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At the start of the 20th century, Moritz von Rohr invented the synopter: a device that removes 3D depth cues that arise from binocular disparities and vergence. In the absence of these visual cues, the observer is less aware of the physical flatness of the picture. This results in a surprisingly increased depth impression of pictorial space, historically known as the 'plastic effect'. In this paper we present a practical design to produce a synopter and explore which elements of a painting influence the plastic effect. In the first experiment we showed 22 different paintings to a total of 35 observers, and found that they rate the synoptic effect rather consistent over the various paintings. Subsequent analyses indicated that at least three pictorial cues were relevant for the synoptic effect: figure-ground contrast, compositional depth and shadows. In experiment 2, we used manipulated pictures where we tried to strengthen or weaken these cues. In all three cases we found at least one effect that confirmed our hypothesis. We also found substantial individual differences: some observers experience little effect, while others are very surprised by the effect. A stereo acuity test revealed that these differences could not be attributed to how well disparities are detected. Lastly, we informally tested our newly designed synopter in museums and found similar idiosyncratic appraisal. But the device also turned out to facilitate discussions among visitors. [ABSTRACT FROM AUTHOR]

At the start of the 20th century, Moritz von Rohr invented the synopter: a device that removes 3D depth cues that arise from binocular disparities and vergence. In the absence of these visual cues, the observer is less aware of the physical flatness of the picture. This results in a surprisingly increased depth impression of pictorial space, historically known as the 'plastic effect'. In this paper we present a practical design to produce a synopter and explore which elements of a painting influence the plastic effect. In the first experiment we showed 22 different paintings to a total of 35 observers, and found that they rate the synoptic effect rather consistent over the various paintings. Subsequent analyses indicated that at least three pictorial cues were relevant for the synoptic effect: figure-ground contrast, compositional depth and shadows. In experiment 2, we used manipulated pictures where we tried to strengthen or weaken these cues. In all three cases we found at least one effect that confirmed our hypothesis. We also found substantial individual differences: some observers experience little effect, while others are very surprised by the effect. A stereo acuity test revealed that these differences could not be attributed to how well disparities are detected. Lastly, we informally tested our newly designed synopter in museums and found similar idiosyncratic appraisal. But the device also turned out to facilitate discussions among visitors. [ABSTRACT FROM AUTHOR]