Phoria adaptation after sustained symmetrical convergence: Influence of saccades

We recorded divergence eye movements after short (4 s) and long (36 s) periods of sustained symmetrical convergence (30 degrees) in nine normal human subjects using the search coil technique. Following the long period of convergence, alignment after the initial 1,250 ms of divergence was more converged than after the short period of convergence, showing short-term "phoria adaptation". The first 1,000 ms of divergence, however, could be slower, faster or relatively unchanged, depending upon the subject. A change in the timing and/or amplitude of associated saccades (which accelerate ongoing vergence) between the long and short stimuli accounted for much of the difference in the rate of divergence. The differences in saccade pattern during early divergence following the long and short periods of convergence may reflect changes in attentional focus (to near or to far).