A new motion illusion based on competition between two kinds of motion processing units: The Accordion Grating

Simone Gori, Enrico Giora, Arash Yazdanbakhsh, Ennio Mingolla

Research output: Contribution to journalArticlepeer-review


Parametric psychophysical investigations are reported for two related illusory effects that occur when viewing an elementary square-wave grating while making "back and forth" head movements along the projection line. Observers report a non-rigid distortion of the pattern, including: (i) an expansion in a direction perpendicular to the stripes, and (ii) a perceived curvature of the stripes. We investigated these two phenomena independently. The first depends on the classical physiological aperture problem that confronts early cells in the vision system. Interactions between ambiguous and unambiguous motion signals, generated at line interiors and line ends, respectively, can explain why the perceived expansion occurs only in directions perpendicular to the stripes. A simple model is presented and successfully tested by a nulling psychophysical experiment with four subjects. The experiment varies key stimulus attributes that generate ambiguous and unambiguous motion signals. Regarding the illusory curvature, a differential geometry model of the optics of our display, which identifies a non-classical three-dimensional (3D) aperture problem, is proposed (Yazdanbakhsh & Gori, 2011). We tested that model by implementing its closed form prediction of distortion to design displays for a second psychophysical experiment that also uses a nulling technique. Results from four subjects allow the quantification of the degree of perceived curvature as a function of speed, distance and stimulus type (blurred vs. unblurred grating) and are compatible with the predictions of the model.

Original languageEnglish
Pages (from-to)1082-1092
Number of pages11
JournalNeural Networks
Issue number10
Publication statusPublished - Dec 2011


  • Accordion Grating
  • Aperture problem
  • Differential geometry
  • Line of sight
  • Motion
  • Projection line
  • Visual illusion

ASJC Scopus subject areas

  • Artificial Intelligence
  • Cognitive Neuroscience


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