Perceptual transparency

Although he was not the first author to study the phenomenon of transparency illusion, the

Gestalt psychologist Metelli was probably the one who made the major contribution to the

problem.

Like his forecomers, Metelli faces the problem from a phenomenical more than from a

physiological point of view. In other words, he did not investigate which are the

physiological algorithms or the brain networks underlying transparency perception, but

studied and classified the conditions under which a transparency illusion is generated. In

doing so, Metelli marks an approach to the problem that will be followed by many scientists

after him.

The model is based on the idea that the perceptual colour scission following transparency

is the opposite of colour fusion in a rotating episcotister, i.e. a rotating disk that alternates

open and solid sectors.

Metelli referred to colour fusion in a physical situation in which an episcotister rotates in

front of an opaque background of reflectance A; the episcotister has an open sector of size t (a

proportion of the total disk) and a solid sector of size (1-t) having reflectance r. The

reflectance of the solid sectors and that of background are fused by rotation to produce a

virtual reflectance value $P$:

$P=t*A\; +(1-t)*R$

that is the weighted sum of background reflectance and episcotister solid sector

reflectance.

An episcotister is not a transparent object. Nevertheless, Beck et al. (1984) proposed an

alternative model, based on transparent filters, that has the characteristic of including the

effects of repetitive reflections between the transparent layer and the underlying surface.

Both the episcotister model and the filter model, in their original formulation, were written

in terms of reflectance values. A consequence is that their validity as physical models is

dependent on the illumination conditions. However, both models can be rewritten in terms of

luminance, as shown by Gerbino et al. (1990).

Although physically correct in a number of situations, the filter model never gained a

significant role in the prediction of perceptual transparency. In spite of being

much more complicated than the episcotister model, it doesn’t lead to significant

improvements in predictions about the occurrence of the illusion.

While Metelli's episcotister model has long remained the preferred framework for the

study of luminance conditions in transparency illusion, its validity as a theory of perception

has been challenged by different studies.

Beck et al. (1984) showed that only constraints (i) and (ii) imposed by the episcotister

model are necessary for illusion of transparency; when constraints (iii) and (iv) are not

fulfilled, the illusion can still be experienced. They also argued that the degree of perceived

transparency depends on lightness more than reflectance.

Masin and Fukuda (1993) proposed as alternative conditions for transparency to (i) and (ii)

the ordinal condition p Є (a, q) [or q Є (p, b)], that was shown to agree better than

episcotister model with transparency-judgements performed by naïve subjects in a yes-no

task (Masin 1997).

Metelli's equations were extended to three-dimensional colour space by D'Zmura et al.

(1997). According to the model, transparency illusion would be generated by coherent

convergence and translation in colour space. However, also in colour space, evidence was

found in which the perceptual appearance does not reflect the physical model. For instance,

D'Zmura et al. (1997) showed that equiluminant convergence and translation in colour space

can elicit an impression of transparency, even if no episcotister nor physical filter can

generate this stimulus configuration. Chen and D'Zmura (1998) showed deviations from the

predictions of convergence model when the transparent regions have complementary hues.

• Beck, J., K. Prazdny, et al. (1984). "The perception of transparency with achromatic colors." Percept Psychophys 35(5): 407-22.
• Metelli, F. (1970). An Algebraic Development of the Theory of Perceptual Transparency. Contemporary Problems in Perception. London, Taylor and Francis.
• Metelli, F. (1974). "The perception of transparency." Scientific American 230: 91-98.
• Tudor-Hart, B. (1928). "Studies in Transparency, Form and Color." Psychologische Forschung X: 255-298.

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Category:Visual perception