Diphasic and Polyphasic Temporal Modulations Multiply Apparent Spatial Frequency

The effects of diphasic and polyphasic flicker on apparent spatial frequency were studied in several experiments through simultaneous spatial-frequency matches. In diphasic flicker the spatial phase of a sinusoidal grating alternated between two values in a counterphase fashion, and in polyphasic flicker the spatial phases of gratings were varied discretely in time in a variable number of steps. Both forms of flicker increased the apparent spatial frequency at low temporal frequencies, in the same manner as low-frequency monophasic flicker has been found to do. At high temporal frequencies, diphasic flicker doubled the apparent spatial frequency, as reported by Kelly (1966). We found that through high-frequency polyphasic flicker the spatial effect that Kelly mentions can be generalised to spatial frequency multiplication: polyphasic flicker produces not only the apparent second harmonic but also the third and the fourth harmonic, depending on the phase parameters. A numerical analysis showed that the spatial high-frequency effects can be explained through temporal integration of nonlinearly filtered input signals if a value of 200 td⁽¹⁾ is assumed for the nonlinearity constant in

where B(I) is the brightness, I is the retinal illuminance, K is a scale constant, and I_½ is the constant of nonlinearity. A minimum value of 60 ms had to be estimated for integration time. An investigation of the integration time with diphasic flicker indicated that spatial integration time decreases when the level of light adaptation increases, and that the integration time for spatial effects is longer than for flicker fusion. The spatial effects of low-frequency and high-frequency flicker differ in so many respects that different neural processes have to be postulated for their explanation.