Contrast sensitivity, first-order motion and Initial ocular following in demyelinating optic neuropathy

Abstract

The ocular following response (OFR) is a measure of motion vision elicited at ultra-short latencies by sudden movement of a large visual stimulus. We compared the OFR to vertical sinusoidal gratings (spatial frequency 0.153 cycles/ degrees or 0.458 cycles/ degrees) of each eye in a subject with evidence of left optic nerve demyelination due to multiple sclerosis (MS). The subject showed substantial differences in vision measured with stationary low-contrast Sloan letters (20/63 OD and 20/200 OS at 2.5% contrast) and the Lanthony Desaturated 15-hue color test (Color Confusion Index 1.11 OD and 2.14 OS). Compared with controls, all of the subject's OFR to increasing contrast showed a higher threshold. The OFR of each of the subject's eyes were similar for the 0.153 cycles/ degrees stimulus, and psychophysical measurements of his ability to detect these moving gratings were also similar for each eye. However, with the 0.458 cycles/ degrees stimulus, the subject's OFR was asymmetric and the affected eye showed decreased responses (smaller slope constant as estimated by the Naka-Rushton equation). These results suggest that, in this case, optic neuritis caused a selective deficit that affected parvocellular pathways mediating higher spatial frequencies, lower-contrast, and color vision, but spared the field-holding mechanism underlying the OFR to lower spatial frequencies. The OFR may provide a useful method to study motion vision in individuals with disorders affecting anterior visual pathways.

Figure 1

Sample mean R-L velocity OFR profiles over time elicited in the subject by successive ¼-wavelength shifts applied to 1f stimuli (left column) and 3f stimuli (right column) of different contrast. A and B: right-eye-viewing sessions (REV). C and D: left-eye-viewing sessions (LEV). E and F: onset latencies of the OFR to stimuli of different contrast. See text for details.

Figure 2

Quantitative dependence of the subject's OFR on contrast, based on mean R-L response measures. A: 1f stimulus; B: 3f stimulus. Data were fitted with the Naka-Rushton equation [27], and the parameters, c₅₀ and n, for these various fits are printed beside the curves.

Figure 3

The initial OFR: dependence on contrast (normalized R-L response measures); comparison of the subject with controls. Plots show the horizontal OFR to pure sine-wave gratings of 1f (open symbols) or 3f (filled symbols) spatial frequency. For clarity responses to the pure 1f and 3f sine waves are shown on opposite sides from zero. Circles and smooth grey lines – 3-subject means and best-fit Naka-Rushton functions, respectively, for controls (3 subjects; error bars are SD's of the means; c₅₀ and n parameters for Naka-Rushton functions are also shown); squares and smooth continuous lines – patient's right eye data and best fitting curves; diamonds and smooth dashed lines – patient's left eye data and best fitting curves.

Figure 4

The subject's perceptual judgments of motion direction with the 1f stimulus. There were close to 100% for all but the lowest contrasts with either eye viewing the stimulus. See text for details.