Changes in Phoria and Convergence Peak Velocity Figure 5. 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 S1. Plots A and B are after the 1u sustained fixation task / far adaptation; plots C and D are after the 16u sustained fixation task / near adaptation. 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.g005 vergence position dependency predicted by Patel and colleagues’ model [3]. The model proposed by Patel and his colleagues predicts that with equal parameters for the convergence and divergence pathways, divergence responses with a far initial vergence position would be slower than divergence responses with a near initial vergence position. Meanwhile, convergence respons- es with a near initial vergence position would be slower than responses located farther from the subject [3]. Patel and colleagues do report a divergence dependence on initial position but do not report significant changes for convergence when studying up to 90 seconds of sustained fixation [17]. When we compared congruent responses within this present study using a paired t-test, we did not observe any significant differences in peak velocity. However, by comparing incongruent responses with congruent responses, we do observe a convergence peak velocity dependence on initial vergence position. We speculate that by presenting the convergence stimulus at the same initial vergence position during a sustained fixation task repeat- edly, we stimulated short term adaptive changes within the convergence system through phoria adaptation. This showed that convergence peak velocity was dependent on initial vergence position. There are differences between the numbers of cells that respond to convergence compared to divergence movements and perhaps this may lead to the differences in the malleability of adaptation between convergence and divergence movements. There are distinct cells that encode for convergence and divergence within the midbrain where more convergence cells have been observed compared to divergence cells. These cells display a discrete burst of activity just before and during a convergence or divergence eye movement that is correlated with instantaneous vergence velocity [33,34]. Since more convergence cells are observed within the midbrain compared to divergence, we speculate that differences in the cell population sizes may contribute to the differences in the ability to change convergence peak velocity compared to divergence movements. The visual system does encode for relative disparity where the brain processes information about relative position of a target with respect to the person. Specifically, there is evidence from the single cell recordings in primates of disparity tuned cells that modulate their activities for near and far positions within the primary visual cortex [35,36,37]. Perhaps the neural activity encoding of near, far and ‘zero disparity’ stimuli observed within primate studies [13] may in part be the stimulus that drives the differences in average disparity-vergence peak velocity that have been demonstrated in this present study for convergence as well as previous research investigating divergence [10,11,12,13,14,15]. PLoS ONE | www.plosone.org 7 June 2011 | Volume 6 | Issue 6 | e20883