Flicker masking depends on the method of measurement

Luminance flicker masks sinusoidal gratings of low spatial frequency (SF), the effect being attributed to the transiency of response within this SF range. An alternative point of view is that the lower number of cycles of such gratings makes them more susceptible to flicker masking. To verify this possibility, flicker masking of low-SF gratings (1 c/deg) of varying size was studied by the methods of human visually evoked potentials (VEP), reaction time (RT) and contrast threshold. Difference between the effects of flicker masking on contrast threshold, RT and VEP was found: RT was delayed and VEP was reduced in amplitude by flicker with both small and large gratings while contrast threshold was only increased with the small gratings. The decrease of grating duration increased the masking effect on contrast threshold. Moreover, contrast threshold was a unimodal function of flicker frequency with brief test gratings and a bimodal one with long-lasting gratings. It is concluded that: (1) the previously reported effects of luminance flicker masking on RT and VEP are not an artefact of the reduced number of cycles; (2) at the SF tested, both RT and VEP depend on the transient components of the response, while contrast sensitivity depends on both transient and sustained components of the response.