Accommodation and flicker: evidence of a role for temporal cues in accommodation control?

Static accommodation responses to sinusoidal grating stimuli that displayed temporal modulations in luminance contrast (i.e. contrast flicker) were measured with a laser speckle optometer. The effects of a variety of temporal waveforms were investigated including square-wave modulations, sinusoidal modulations, and band-pass filtered noise. The effects of altering both the amplitude and the temporal frequency (0.4-30 Hz) of the contrast flicker and the spatial frequency of the stimulus (0.77-9.2 c/deg) were also examined. All the flicker waveforms investigated (square wave, sinusoidal and band-pass noise) reduced accommodative accuracy, the effect being most apparent at lower spatial frequencies (0.77-1.15 c/deg). With band-pass filtered noise the effects of flicker were most apparent with frequencies in the range 1-4 Hz, at both lower (0.4 Hz) and higher flicker frequencies (8-32 Hz) accommodations was less affected. It was found that flicker impaired accommodation under conditions where the contrast was at all times suprathreshold. This is incompatible with the proposal that flicker reduces accommodation responses because for part of each flicker cycle the stimulus was below threshold. However, these results are compatible with the alternative hypothesis that flicker impairs the ability of the accommodation system to utilize temporal cues such as those derived from the higher frequency component (1-2 Hz) of accommodative oscillations.