Amplitude and phase in the Müller-Lyer illusion

Area of Science:

• Visual perception
• Computational neuroscience
• Psychophysics

Background:

• The Müller-Lyer illusion is a well-known optical illusion where two lines of equal length appear to be different lengths.
• A prominent theory suggests this illusion arises from low-pass spatial filtering in the visual system.
• This theory posits that the distribution of spatial frequency amplitudes is key to generating the illusion.

Purpose of the Study:

• To investigate whether the Müller-Lyer illusion is dependent on the amplitude spectrum of the visual stimuli.
• To test the hypothesis that low-pass spatial filtering explains the Müller-Lyer illusion.
• To dissociate the roles of phase and amplitude spectra in creating the Müller-Lyer illusion.

Main Methods:

• Computed 2-D Fourier transforms of Müller-Lyer stimuli to obtain phase and amplitude spectra.
• Created hybrid stimuli by combining the phase spectrum of one figure with the amplitude spectrum of the other.
• Reconstructed images using inverse Fourier transforms of the hybrid spectra.

Main Results:

• Hybrid stimuli with the phase spectrum of outward-pointing fins consistently appeared longer, regardless of the amplitude spectrum.
• This effect persisted even when using flat amplitude spectra, which lack specific frequency distributions.
• The illusion remained largely intact when analyzing very small patches of the stimuli.

Conclusions:

• The Müller-Lyer illusion does not rely on a specific distribution of spatial frequency amplitudes.
• These findings do not support the theory that low-frequency filtering is the primary mechanism behind the Müller-Lyer illusion.
• The phase spectrum appears to play a more critical role in generating the illusion than the amplitude spectrum.