The Spatiotemporal Dynamics of Illusory Contour Processing: Combined High-Density Electrical Mapping, Source Analysis, and Functional Magnetic Resonance Imaging

The Cognitive Neurophysiology Laboratory, Program in Cognitive Neuroscience and Schizophrenia, Nathan S. Kline Institute for Psychiatric Research, Orangeburg, New York 10962, USA.
The Journal of Neuroscience : The Official Journal of the Society for Neuroscience (Impact Factor: 6.34). 07/2002; 22(12):5055-73.
Source: PubMed

ABSTRACT Because environmental information is often suboptimal, visual perception must frequently rely on the brain's reconstruction of contours absent from retinal images. Illusory contour (IC) stimuli have been used to investigate these "filling-in" processes. Intracranial recordings and neuroimaging studies show IC sensitivity in lower-tier area V2, and to a lesser extent V1. Some interpret these data as evidence for feedforward processing of IC stimuli, beginning at lower-tier visual areas. On the basis of lesion, visual evoked potentials (VEP), and neuroimaging evidence, others contend that IC sensitivity is a later, higher-order process. Whether IC sensitivity seen in lower-tier areas indexes feedforward or feedback processing remains unresolved. In a series of experiments, we addressed the spatiotemporal dynamics of IC processing. Centrally presented IC stimuli resulted in early VEP modulation (88-100 msec) over lateral-occipital (LOC) scalp--the IC effect. The IC effect followed visual response onset by 40 msec. Scalp current density topographic mapping, source analysis, and functional magnetic resonance imaging results all localized the IC effect to bilateral LOC areas. We propose that IC sensitivity described in V2 and V1 may reflect predominantly feedback modulation from higher-tier LOC areas, where IC sensitivity first occurs. Two additional observations further support this proposal. The latency of the IC effect shifted dramatically later (approximately 120 msec) when stimuli were laterally presented, indicating that retinotopic position alters IC processing. Immediately preceding the IC effect, the VEP modulated with inducer eccentricity--the configuration effect. We interpret this to represent contributions from global stimulus parameters to scene analysis. In contrast to the IC effect, the topography of the configuration effect was restricted to central parieto-occipital scalp.

