Changes in Phoria and Convergence Peak Velocity Figure 6. Typical ensemble convergence responses from a far initial vergence position of 16 (left) and from a near initial vergence position of 126 (right) after two sustained fixation conditions from subject S2. Plots A and B are after the 1u sustained fixation task; plots C and D are after the 16u sustained fixation task. The subject fixated on a sustained fixation target for 3 minutes and then each step response was recorded after 30 seconds of sustained fixation. doi:10.1371/journal.pone.0020883.g006 Modification of the Transient Convergence Due to Sustained Fixation and Subject Variability Another goal of this research was to study whether convergence peak velocity was modified due to phoria adaptation stimulated through sustained fixation. We observed that subjects demon- strated either hypermetric or hypometric movement in the transient portion of convergence responses depending upon the type of the incongruent stimulus. During the 16u sustained fixation task with a far convergence step, hypermetric responses were present in the transient portion. During the 1u sustained fixation task with a near convergence step, hypometric responses were present during the transient portion. This study supports that when the immediately preceding sustained fixation is mismatched to the initial vergence position of the convergence step, the transient portion of the convergence response is modified compared to responses when the sustained fixation is similar to the initial vergence position of the convergence step. Another similar study to ours by Satgunam et al. reported that an esophoric shift in phoria increases both the amplitude and peak velocity of convergence step responses [18]. Our current study extends Satgunam and colleagues’ research by showing that an esophoric shift in phoria (due to 16u sustained task) results in an increase in convergence peak velocity at both near (12u) and far (1u) initial vergence positions. We also observed that an exophoric shift in phoria (due to 1u sustained task) resulted in a decrease in convergence peak velocity at both near and far initial vergence positions. Interestingly, the change in convergence dynamics was more apparent in some subjects compared to others. For example, S1 and S2 who were both emmetropes at this viewing distance showed greater change in the transient portion of convergence responses while other subjects exhibited a similar behavior but to a lesser extent. The variability in convergence dynamics among the six subjects suggests that some subjects may possess an innately faster vergence system compared to others. While different factors may attribute to the subject variability in vergence dynamics, we speculate that phoria may be a factor, explaining in part why some subjects have greater changes in convergence dynamics compared to other subjects. In addition, we do not feel that age was a contributing factor because we redid our analysis with only the five younger subjects and the results do not substantially change. Correlation between Change in Phoria and Convergence Dynamics To better assess the relationship between phoria and conver- gence dynamics, a correlation analysis was conducted between the change in phoria and the change in convergence peak velocity. The result demonstrated that the change in convergence peak velocity was highly correlated to the change in phoria for far steps PLoS ONE | www.plosone.org 8 June 2011 | Volume 6 | Issue 6 | e20883