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Rick Cai, Jerry Federspiel, Kerstin Priest, Mike Zenz; Emmert's law cannot be generated by relative size cues even when these cues contain sufficient information. Journal of Vision 2007;7(9):833. doi: 10.1167/7.9.833.
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© ARVO (1962-2015); The Authors (2016-present)
In Emmert's law, the apparent size of an afterimage changes proportionally with the distance between the observer and the surface onto which it is projected (the ‘projection surface’). In most cases in which the law was tested, relative size cues were available in addition to distance cues. For example, if an afterimage is projected onto a nearby chair, it may appear as a speck on the chair's fabric, but when the same afterimage is projected across a large room, several chairs can submerge into it. Such relative size cues contain sufficient information to scale an afterimage according to Emmert's law. It has been shown previously that distance cues alone, in the absence of any relative size cue, can lead to Emmert's law. Here, we tested the inverse condition to find out whether relative size cues alone can also generate Emmert's law. In Exp. 1, an object was presented on a computer monitor set at a fixed distance from the observer. The object's size changed smoothly, simulating the change in its retinal image as it moved closer or further away. An afterimage was projected onto the object. Thus, the relative size change between the two was achieved without any change in the apparent distance of the afterimage itself. The perceived size of the afterimage remained essentially the same. In Exp. 2, the changing object was placed within a virtual environment on the monitor, such that it was perceived as a looming or receding. The perceived size of the afterimage projected onto it remained unaltered. In Exp.3, the object' size was fixed on the monitor, but the monitor itself moved forward or backward. Emmert's law was obtained. Our results suggest that Emmert's law is due to a direct scaling of the afterimage itself by the size constancy mechanism.
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