Sustained Fixation Induced Changes in Phoria and Convergence Peak Velocity Eun H. Kim1, Vincent R. Vicci2, Sang J. Han1, Tara L. Alvarez1* 1 Department of Biomedical Engineering, New Jersey Institute of Technology, Newark, New Jersey, United States of America, 2 Department of Vision, Private Practice, Westfield, New Jersey, United States of America, 3 Department of Vision Rehabilitation, Kessler Institute, West Orange, New Jersey, United States of America Abstract Purpose: This study sought to investigate the influence of phoria adaptation on convergence peak velocity from responses located at different initial vergence positions. Methods: Symmetrical 4u convergence step responses and near dissociated phoria (measured at 40 cm from the subject’s midline) were recorded from six subjects with normal binocular vision using an infrared limbus tracking system with a haploscope. Two different sustained fixations (1u and 16u convergent rotation along the subject’s midline) were used to study whether phoria had an influence on the peak velocity of convergence responses located at two initial vergence positions (1u or ‘far’ steps and 12u or ‘near’ steps). Results: Phoria was significantly adapted after a sustained fixation task at near (16u) and far (1u) (p,0.002). A repeated measures ANOVA showed that convergence far steps were significantly faster than the near steps (p,0.03). When comparing convergence steps with the same initial vergence position, steps measured after near phoria adaptation were faster than responses after far adaptation (p,0.02). A regression analysis demonstrated that the change in phoria and the change in convergence peak velocity were significantly correlated for the far convergence steps (r = 0.97, p = 0.001). A weaker correlation was observed for the near convergence steps (r = 0.59, p = 0.20). Conclusion: As a result of sustained fixation, phoria was adapted and the peak velocity of the near and far convergence steps was modified. This study has clinical considerations since prisms, which evoke phoria adaptation, can be prescribed to help alleviate visual discomfort. Future investigations should include a systematic study of how prisms may influence convergence and divergence eye movements for those prescribed with prisms within their spectacles. Citation: Kim EH, Vicci VR, Han SJ, Alvarez TL (2011) Sustained Fixation Induced Changes in Phoria and Convergence Peak Velocity. PLoS ONE 6(6): e20883. doi:10.1371/journal.pone.0020883 Editor: Joel M. Miller, Smith-Kettlewell Eye Research Institute, United States of America Received February 2, 2011; Accepted May 11, 2011; Published June 17, 2011 Copyright: 2011 Kim et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported in part by a CAREER award from the National Science Foundation (BES-0447713). No additional external funding was received for this study. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: tara.l.alvarez@njit.edu Introduction Vergence, a binocular eye movement, is responsible for attaining visual information located at different distances. The two primary inputs to the vergence system are retinal blur and disparity[1]. Retinal blur is the defocusing of images which stimulates the accommodative vergence system, whereas dispar- ity is the difference between where a new target is projected onto the retina and the fovea. Disparity stimulates the disparity- vergence system and allows a person to perceive and maintain a single binocular vision. The eyes may rotate inward (conver- gence) or outward (divergence) to project the line of sight onto the same point of interest. Through the use of different instruments, one can study accommodative-vergence or dispar- ity-vergence independently. A haploscope can systematically change where an object is projected onto the retina while maintaining a fixed focal length. Hence, when using a haploscope, any changes within eye movements can be assumed to be associated with the disparity-vergence system since the retinal blur stimulus is constant. When a binocular stimulus is eliminated, such as when one eye is occluded while the other eye is fixating on a target located along midline, the occluded eye will decay to its dissociated phoria level. Near-dissociated phoria is quantified when the viewing or non- occluded eye is fixated on a target located at 40 cm from the subject’s midline. Phoria can be classified as orthophoria, esophoria or exophoria, which are the absence, nasal or temporal rotation of the occluded eye, respectively. Phoria adaptation is a persistent modification in the eye alignment which can be due to a prolonged exposure to a binocular stimulus at different spatial depths. A rich history exists in the literature that describes model- representation of the neural control of disparity-vergence [2,3,4,5,6,7,8,9]. These models can be classified into two types: 1) negative feedback control or 2) preprogrammed control operating synergistically with a negative feedback control. The models with only negative feedback control are continuously modified such that the error between the input and output is adjusted until it is negligible. Negative feedback control models can be further separated into a single negative feedback control, PLoS ONE | www.plosone.org 1 June 2011 | Volume 6 | Issue 6 | e20883