velocity between the 1u and 16u sustained fixation tasks. The analysis was calculated using MATLABTM. A paired t-test was used to determine whether the baseline phoria was significantly modified after the near and far sustained fixation tasks, and whether the baseline phoria was significantly different between the two days of experimentation. A repeated measures ANOVA was used to determine whether the peak velocity of the convergence steps was significantly different depending upon the initial vergence position of the convergence step (two initial vergence positions were investigated: 1u and 12u) and the sustained fixation task (near / 16u adaptation and far / 1u adaptation) using NSC2004 (Kaysville, UT, USA). Figures were generated using MATLABTM and Excel software. Results Sustained Fixation Induced Phoria Adaptation Typical right eye movement responses decaying to the subject’s baseline phoria level, from S4 at the start of the experimental sessions, are shown in figure 3, plots A and C. These responses are typical single recordings. The right eye movement response decaying to the subject’s phoria level was measured after three minutes of the sustained fixation task and is shown as a long dashed black line on the right plots. The right eye position response decaying to the phoria level was measured following the 30 convergence steps to determine whether phoria was still adapted. It is shown as a solid black line for the phoria response after the near convergence steps and as a dashed line for the phoria response after the far convergence steps. The sustained fixation tasks altered the phoria level depending on the vergence position. A summary of the phoria measurements Changes in Phoria and Convergence Peak Velocity of all six subjects is shown in figure 4. All subjects became significantly more esophoric after the 16u sustained convergence task (p = 0.002). Similarly, after the far convergence task (1u vergence fixation), all six subjects became significantly more exophoric (p = 0.0008). The baseline phoria measurements were not significantly different between the two days of recording (p = 0.65). Effect of Sustained Fixation on Convergence Dynamics Typical 4u convergence step responses from the four conditions (after sustained fixation of 16u or 1u vergence positions and convergence steps with an initial vergence position of 12u or 1u) of subjects 1 and 2 are shown in figures 5 and 6, respectively. The visual stimulus is the same, yet the dynamics of the responses varies depending on the initial vergence position and where the person was visually sustaining prior to the convergence steps (near phoria adapted compared to far phoria adapted). When the phoria was adapted to the same location, the peak convergence steps were dependent on the initial vergence position. For example, when the subject’s phoria was near-adapted using the 16u sustained fixation task, the peak velocities of the far convergence steps were faster compared to the near convergence steps. Results are plotted in figure 7A. Similarly, when the subject’s phoria was far-adapted using the 1u sustained fixation, the far convergence steps were faster compared to the near convergence steps as plotted in figure 7B. A repeated measures ANOVA investigated whether the initial vergence position (near and far) and the sustained fixation adaptation (near and far) had a significant influence on the peak velocity of all 4u convergence steps. The peak velocities of the far steps were significantly greater than the peak velocities of the near Figure 3. Right eye movement responses decaying to the phoria position during the baseline recording (left plots A and C). Adapted phoria responses are plotted on the right plots (plot B and D). Right eye movement responses decaying to the phoria position after 3 minutes of sustained fixation (long dashed line), after 30 seconds of sustained fixation during the near step phase (solid line) and after 30 seconds of sustained fixation during the far step phase (short dashed line) are shown in the right plots. The right eye movement response decaying to phoria is shown under two sustained fixation conditions, 16u or near adaptation (Plot B) and 1u or far adaptation (Plot D). doi:10.1371/journal.pone.0020883.g003 PLoS ONE | www.plosone.org 5 June 2011 | Volume 6 | Issue 6 | e20883