Increment sensitivity in humans with abnormal visual experience.

1. Visual acuity is lower for gratings oriented diagonally than for those of horizontal and vertical orientations. In addition to this oblique effect, some subjects show substantial deficits in acuity for horizontal or vertical targets (meridional amblyopia). These subjects are invariably astigmatic, but the condition has a neuronal basis and is thought to arise from faulty post-natal neural development. 2. Foveal increment sensitivites have been determined for normal subjects and meridional amblyopes using bar-shaped targets of various lengths, widths and orientations.3. Normal subjects do not exhibit differences in sensitivity as a function of orientation. No oblique effect is found for 1-5' wide bars ranging in length from 10 to 60'. On the other hand, meridional amblyopes have substantial differences in increment sensitivity which depend on test target orientation. Invariably, when there is a deficit in acuity for a particular grating orientation, there is also a reduction in increment sensitivity for a bar of the same orientation. This effect is diminshed or eliminated when the background illuminance is lowered from 70 to 7td. 4. The orientational differences in increment sensitivity found in meridional amblyopes do not increases for bars longer than about 10'. As the bar is shortened, the differences are reduced, and they are absent when the test bar is 6' or less. 5. In normal subjects, for a 1 degree long bar, increment sensitivity increases with width up to about 4' where the width-sensitivity curve levels off. No orientation differences are exhibited. Prominent orientation differences are found with meridional amblyopes when the bar target width is altered. The normal meridian is similar to those of the control subjects but the deficity meridian has very low sensitivity and summation is present for widths up to about 11'.