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From Discoveries in Sight, Devers Eye Institute, Portland Oregon.
Purpose.
The frequency-doubling (FD) phenomenon describes the increase in apparent spatial frequency occurring when low-spatial-frequency sine wave gratings undergo rapid counterphase flicker. It is unclear whether the visual mechanisms isolated when pattern appearance is used as a threshold criterion are the same as when a simple detection criterion (as in FD perimetry) is used. It is also unclear whether the FD stimulus isolates mechanisms that differ from those isolated by spatially uniform flicker. In the current study, adaptation and spatiotemporal tuning functions were determined, using the FD stimulus, uniform flicker, and static (nonflickering) grating targets, to establish whether distinct mechanisms are isolated by the FD stimulus.
METHODS. Spatiotemporal tuning functions were determined in six observers, using an FD stimulus under conditions of detection and resolution and using spatially uniform flickering stimuli and static grating stimuli. The effect of light adaptation on these stimulus classes was also assessed. All stimuli were 10°-wide squares.
RESULTS. Spatiotemporal tuning functions and adaptation characteristics were identical for both the FD detection and resolution paradigms. Spatially uniform flicker gave indistinguishable tuning functions and adaptation characteristics to the FD stimulus at 25 Hz and higher, but differed below this frequency. Static grating stimuli differed from FD stimuli in both tuning functions and adaptation characteristics.
CONCLUSIONS. Absolute detection of the FD stimulus involves mechanisms that are indistinguishable from those involved when a criterion based on spatial form (i.e., resolution of a pattern) is used, indicating that a simple detection criterion can be used in FD perimetry. The FD stimulus isolates similar mechanisms to spatially uniform flickering stimuli at high temporal frequencies.
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