Visual evoked potentials elicited by subjective contour figures

Abstract

In order to investigate the relationship between the appearance of illusory figures and the wave form of visual evoked potentials (VEPs), 8 different visual pattern stimuli were presented to 8 normal subjects. Four of the stimuli (experimental stimuli) produced subjective figures and contours (squares and discs). The 4 other stimuli (reference stimuli), although equal to the experimental stimuli in the amount of physical energy, did not produce the illusion of squares or discs. Electrodes were placed on the scalp at central and occipital locations. Three prominent peaks in the occipital record were observed in all subjects. An amplitude difference of VEP N180 (N2) between the subjective figures and the reference stimuli was found in the values for each subject. Enhancement of the VEP of the illusory figure stimuli was observed for a specific component (N2), whereas the amplitude values at the central components and the occipital P120 (P2) and P280 (P3) were almost the same as the reference values. The VEP (N2 component) amplitude enhancement at the occipital area for subjective figure stimuli suggests that illusory contour formation takes place at higher levels in the visual system. This was known from experiments using dichoptic presentation.

Illusory contours: a window onto the neurophysiology of constructing perception

“…However, "close" could presumably refer either to spatial or temporal proximity (see Box 3), allowing for IC sensitivity to be initially mediated in regions such as LOC. The IC effect is likewise observed when using misaligned gratings instead of Kanizsa-type stimuli [38,39,44]. The timing of the IC effect shifts ~100ms later (i.e.…”

“…This counter-claim was formalized to suggest that the IC effect reflects sensitivity to the detection of salient regions bound by the inducers rather than to IC processes per se [7] (Figure 1). This claim was based on fMRI results showing equivalent depth of modulation within the LOC to Kanizsa-type ICs as well as rounded versions of these stimuli that lack a perception of bound contours, but nonetheless demarcate a territory at which a shape could appear [39]. However, whereas this model proposes a differential response within V1/V2 to ICs versus these rounded versions, no direct evidence for such nor any V1/V2 responses were shown.…”

“…In addition, the IC effect is insensitive to variations in the contrast polarity (i.e. whether inducers are black on a gray background or vice versa) [8] (see also [39]), ruling out an explanation in terms of perceived brightness enhancement (see also [40]). The IC effect is mediated by the presence of the illusory figure but not collinear lines [41].…”

“…Other evidence speaks against models that prioritize salient region detection over IC sensitivity. Sugawara and Morotomi [39] not only presented Kanisza-type stimuli but also Ehrenstein-type stimuli (while also varying contrast polarity). Both types of stimuli led to similarly robust ERP modulations highly similar to the above-described IC effect.…”

Electrophysiological indexes of illusory contours perception in humans

“…Furthermore, in these studies the reduced number of electrodes used to collect VEP data does not guarantee reliable experimental proof of hemispheric asymmetry. Sugawara and Morotomi [31] recorded visual evoked potentials (from CZ, OL and OR sites) to passively viewed illusory and control stimuli on a white or black background. Results showed an enhancement of N2 occipital deflection for subjective figures, with no effect of background on VEP amplitudes.…”