Modification of pattern reversal VERs by ocular accommodation.
The amplitudes and implicit times of the P-1 component of monocular transient pattern VERs elicited by reversing checkerboard targets were measured in 6 conditions: (1) Ocular accommodation (Ao) elicited by minus power ophthalmic lenses; (2) Ao stimulated by minus lenses with the natural pupil of the test eye dilated and replaced by an artificial pupil; (3) accommodative demand and pupil size fixed with retinal image size (Ir) changed by afocal minifying lenses, (4) accommodative demand and Ir fixed but pupil size changed; (5) Ao elicited by minus lenses for three sizes of reversing checkerboards, and (6) Ao elicited by minus lenses for a constant checkerboard size with three check sizes. The observed decrease in VER amplitude with Ao could not be attributed entirely to the effects of pupil size or Ir. The rate at which the amplitude of P-1 decreased with Ao was not affected by checkerboard check size, but was closely correlated with checkerboard field size. The implicit time of P-1 first decreased slightly below baseline values at low and moderate levels of Ao and then tended to increase significantly at higher levels. The increase in implicit time at high levels of Ao could be explained adequately by the effects of pupil size alone on the VER. However, where Ao occurred without pupillary miosis, the increase in implicit time could only be explained by focusing error.