appendix 1, available at http://links.lww.com/OPX/A358: video eye tracking systems with 100 frame/s cameras are very common, particularly in clinical integrated vestibular/oculomotor recording systems, but are considered too slow for saccades. Sampling frequencies of less than 200 to 300 Hz cause an underestimation of saccadic peak velocity. Mack et al.15 have demonstrated that numerical oversampling can reduce this effect. In the Appendix, we analyzed the effect of 10-point oversampling of the eye position traces on the V max and SAT estimates.

Submitted: September 16, 2017

Accepted: June 19, 2018

Funding/Support: UAB Health Service Foundation General Endowment Fund; National Eye Institute (P30 EY-03039) Vision Science Research Center Core Grant; and Alabama Department of Commerce.

Conflict of Interest Disclosure: None of the authors have reported a financial conflict of interest.

Author Contributions and Acknowledgments: Conceptualization: CB, JBC; Data Curation: CB, JBC; Formal Analysis: CB, JBC; Funding Acquisition: CB, JBC; Investigation: CB, JBC; Methodology: CB, JBC; Project Administration: CB, JBC; Resources: CB, JBC; Software: CB; Supervision: CB, JBC; Validation: CB, JBC; Visualization: CB, JBC; Writing – Original Draft: CB, JBC; Writing – Review & Editing: CB, JBC.

The authors thank Drs. James Johnston, Katherine Weise, and Mark Swanson for their assistance as members of the UAB VOR Clinic and Dr. Kevin Schultz for the helpful discussions and review of the article.

Supplemental Digital Content: Direct URL links are provided within the text.

© 2018 American Academy of Optometry