Short version. How many black stripes do you see on the right? Probably four. Now select from the pop-up menu (bottom center) ‘low spatial frequency, slow’. How many are there now? – If you saw fewer: I did not change the number of stripes, just the speed with which they shift.
Longer version. The grating on the right comes up set to a low spatial frequency (=broad stripes), rapidly phase inverting (=high temporal frequency). To my chagrin the phase reversal has many hiccups, it stammers (explanation here). Pick times where phase reversal is regular and ask yourself “how many vertical black stripes do I see?”. You will probably see four. Now switch to a low temporal frequency (=slow) and observe that there are actually only two vertical black stripes. That’s frequency doubling – you see twice as many stripes as there are if the following conditions are met (1) temporal frequency is high, (2) spatial frequency is very low and (3) contrast is high.
Choose a high spatial frequency (=fine stripes) and then switch the temporal frequencies between slow and fast – clearly no frequency doubling occurs for such narrow-striped (=high spatial frequency) gratings.
The phenomenon was first reported by Kelly 1981. The figure on the right depicts visual sensitivity vs. temporal and spatial frequency (combining the de Lange curve with the contrast sensitivity function CSF). The striped gray area roughly outlines the range where frequency doubling occurs.
For explanation, “only a bit of non-linearity” is required.
The frequency doubling phenomenon has made it into a clinical test –not a bad career– with the following quirks:
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Maddess T, Goldberg I, Dobinson J, Wine S, Welsh AH, James AC (1999) Testing for glaucoma with the spatial frequency doubling illusion. 39:4258–4273
Zeppieri & Johnson “Frequency Doubling Technology (FDT) Perimetry” (at the International Perimetric Socienty)
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