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Available from: John J Foxe, Sep 28, 2015
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    • "Statistical analyses were performed on the mean amplitude values acquired in the time window between 60 ms and 85 ms after the delivery of the TMS pulse. This time window was set after an exploratory analysis of the data across the entire epoch, using successive T-test comparisons (Murray et al., 2002; Guthrie and Buchwald, 1991), which showed no earlier effects of the experimental variables. During 30 min of Sham sessions we acquired EEG activity locked to the placebo TMS pulse, in order to monitor the unspecific effects (e.g., flow of time; acoustic sensations) that could account for modulations of cortical excitability, but are not directly due to the magnetic stimulation. "
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    ABSTRACT: Spatial neglect is traditionally explained as an imbalance of the interhemispheric reciprocal inhibition exerted by the two hemispheres: after a right lesion, the contralesional hemisphere becomes disinhibited and its enhanced activity suppresses the activity in the lesioned one. Even though the hyperexcitability of the left hemisphere is the theoretical framework of several rehabilitation interventions using non-invasive brain stimulation protocols in neglect, no study has yet investigated directly the actual state of cortical excitability of the contralesional hemisphere immediately after the brain lesion. The present study represents the first attempt to directly assess the interhemispheric rivalry model adopting a novel approach based on the induction of neglect-like biases in healthy participants. Applying repetitive transcranial magnetic stimulation (rTMS) over the right posterior parietal cortex while concurrently recording the EEG activity allows to measure specific neurophysiological markers of cortical activity (i.e. TMS-evoked potentials, TEPs) both over the stimulated right hemisphere and over the contralateral homologous area. Besides the effectiveness of the protocol used in modulating behavior, our results show an inhibition of the cortical excitability of the directly stimulated parietal cortex (right hemisphere) and, most importantly, a comparable reduction of cortical excitability of the homologous contralateral (left) area. TEPs and additional electrophysiological measures reliably provide strong evidence for a bilateral hypo-activation following TMS induction of neglect-like biases. These results suggest that the parietal imbalance typically found in neglect patients could reflect a long-term maladaptive plastic reorganization that follows a brain lesion. Copyright © 2015. Published by Elsevier Ltd.
    Neuropsychologia 04/2015; 72. DOI:10.1016/j.neuropsychologia.2015.04.010 · 3.30 Impact Factor
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    • "The minimum number of consecutive significant data points (cluster) needed to control for the family-wise error rate was set at 101 ( 420 ms at a 5000 Hz digitizing rate). The results of this analysis are represented as intensity plots (Murray et al., 2002) in which only cluster-corrected significant results are shown (see Fig. 1D and F) to depict the topographic distribution of differential activation associated with phosphene-present trials versus phosphene-absent trials and to identify the onset of these differences across time. Scalp current density (SCD) maps were used to determine the generators contributing to the phosphene present/absent effect. "
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    ABSTRACT: Transcranial magnetic stimulation (TMS) of the occipital cortex is known to induce visual sensations, i.e. phosphenes, which appear as flashes of light in the absence of an external stimulus. Recent studies have shown that TMS can produce phosphenes also when the intraparietal sulcus (IPS) is stimulated. The main question addressed in this paper is whether parietal phosphenes are generated directly by local mechanisms or emerge through indirect activation of other visual areas. Electroencephalographic (EEG) signals were recorded while stimulating left occipital or parietal cortices inducing phosphene perception in healthy participants and in a hemianopic patient who suffered from complete destruction of the early visual cortex of the left hemisphere. Results in healthy participants showed that the onset of phosphene perception induced by occipital TMS correlated with differential cortical activity in temporal sites while the onset of phosphene perception induced by parietal TMS correlated with differential cortical activity in the stimulated parietal site. Moreover, IPS-TMS of the lesioned hemisphere of the hemianopic patient with a complete lesion to V1 showed again that the onset of phosphene perception correlated with differential cortical activity in the stimulated parietal site. The present data seem thus to suggest that temporal and parietal cortices can serve as different local early gatekeepers of perceptual awareness and that activity in the occipital cortex, although being relevant for perception in general, is not part of the neural bases of the perceptual awareness of phosphenes. Copyright © 2015. Published by Elsevier Ltd.
    Neuropsychologia 02/2015; 70. DOI:10.1016/j.neuropsychologia.2015.02.021 · 3.30 Impact Factor
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    • "Similar results were subsequently reported by Waberski et al. (2008). Foxe et al. (2003) found that the onset of the line-bisection effect systematically tracked the latency of the N1 visual-evoked potential, known to reflect object processing in the ventral visual pathway (e.g., Allison et al., 1999; Murray et al., 2002). Critically, however, the onset of the line-bisection effect followed N1 peak latency. "
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    ABSTRACT: Traditionally, the right cerebral hemisphere has been considered to be specialized for spatial attention and orienting. A large body of research has demonstrated dissociable representations of the near space immediately surrounding the body and the more distance far space. In this study, we investigated whether right hemisphere activations commonly reported for tasks involving spatial attention (such as the line bisection and landmark tasks) are specific to stimuli presented in near space. In separate blocks of trials, participants judged either whether a vertical transector was to the left or right of the centre of a line (landmark task) or whether the line was red or blue (colour task). Stimuli were seen from four distances (30, 60, 90, 120 cm). We used EEG to measure an ERP component (the ‘line-bisection effect’) specific to the direction of spatial attention (i.e., landmark minus colour). Consistent with previous results, spatial attention produced a right-lateralized negativity over occipito-parietal channels. The magnitude of this negativity was inversely related to viewing distance, being largest in near space and reduced in far space. These results suggest that the right occipito-temporal cortex may be specialized not just for the orientation of spatial attention generally, but specifically for orienting attention in the near space immediately surrounding the body.
    Neuropsychologia 11/2014; 70. DOI:10.1016/j.neuropsychologia.2014.10.035 · 3.30 Impact Factor
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