Assimilation: Central and Peripheral Effects

Abstract

Assimilation and contrast have opposite effects: Contrast leads to an increase of perceived differences between neighbouring fields, whereas assimilation leads to a reduction. It is relatively easy to demonstrate these effects, but the precise localisation of these effects in the perceptual system is not yet possible. In an experiment the strength of assimilation effects was modified by adding spatial noise. By varying the localisation in perceived space of the added noise (by presentation of the noise pattern with different binocular disparities) the masking effect of this noise can be influenced. Masking caused by binocularly disparate noise is less than masking caused by binocularly non-disparate noise. Our conclusion is that the effect at least partly occurs beyond the (binocular) locus of separation in different depth planes. A similar approach, involving moving noise, will also be presented. Finally, several demonstrations show that images that are peripherally similar can give rise to differences in the perceived amount of assimilation. These effects further indicate that a central mechanism is involved in assimilation. Assimilation is a perceived change in colour and/or brightness in one part of a picture in the direction of the colour and/or brightness of another part or other parts in the picture. The simplest case is a change in perceived colour of the background in the direction of the colour parts belonging to the foreground. Assimilation is more common in daily visual scenes than the better known simultaneous contrast effect DeValois and DeValois, 1988). Since the discovery of receptive fields with a centersurround structure, lateral inhibition has been the most popular tool for explaining basic visual processes. Undoubtedly it is an important mechanism. The shape of the luminance contrast modulation transfer function can easily be understood on the basis of the mechanism of lateral inhibition, which in turn, finds its physiological basis in spatial opponency of the center-surround receptive fields. Assimilation has been known for a long time (Von Bezold, 1874;Helson, 1963). Kanizsa (1979) described a particular aspect of the assimilation effect, namely the influence of the observer s attitude. Assimilation effects are most distinct if the observer does not fixate steadily. In fact, he stated that an untrained observer clearly perceives assimilation, whereas a trained observer like a painter, is less certain whether contrast or assimilation occurs (Kanizsa was both a painter and a scolar of visual perception. Kanisza ascribed this difference to the distinction that trained

Von Bezold Assimilation Effect Reverses in Stereoscopic Conditions

“…The present research tested the hypothesis that central mechanisms are involved in both contrast and assimilation, and it was inspired by an observation reported by de Weert and van Kruysbergen (1997). By means of a stereo display, de Weert and van Kruysbergen observed that when some red and green discs are painted on a white target, the latter appears greenish when the green discs are perceived at its same depth and the red discs stratify.…”

“…According to the level of the visual process they refer to, researchers have provided three interpretations of this relationship. The first asserts that both the phenomena occur at a peripheral level of the visual system (Helson, 1964;DeValois & DeValois, 1975;Hurvich & Jameson, 1966;1974;Jameson & Hurvich, 1975;Reid & Shapley, 1988); the second attributes their occurrence to more central mechanisms (Musatti, 1931;1953;Festinger, Coren & Rivers, 1970;de Weert and Van Kruysbergen 1997;Kanizsa, 1979), and the third claims that contrast involves central mechanisms whilst assimilation involves peripheral ones (Gilchrist et al, 1999). …”

“…The second interpretation suggests that this shift is generated by more central mechanisms of visual processing, such as figure/ground segmentation (Musatti, 1931;1953;Festinger, Coren & Rivers, 1970;de Weert and Van Kruysbergen 1997) and observer expertise (Kanizsa, 1979).…”

“…For example, de Weert and van Kruysbergen (1997) found that the strength of assimilation reduces when spatial noise is added to an assimilation-eliciting display (i.e. to a display in which the inducers spatial frequency is high).…”

“…The aim of the present study is to further investigate this observation and to extend it in the achromatic domain with the following important modifications. In de Weert and van Kruysbergen's (1997) displays, both the green and red inducers where superimposed to the target-background at the same time. Because of this, it may be argued that the greenish and reddish appearance of the white target could be caused by a contrast effect of the segregated discs instead of an assimilation effect of the coplanar ones.…”