Temporal frequency discrimination in optic neuritis and multiple sclerosis.

The temporal contrast sensitivity function and temporal frequency discrimination have been studied in normals and in 7 cases of multiple sclerosis or optic neuritis. Sinusoidal gratings of two spatial frequencies (0.2 and 2.0 cycles/deg) were used. The abnormalities demonstrated in the patients varied between individuals but overall a picture has emerged of four principal anomalies. The temporal transfer function of the 2 cycles/deg medium spatial frequency grating has a band-pass appearance, whereas it is low-pass in normal subjects (5 out of 8 eyes). The peak of temporal frequency discrimination function (i.e. the temporal frequency at which discrimination is most acute) occurs at a lower temporal frequency in the patients than in normal subjects particularly at the higher spatial frequency (5 out of 8 eyes for a spatial frequency of 2.0 cycles/deg; 2 out of 8 eyes for a spatial frequency of 0.2 cycles/deg). It was found that normal subjects were able to discriminate between the maximum temporal frequency used as a standard in the discrimination experiments (16 Hz) and a higher temporal frequency. In the patients, however, a cut-off in the discrimination function occurred at a temporal frequency well below this (8 out of 8 eyes for a spatial frequency of 2.0 cycles/deg; 2 out of 8 eyes for a spatial frequency of 0.2 cycles/deg). Further to the above result, in a number of patients an attempt was made to ascertain how the gratings were perceived in this abnormally extensive region of ambiguity in the high temporal frequency range. It was found that high temporal frequencies, at a constant contrast level above threshold and constant apparent contrast, were perceived either as flickering at a slower rate than did an intermediate temporal frequency or even appeared stationary. This effect was rare at the lower spatial frequency (1 out of 8 eyes) but was demonstrated in 4 out of 8 eyes at the spatial frequency of 2.0 cycles/deg. It was not specifically sought in all individuals. These results are discussed in the context of an information channelling model of temporal frequency processing in the visual system.