Furthermore, our analysis suggests that subjects who showed a greater change in phoria also showed a greater change in convergence peak velocity. Single cell primate studies have researched phoria and disparity vergence eye movements. Cells located within the vicinity of the midbrain have been identified to modulate their activity based upon phoria adaptation [38] as well as on convergence velocity [39]. Lesions to the oculomotor vermis in primates have resulted in a decrease in vergence velocity and a reduction in phoria adaptation [40,41]. A direct link between the dorsal vermal outputs of the cerebellum to the midbrain via the caudal fastigial nucleus has been reported, suggesting structural connectivity between the two regions [42]. In addition, human case studies report reduced vergence dynamics in patients with cerebellar lesions, particularly those within the vermis [43]. The signal that drives and controls the modification in vergence dynamics as well as phoria needs further study. Likewise, the rationale of why some subjects have a greater change in phoria leading to an increased change in convergence dynamics requires additional investigation. Clinical Implications of Phoria Adaptation and Vergence Eye Movements A conventional treatment for binocular dysfunctions such as convergence insufficiency (CI) is using a prism correction within a spectacle which stimulates a phoria / prism adaptation; however a common clinical observation for CI patients is that the prism reduces the patient’s symptoms initially, but it does not have sustained effects [44]. In addition, Brautaset and Jennings report phoria adaptation is reduced in CI patients where they suggest CI patients may have a generally reduced horizontal phoria adaptation mechanism [45]. However, these patients demonstrate improvements in their ability to perform phoria adaptation [46,47] and increases in convergence peak velocity after oculomotor training [23,48]. Future studies are needed to understand how oculomotor training improves the ability of both the disparity- vergence and the phoria systems to adapt in binocular dysfunc- tions such as convergence insufficiency. Future Direction of Disparity-Vergence Models Some disparity-vergence models do include components that can adjust and suggest that adaptation occurs via a recruitment mechanism or a time-constant modulation [8,19]. However, these models do not incorporate any mechanism by which vergence peak velocity and phoria can adapt together as a result of sustained fixation. Therefore, a model is needed to describe convergence and divergence independently and account for the influence of References 1. Leigh RJ, Zee DS (2006) The Neurology of Eye Movements; Gilman S, ed. 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Chumerin N, Gibaldi A, Sabatini SP, Van Hulle MM (2010) Learning eye vergence control from a distributed disparity representation. Int J Neural Syst 20: 267–278. Changes in Phoria and Convergence Peak Velocity sustained fixation on disparity-vergence peak velocity. One possibility is to modify the transient / preprogrammed (open loop) component of the Dual Mode Model [8] by altering the width and / or height of the transient component dependent upon changes in phoria and on the initial vergence position of the vergence step. This current study supports that a new model- representation of the disparity-vergence system, one that incorpo- rates phoria in addition to the direction and initial vergence position of the stimulus, is needed to account for changes in disparity-vergence peak velocity observed within our experimental data. Differences in Baseline Phoria Measurements Lastly, although we did not observe a significant difference, the baseline phoria measurements were not identical on each day. Lee et al. also observed differences in phoria when data were recorded on different days [13]. Phoria can be influenced by many factors, such as near work [49]. Hence, the changes in baseline phoria could potentially be due to the amount of near work the subject was performing prior to the experiment. Other potential factors that may lead to variability of the baseline phoria include physiological changes, such as the amount or degree of fatigue, inattention, cognitive demand and / or accommodation from different days [22,50]. Conclusion In summary, phoria was altered depending upon the far and near adapting sustained fixation positions. 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Optom Vis Sci 76: 656–663. PLoS ONE | www.plosone.org 10 June 2011 | Volume 6 | Issue 6 | e20883