Changes in Phoria and Convergence Peak Velocity Figure 2. Experimental Protocol (see text). doi:10.1371/journal.pone.0020883.g002 ending at 16u (near convergence step). The 30 seconds of sustained fixation after each convergence step was used to ensure that all the parameters of the vergence system that may have adapted after the initial three minutes of sustained fixation remained adapted during the collection of 4u convergence step responses. This was repeated 30 times. Subjects were asked to inform the experimenter if fatigue occurred. If the subject began to report fatigue, the experiment would end for that session. However, none of the six subjects reported fatigue. Phoria was recorded again after 30 near convergence steps to ensure that the phoria remained adapted; this is referred to as phoria after near steps throughout the paper. The subject then performed 4u convergence steps beginning at an initial vergence position of 1u and ending at 5u (far convergence step). Phoria was recorded again after 30 far convergence steps to ensure that phoria remained adapted; this is referred to as phoria after far steps throughout the paper. Patel’s model predicts that convergence peak velocity is faster for responses with a farther initial vergence position compared to responses closer to the subject or located near [3]. To ensure fatigue was not a confounding factor influencing convergence dynamics, near steps were recorded prior to the far steps. Fatigue in the form of repetitive eye movements has been reported to decrease peak velocity [30]. We presented far convergence steps in the latter part of the experiment because fatigue is more evident during the latter phase compared to the initial phase of the experiment. If fatigue had an effect in the experiment, it should decrease the convergence peak velocity. We recorded convergence at far during the latter part of the experiment because if the data support Patel’s model, then the responses should be faster whereas fatigue would decrease the peak velocity. Convergence steps were presented after a 0.5 second delay with an additional random delay of up to 1.5 seconds to avoid prediction. Predictive cues have been shown to increase the peak velocity of vergence eye movements [31,32]. Responses were recorded for 3 seconds and data were saved for off-line analysis. In summary, four types of 4u convergence step responses were recorded: 1) near convergence steps after a sustained fixation located at a 1u vergence position / far adaptation (incongruent stimulus), 2) near convergence steps after a sustained fixation position located at a 16u vergence position / near adaptation (congruent stimulus), 3) far convergence steps after a sustained fixation position located at a 16u vergence position / near adaptation (incongruent stimulus) and 4) far convergence steps after a sustained fixation position located at a 1u vergence position / far adaptation (congruent stimulus). Statistical Analysis A linear regression analysis was used to assess the correlation between the change in phoria and the change in convergence peak PLoS ONE | www.plosone.org 4 June 2011 | Volume 6 | Issue 6 | e20883