[4,5,6], or into multiple channels where each channel is operated using separate negative feedback controls [3,7]. One model that incorporates preprogrammed control is the Dual Mode Model [2,8]. It is composed of a preprogrammed (transient) component and a feedback (sustained) component where the transient component influences the peak velocity of the response and the sustained component results in the accuracy of the final position [2,8]. Most disparity-vergence models do not distinguish between convergence and divergence except for a positive or negative input stimulus. Hence, these models cannot predict any differences in convergence and divergence peak velocities. Previous studies have shown that divergence peak velocity is strongly dependent upon initial vergence position [10,11,12,13,14,15,16]. Depending upon where the responses are recorded, divergence peak velocity can be faster, slower or approximately the same as convergence peak velocity [11]. One model that does account for the differences between convergence and divergence movements was developed by Patel et al. using the Hodgkin-Huxley equation for membrane dynamics to predict vergence peak velocity [3,10]. This model demonstrates that convergence and divergence responses have distinctive peak velocities at different ranges of spatial depth. Specifically, Patel’s model predicts that with equal parameters for the convergence and divergence pathways, divergence responses at far would be slower (reduced peak velocity) than divergence responses at near while convergence responses at near would be slower than those at far. However, our current results and other studies suggest that the duration of time that a person is fixating on a target can influence the peak velocity in a disparity-vergence response [13,17,18]. Patel’s model does not allow any of its parameters to change based upon how long a person is sustaining fixation on a prior target. A few models have incorporated phoria adaptation in their design to account for modification of the disparity-vergence responses. Schor’s model contains two portions, the fast-disparity vergence portion and the slow-disparity vergence portion [6,19]. With sustained fixation, there is an increase in the output of the slow-disparity vergence portion that acts to change the phoria toward the current state of convergence angle; thereby reducing the load of the fast-disparity vergence portion [6,19]. An alternate model proposed by Hung et al. employs a variable time-constant mechanism in which the neurons increase their time-constants proportionally to the duration of phoria adaptation [20]. Saladin’s model has been also shown to account for direction during phoria adaptation by using a separate sensorimotor pathway for convergence and divergence [21]. However, these models do not accurately predict the influence of phoria adaptation on convergence and divergence peak velocity. Studies have reported that divergence peak velocity is influenced by sustained fixation [13,17,22]. In one study, vergence responses were measured after 5, 30, 60 and 90 seconds of 6u sustained fixation where divergence peak velocity decreased significantly after 30 seconds or longer, compared to only 5 seconds of sustained fixation. Yet, the convergence peak velocity was unchanged for all the exposure durations up to 90 seconds [17]. Satgunam and colleagues (2009) studied the changes in 4u disparity-vergence step responses with an initial vergence position of 12u after five minutes of 12u (near) sustained fixation. They reported a decrease for divergence amplitude and peak velocity, and an increase for convergence amplitude and peak velocity, when comparing the data from before and after the near sustained fixation [18]. Satgunam and colleagues adapted for 5 minutes, (whereas Patel and colleagues adapted for up to 90 seconds) and demonstrated that when the sustained fixation was longer, it did influence the convergence peak velocity. However, neither study Changes in Phoria and Convergence Peak Velocity investigated the influence of the initial vergence position of the convergence step stimulus. Patel’s neural network model predicted that convergence peak velocity would be faster when the initial vergence position was presented farther away from a subject’s midline compared to an initial vergence position closer to the subject’s midline [3,10]. A systematic study to investigate whether the peak velocity of convergence is dependent on initial vergence position is warranted to test Patel and colleagues’ model. In addition, using a sustained fixation position which is located at a different spatial depth compared to the initial vergence position of the convergence steps will allow us to study whether changes in phoria are correlated to changes in convergence peak velocity. Satgunam and colleagues report that an esophoric shift in phoria as a result of near sustained fixation, leads to an increase in convergence dynamics [18]. They use a congruent stimulus; the position of the sustained fixation (direction of phoria shift) is similar to the initial vergence position of the 4u disparity-vergence steps. However, our study is designed to analyze the effects of both congruent and incongruent stimuli. For a congruent stimulus, the phoria is adapted to a similar visual location where the symmetrical vergence steps will be recorded. For incongruent stimuli, the position of the sustained fixation is mismatched with the initial vergence position of the 4u disparity-vergence steps. For example, the person’s phoria is adapted at far and the convergence steps are recorded at near. Incongruent stimuli allow us to study greater changes in the phoria and convergence peak velocity from different initial vergence positions. The purpose of our study is to test two different sustained fixation adapting positions (16u and 1u) and measure 4u convergence responses for two different initial vergence positions (1u and 12u). This experimental design allows us to investigate whether: 1) phoria influences convergence peak velocity, 2) convergence peak velocity is dependent on initial vergence position and 3) the change in phoria is correlated to the change in the peak velocity of convergence steps. In our study, change is defined as the phoria, or peak velocity, after near sustained fixation minus the phoria, or peak velocity, after far sustained fixation. Materials and Methods Subjects The New Jersey Institute of Technology (NJIT) Institution Review Board (IRB) approved this study. All subjects signed written informed consent forms approved by the NJIT IRB in accordance with the Declaration of Helsinki. Six subjects, 22 to 65 years of age, who could easily perform the experiment described here participated in the study. All subjects had normal binocular vision defined as better than 50 seconds of arc by the Randot Stereopsis test and a near point of convergence (NPC) of less than 6 cm as described in our previous study [23]. Subjects were screened to ensure none of the subjects had anisometropia. Four subjects were emmetropes while two were myopes. Subjects S5 and S6 were myopic. Subject S59s refraction was 2.75D and S69s refraction was 2.25D for both the left and right eye. Both wore their refractive correction during the experiment. Our eldest subject (S1) had similar vergence peak velocities compared to the other younger subjects. Yang and colleagues also report no aging effects on the peak velocity of vergence [24]. Hence, subject S19s data were included in the current study. Haploscope Setting and Experimental Setup Visual stimuli were displayed using a haploscope. Two computer screens were used to generate a symmetrical disparity PLoS ONE | www.plosone.org 2 June 2011 | Volume 6 | Issue 6 | e20883