A sinusoidal grating appears of higher spatial frequency when briefly flashed than when presented for longer durations. This effect is restricted to gratings of low spatial frequency (1 cycle/deg). Time-dependent changes in the response of spatial frequency channels are a likely cause.
Paul Tynan and Robert Sekuler, "Perceived spatial frequency varies with stimulus duration," J. Opt. Soc. Am. 64, 1251-1255 (1974)