Origin of notches in CSF: optical or neural?

Grating contrast sensitivity was measured across a range of 1 to 32 cycles per degree (c/deg) in normal observers with a computer-automated method of ascending limits. Monocular contrast sensitivity functions (CSF) were obtained for vertical, oblique and horizontal orientations, with or without full refractive correction. Small amounts of astigmatic error resulted in loss of sensitivity at selective spatial frequencies. Coincident with these CSF "notches" was the presence of monocular diplopia induced, in this study, by the condition of astigmatic error. Experimental manipulation of the selective spatial frequency losses was possible by the introduction of slight cylindrical defocus and by changes in grating orientation. Determination of the angular displacement and orientation of the monocular double images allowed prediction of the spatial frequencies which would show reduced sensitivity due to partial cancellation of contrast. The close fit between the predicted and measured sensitivity loss supports the suggestion that refractive error can affect narrowly-tuned notches. These results indicate that before the presence of a notch in the CSF can be attributed to neural abnormality, an optical cause must be eliminated.