Most visual illusions are produced using carefully contrived drawings or gadgets to fool the visual system into thinking impossible things. As described in an earlier post, while waiting at a train station, I encountered an apparently real-life Ponzo illusion. Here I explain why I think the new illusion is more strange and more interesting than the classical Ponzo Illusion.
The Ponzo Illusion
The traditional Ponzo illusion is produced by drawing a pair of receding railway lines. The context suggests different depths in the drawing. An object towards the top of the drawing appears larger than an identical object near the bottom of the drawing. Using a principle of size constancy, the visual system estimates the size of any object as its retinal size multiplied by the assumed distance. Thus, the ‘most distant’ of the two identical yellow lines appears to be longer than the ‘nearer’ yellow line.
The Setting for the New Depth-Rescaling Illusion
The setting of this new illusion is a railway station situated at Vitrolles Airport, Marseille (see photo below). The station has glass panelled shelters on the platforms on each side. The glass panel at the front of each shelter displays two rows of grey rectangles. Apart from their decorative function, one assumes that these rows of rectangles are intended to help prevent people from walking into the glass panel as they move in and around the shelter. The photo below shows the arrangement of the two rows of rectangles on the shelter.
The stimuli for the illusion consist of rectangles that are slightly longer than a credit card, approximately 10.0 cm long x 1.5 cm wide with a separation of about 3.0 cm between successive rectangles. The plate glass window is about 5 mm thick and is marked with rectangles on both sides of the glass in perfect alignment so that a 3-D effect is created indicating a false sense of solidity to these rectangles. This ‘3-D look’ may strengthen the illusory effect illustrated below.
An Illusion of Depth and Scaling
The new illusion has two intimately related features: illusory magnification and illusory depth. Looking at the rectangles on the glass, the rectangles appear to be projected into the space behind the glass at a greatly magnified size. The rectangles appear as a set of ‘illusory posts’ on the station platform.
The ‘illusory posts’ appear huge in comparison to the actual size of the rectangles on the shelter window (10 cm). When viewed against the background of the nearby platform, the new illusory size is re-scaled to around 10-15 times the actual size.
The perceived depth of an object or picture is the perceived distance from the nearest to the farthest point. If an object is perceived to be further away, then its magnitude is re-scaled accordingly. The depth and re-scaling are directly proportionate. The further the illusory depth, the greater the illusory magnification. When viewed against the background of the platform on the other side of the railway lines, the rescaling is 40-50 times. The scaling of the lines is driven by the illusory depth.
The Context is Crucial
This new illusion does not occur when the context is incompatible with the existence of ‘rectangular posts’ in the aligned position. When the rectangles appear against incompatible backgrounds, there is no clearly formed illusion, as illustrated below. In the first picture, the rectangles are shown against an incompatible background where ‘posts’ cannot normally float in space above a field of reeds. Hence the two rows of rectangles are perceived in a single plane of a reflecting window.
In the case below, the photograph shows rectangles on the side window of the shelter, which cannot rest as ‘posts’ all the way along the row from the left- to right-hand sides. The illusion does not appear on the left side, but there is a weak magnification effect at the right-hand end of the row, where the platform could be compatible with the existence of ‘posts’.
This powerful new illusion is produced by a combination of illusory depth and illusory rescaling. The illusion is possible when the existence of the projected stimuli is congruent with the observed background. With an incongruent background, the illusion does not occur. The illusion is likely induced by the operation of constancy mechanisms in the visual system of the brain.
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