Journal Article

Early electrophysiological indices of illusory contour processing within the lateral occipital complex are virtually impervious to manipulations of illusion strength

Russo, Natalie N., Brandwein, Alice B., Altschuler, Ted S., Molholm, Sophie, Snyder, Adam C., Foxe, John J., Blanco, Daniella

2012

Overview

The visual system can automatically interpolate or “fill-in” the boundaries of objects when inputs are fragmented or incomplete. A canonical class of visual stimuli known as illusory-contour (IC) stimuli has been extensively used to study this contour interpolation process. Visual evoked potential (VEP) studies have identified a neural signature of these boundary completion processes, the so-called IC-effect, which typically onsets at 90–110ms and is generated within the lateral occipital complex (LOC). Here we set out to determine the delimiting factors of automatic boundary completion with the use of illusory contour stimuli and high-density scalp recordings of brain activity. Retinal eccentricity, ratio of real to illusory contours (i.e. support ratio), and inducer diameter were each varied parametrically, and any resulting effects on the amplitude and latency of the IC-effect were examined. Somewhat surprisingly, the amplitude of the IC-effect was found to be impervious to all changes in these stimulus parameters, manipulations that are known to impact perceived illusion strength. Thus, this automatic stage of object processing appears to be a binary process in which, so-long as minimal conditions are met, contours are automatically completed. At the same time, the latency of the IC-effect was found to vary inversely with support ratio, likely reflecting the additional time necessary to interpolate across the relatively longer induced boundaries of the implied object. These data are interpreted in the context of a two stage object-recognition model that parses processing into an early automatic perceptual stage that is followed by a more effortful conceptual processing stage. ► VEPs delineate a 2-stage model of object processing. ► In the first, gaps in object contours are automatically closed. ► We tried to modulate a VEP index of this phase by varying contour parameters. ► Peak latency varied inversely with one manipulation, but amplitude was invariant.

• Publisher: Elsevier Inc,Elsevier Limited
• Subject: Adult / Electrophysiological Phenomena / Electrophysiology / Experiments / Female / Humans / Illusory-contours / Male / Object processing / Optical Illusions - physiology / Studies / Visual Cortex - physiology / Visual evoked potentials / Visual Perception - physiology / Visual processing / Young Adult