Accommodation responses to flickering stimuli.

It has been suggested that accommodation to visual display unit (VDU) symbols or to stimuli lit by fluorescent tubes or similar temporally modulated light sources may be less accurate due to flicker. Furthermore, if the microfluctuations of accommodation play a part in the control of steady-state accommodation, the effects of stimulus flicker might affect the quality of this control. Experiments are described in which monocular accommodation stimulus-response curves for sinusoidal grating stimuli (1.0, 5.3 and 8.9 c/deg) were measured under the following conditions of temporal modulation: (1) square-wave on-off modulation to give spatial modulation varying between 0 and 37%; (2) sinusoidal modulation to give spatial modulation varying between 0 and 37%; (3) sinusoidal modulation to give spatial modulation varying between 16 and 32%; and (4) on-off modulation with varying mark/space ratio. Temporal modulation frequencies were between 0.5 and 7.8 Hz and above the critical fusion frequency (CFF) i.e., > or = 40 Hz. In general, the results suggest that while lower-frequency flicker may adversely affect the accuracy and stability of the accommodation response, the latter are very little affected by flicker at frequencies > or = 40 Hz. Thus flicker from fluorescent lamps or VDUs is unlikely to cause systematic accommodation difficulties. The possible relevance of the results to theories which ascribe a role for the higher-frequency (> 0.5 Hz) accommodative microfluctuations in the control of the response is considered.