Changes in Phoria and Convergence Peak Velocity Figure 4. Summary per subject of baseline phoria before 166 sustained fixation (dark gray bar), baseline phoria before 16 sustained fixation experiment (light gray bar), adapted phoria after 166 sustained fixation task (medium gray bar) and adapted phoria after 16 sustained fixation task (white bar) for all six subjects. Adapted phoria measurements are the average of phoria measurements after the three minutes of sustained convergence, after near steps with an initial vergence position of 12u and after far steps with an initial vergence position of 1u. The error bars are one standard deviation from the mean. doi:10.1371/journal.pone.0020883.g004 steps [F (1, 5) = 8.86, p,0.03]. Post-hoc analysis using the Bonferroni all-pairwise test indicated that the peak velocities of the convergence steps at near were significantly different than the peak velocities of convergence steps with a far initial vergence position. In addition, the average peak velocity of the steps after sustained near fixation was significantly faster than that of the steps after sustained far fixation [F (1,5) = 11.84, p = 0.02]. These differences were confirmed with the post-hoc Bonferroni all-pairwise test. A paired t-test was used to compare the congruent stimuli. We compared the convergence peak velocity data between the two congruent stimuli (near steps after near adaptation and far steps after far adaptation). No statistical differences were observed (T = 1.0453, p = 0.3438) between convergence peak velocity at far after the far sustained fixation task and convergence peak velocity at near after the near sustained fixation task. The average convergence peak velocity with the corresponding standard deviation of near and far steps responses after the near and far sustained fixation tasks is summarized in figure 7. We did not observe significant differences between the latency of the responses for the different visual tasks. Correlation between Changes in Phoria and Convergence Peak Velocity The correlation analysis between the change in phoria and the change in convergence peak velocity is plotted in figure 8. The baseline phoria level is not used for the correlation analysis. The change in phoria is described as the difference between phoria recorded after steps during the near sustained fixation task and the far sustained fixation task. This is shown in the experimental design in figure 2 where it is the data collected during the first experiment, or figure 2A, minus the data collected during the second experiment, or figure 2B. Hence, the change in peak convergence velocity for steps with a near initial vergence position is the difference in convergence peak velocity between steps after the 16u and the 1u sustained convergence task. The analysis compares the differences between the congruent responses (near steps with phoria adapted at near) to the incongruent responses (near steps with phoria adapted far). The analysis is repeated for far steps by comparing the incongruent responses (far steps with phoria adapted at near) to the congruent responses (far steps with phoria adapted far). Using the Pearson’s correlation coefficient to quantify the correlation analysis demonstrates that the change in phoria is significantly correlated to the change in convergence peak velocity for far steps (R = 0.97, p = 0.001) where a weaker non-significant correlation is observed for the near steps (R = 0. 59, p = 0.20). Discussion 4u Convergence Step Responses are Dependent on Initial Vergence Position The first purpose of the current study was to examine whether there were differences in convergence peak velocity at two initial vergence positions (1u and 12u). We observed that convergence peak velocities were faster at an initial vergence position of 1u (far steps) compared to an initial vergence position of 12u (near steps) recorded immediately after 16u (near) and 1u (far) sustained fixation tasks. Other studies have not reported initial vergence position dependence for convergence responses where the model from Patel and colleagues predicts convergence movements should be faster when located further from the subject [3]. Alvarez et al. (2005) studied the dependence of 4u disparity- vergence peak velocity on initial vergence position [11]. In this study, all subjects were presented with a variety of convergent and divergent stimuli. The initial vergence position of convergent and divergent stimuli ranged between 20u to 4u. They reported that divergence peak velocity was significantly dependent on the initial vergence position but convergence peak velocity did not significantly change. During this experiment, the stimuli were randomly intermixed. Hence, the phoria was not held relatively constant at one position. Similar findings were observed when studying converging and diverging ramp responses [12]. These experimental protocols may have obscured the convergence initial PLoS ONE | www.plosone.org 6 June 2011 | Volume 6 | Issue 6 | e20883