Boundary Completion Is Automatic and Dissociable from Shape Discrimination

Abstract

Normal visual perception readily overcomes suboptimal or degraded viewing conditions through perceptual filling-in processes, enhancing object recognition and discrimination abilities. This study used visual evoked potential (VEP) recordings in conjunction with electrical neuroimaging analyses to determine the spatiotemporal brain dynamics of boundary completion and shape discrimination processes in healthy humans performing the so-called "thin/fat" discrimination task (Ringach and Shapley, 1996) with stimuli producing illusory contours. First, results suggest that boundary completion processes occur independent of subjects' accuracy on the discrimination task. Modulation of the VEP to the presence versus absence of illusory contours [the IC effect (Murray et al., 2002)] was indistinguishable in terms of response magnitude and scalp topography over the 124 -186 ms poststimulus period, regardless of whether task performance was correct. This suggests that failure on this discrimination task is not primarily a consequence of failed boundary completion. Second, the electrophysiological correlates of thin/fat shape discrimination processes are temporally dissociable from those of boundary completion, occurring during a substantially later phase of processing (ϳ330 -406 ms). The earlier IC effect was unaffected by whether the perceived contour produced a thin or fat shape. In contrast, later time periods of the VEP modulated according to perceived shape only in the case of stimuli producing illusory contours, but not for control stimuli for which performance was at near-chance levels. Collectively, these data provide further support for a multistage model of object processing under degraded viewing conditions.

Illusory contours: a window onto the neurophysiology of constructing perception

“…The IC effect is likewise unaffected by participants' accuracy in discriminating the curvature of the induced contours and also occurs prior to modulations of the concavity/convexity of the perceived contour [37], prompting at least two conceivable interpretations. First, IC sensitivity may dissociate from and precede shape discrimination.…”

“…Murray et al [25] found that sensitivity to Kanizsa-type ICs onsets at ~90ms post-stimulus onset (see also [33] for initial evidence for such timing and [34] for an alternative paradigm yielding similar results), lagged ERP onset by ~40ms, and is localized first to the LOC (Figure 2). This "IC effect" has been shown to be an amplitude rather than a topographic modulation of the ERP that is superimposed upon the so-called N170 component [28]( [8,[35][36][37][38]), which is a peak of the visual evoked potential at ~150-200ms with a negative distribution of the parieto-occipital scalp with predominant sources within ventral and lateral occipital cortices. In addition, the IC effect is insensitive to variations in the contrast polarity (i.e.…”

Glutathione Precursor, N-Acetyl-Cysteine, Improves Mismatch Negativity in Schizophrenia Patients

“…This clustering procedure identifies the topographies (ie maps) dominating the group-averaged AEPs across populations/ conditions. The pattern observed at the group-average level was then statistically assessed at the individual participant level using a fitting procedure based on spatial correlation (Brandeis et al, 1995; see also Murray et al, 2006, for a recent publication of formulae), yielding a measure of map presence. These values are then submitted to ANOVA, revealing whether and when different maps explain CGI and PANSS scales: S, severity; I, improvement; P, positive; N, negative; G, general psychopathology; T, total score.…”

Learning-Induced Plasticity in Auditory Spatial Representations Revealed by Electrical Neuroimaging

“…the topography of the electric field at the scalp) as well as latency shifts in brain processes across experimental conditions. Because the electrophysiological methods have been extensively detailed previously Murray et al, 2004;De Santis et al, 2007; for a recent publication of formulas, see Murray et al, 2006), we provide only the essential details here.…”

“…In other words, the methods described above generate a hypothesis concerning the sequence of maps observed in the AEPs and any differences between stimulus positions and/or sessions. To statistically assess this hypothesis, each time point of each AEP from each subject was labeled according to the map with which it best correlated spatially (Brandeis et al, 1995;Murray et al, 2006). We, hereafter, refer to this procedure as "fitting."…”