Horizontal and vertical prism adaptation are different mechanisms.

As part of a study on subjects with convergence insufficiency (CI), their vertical and horizontal vergence adaptation was assessed and compared with age matched controls in order to investigate whether the horizontal adaptation system can be regarded as being independent of the vertical adaptation system. Using a flashed Maddox rod technique horizontal vergence adaptation was found to be reduced in CI subjects whereas no difference could be found in vertical adaptation. These results confirm that the vertical and horizontal adaptation systems can be treated as independent mechanisms.