To call attention to the danger of extinction of the panda bear, the Lithuanian artist Ilja Klemencov created the artwork "They can disappear". The illustration is composed of black-and-white zigzagged lines, which form the famous panda logo of the World Wild Fund For Nature (WWF) when seen from a distance. If one is too close to the artwork, it is difficult to spot the bear, however, if one steps back or takes off one's glasses the panda suddenly appears. This led us to ask if the ability to see the panda is related to the visual acuity of the observer and if therefore, the panda illusion can be used to assess the spatial resolution of the eye. Here we present the results of the comparison between visual acuity determined using the Landolt C and that predicted from the panda illusion in 23 healthy volunteers with artificially reduced visual acuity. Furthermore, we demonstrate that the panda illusion is based on a 2D pulse-width modulation, explain its technical history, and provide the equations required to create the illusion. Finally, we explain why the illusion indeed can be used to predict visual acuity and discuss the neural causes of its perception with best-corrected visual acuity.

Institute for Ophthalmic Research Centre for Ophthalmology University of Tuebingen Elfriede Aulhorn Straße 7 72076 Tuebingen Germany

Institute for Ophthalmic Research Centre for Ophthalmology University of Tuebingen Elfriede Aulhorn Straße 7 72076 Tuebingen Germany University Eye Hospital Hradec Králové Czech Republic

Institute for Ophthalmic Research Centre for Ophthalmology University of Tuebingen Elfriede Aulhorn Straße 7 72076 Tuebingen Germany University Eye Hospital Tuebingen Elfriede Aulhorn Straße 5 72076 Tuebingen Germany

Institute for Ophthalmic Research Centre for Ophthalmology University of Tuebingen Elfriede Aulhorn Straße 7 72076 Tuebingen Germany Werner Reichardt Centre for Integrative Neuroscience Otfried Mueller Str 25 72076 Tuebingen Germany

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