A few observations on the amplitude spectra of Ponzo Illusion stimuli

Ponzo Illusion stimuli were filtered with a spatial filter centered on either the upper (the bar closer tothe oblique lines) or lower bar (the bar farther from the oblique lines). It was found that the image filteredwith the filter centered on the upper bar had an amplitude spectrum consistent with the perception of alonger bar relative to the spectrum of the image filtered with the filter centered on the lower bar. This isin agreement with the Ponzo Illusion. Interference was assessed by comparing the amplitude spectrum ofthe filtered oblique lines plus the amplitude spectrum of a filtered bar versus the spectrum of the filteredimage of the two stimuli together. This analysis showed that interference, which takes place in the stimuliand does not depend on vision, may not only cause amplitude reductions but may also cause the shape ofthe amplitude spectra to be altered. In the present case the changes in the spectra were consistent with thechanges in perceived length experienced in the Ponzo Illusion. This suggests, therefore, that interferencemay have the potential to alter the appearance of visual stimuli.