Illusory form with inducers of opposite contrast polarity: Evidence for multistage integration

Abstract

The perception of brightness differences in Ehrenstein figures and of illusory contours in phaseshifted line gratings was investigated as a function of the contrast polarity of the inducing elements. Wepresented either continuous lines or line-like arrangements composed of aligned dashes or dots whose spacing was varied. A yes/no procedure was used in which naive observers had to decide whether or not they perceived a brightness difference in a given Ehrenstein figure or an illusory contour in a phase-shifted line grating. The results show that brightness differences are perceived to some extent in Ehrenstein figures with inducers of opposite polarity of contrast; however, the percentage of yes responses was systematically lower and response times were longer than for figures with inducers of the same polarity. Phase-shifted line gratings with lines of opposite polarity of contrast yielded stronger illusory contours and shorter response times than those with lines of the same polarity. When the sign of contrast was not the same within a given line of induction, neither differences in brightness nor illusory contours were perceived. The results suggest that the mechanisms that lead to apparent differences in brightness are more sensitive to input of the same contrast polarity, the mechanisms generating illusory contours more sensitive to input of opposite polarity. The data are discussed in the light of a multistage approach to illusory form perception and some implications for cortical models of illusory contour integration are discussed.

Cue-dependent circuits for illusory contours in humans

“…For example, Murray et al (2002) identified the onset of the IC effect at ~90ms and localized it to the LOC. This 'brain signature' of IC sensitivity is robust across a range of stimulus/perceptual variations: contrast polarity ; see also Dresp et al, 1996 for psychophysical findings), modal vs. amodal completion (i.e. whether or not there was a perceived brightness enhancement; Murray et al, 2004), accuracy in discriminating convex versus concave shapes created by the ICs (Murray et al, 2006), or the stimulus type eliciting the ICs (Kanizsa-type stimuli or misaligned gratings; Knebel and Murray, 2012;Mendola et al, 1999).…”

Response times to Ehrenstein illusions of varying subjective magnitude: Complementarity of psychophysical measures

“…In earlier experiments (Dresp et aI., 1996), such figures were used to study illusory brightness perception as a function of local variations in stimulus geometry, with an emphasis on assumptions concerning the underlying feature integration mechanisms. These figures were found to produce a variety ofillusory percepts covering a wide range ofdifferent, discriminable, illusory strengths (see Dresp et a\., 1996).…”

“…Subjective magnitude was determined in a first experiment by a cross-modality matching procedure like the one used by Dresp, Lorenceau, and Bonnet (1990) in a study of the perception of apparent brightness in the Kanizsa square. In a second experiment, we measured the time necessary to perceive the presence or the absence of an apparent surface in these same illusory figures, replicating the procedure used by Dresp et al (1996). We found that subjective magnitude, response time, and frequency of perception provide complementary data reflecting different aspects of information processing involved in the perception of brightness illusions.…”

“…Dresp et al (1996) have reported experiments with configurations giving rise to the perception of Ehrenstein illusions with a surprising variation in phenomenal strength. This was achieved by varying the spacing, the length, and the contrast polarity of the inducing element defining the configurational context.…”

“…ulus luminance or contrast polarity produce sometimes paradoxical effects (see, e.g., Dresp, Salvano-Pardieu, & Bonnet, 1996;Spillmann, Fuld, & Neumeyer, 1984) indicating that the relationship between the physical stimulus and the induced brightness percept is far from being straightforward. Attentional factors and psychological processes related to stimulus uncertainty may explain some of these observations; however, the question of which would be the appropriate method to bring them to the fore remains a critical issue (see, e.g., the review by Spil\mann & Dresp, 1995).…”