MEG Responses to the Perception of Global Structure within Glass Patterns

Abstract

BackgroundThe perception of global form requires integration of local visual cues across space and is the foundation for object recognition. Here we used magnetoencephalography (MEG) to study the location and time course of neuronal activity associated with the perception of global structure from local image features. To minimize neuronal activity to low-level stimulus properties, such as luminance and contrast, the local image features were held constant during all phases of the MEG recording. This allowed us to assess the relative importance of striate (V1) versus extrastriate cortex in global form perception.Methodology/Principal FindingsStimuli were horizontal, rotational and radial Glass patterns. Glass patterns without coherent structure were viewed during the baseline period to ensure neuronal responses reflected perception of structure and not changes in local image features. The spatial distribution of task-related changes in source power was mapped using Synthetic Aperture Magnetometry (SAM), and the time course of activity within areas of maximal power change was determined by calculating time-frequency plots using a Hilbert transform. For six out of eight observers, passive viewing of global structure was associated with a reduction in 10–20 Hz cortical oscillatory power within extrastriate occipital cortex. The location of greatest power change was the same for each pattern type, being close to or within visual area V3a. No peaks of activity were observed in area V1. Time-frequency analyses indicated that neural activity was least for horizontal patterns.ConclusionsWe conclude: (i) visual area V3a is involved in the analysis of global form; (ii) the neural signature for perception of structure, as assessed using MEG, is a reduction in 10–20 Hz oscillatory power; (iii) different neural processes may underlie the perception of horizontal as opposed to radial or rotational structure; and (iv) area V1 is not strongly activated by global form in Glass patterns.

Cortical and behavioral sensitivity to eccentric polar form

“…However, the enhanced activity to concentric patterns in V3A/B conflicts with the findings of Mannion et al (2010b), who reported an anisotropy only for starburst patterns. Changes in anisotropic response in V3A/B to polar form centered at eccentricity could underlie this difference, although Swettenham et al (2010) reported qualitatively similar magnitudes of response in V3A/B during observation of starburst and concentric Glass patterns centered at fixation and at eccentricity.…”

“…Indeed, neurophysiological recordings have reported preferences for polar orientation in V1 and V2 neurons and an overrepresentation of neurons preferring concentric orientations (Hegdé & Van Essen, 2007;Mahon & De Valois, 2001). However, Smith, Kohn, and Movshon (2007) found no modulation in activity by polar Glass pattern orientation in V2 neurons, and previous neuroimaging studies have not found differences in polar form response in V1 and V2 (Wilkinson et al, 2000), including to stimuli presented at eccentricity (Swettenham, Anderson, & Thai, 2010). Hence, the role of V1 and V2 in polar form processing remains uncertain.…”

Global shape processing: A behavioral and electrophysiological analysis of both contour and texture processing

“…In this first experiment, we predict that the presence of any global arrangement within a field of randomly oriented Gabor patches should lead to an enhanced N1 component because the N1 has been linked to the presence of a coherent structure (Ohla et al, 2005;Swettenham et al, 2010). Any effect of specific shape attributes, such as the number and placement of corners on the contour, would more likely be evident in later components, including the N220.…”

“…The global processing associated with Glass patterns has previously been localized to areas external to the primary visual cortex (Ohla et al, 2005;Vreven & Berge, 2007) with a recent highdensity MEG study suggesting a generator in area V3a (Swettenham et al, 2010). Mannion and Clifford (2011) have shown that both V3 and V4 were involved in processing global arrangements in Glass patterns.…”

“…Similar studies have been conducted using stimuli in which shape is defined by texture rather than a contour. Coherent structure in a Glass pattern (fields of randomly placed dot stimuli in which dot pairs are oriented to form a coherent global structure; Glass, 1969) increases the N1 amplitude more than a pattern composed of randomly oriented elements at occipital electrode sites, and a concentric coherent arrangement elicits a larger amplitude than either radial or parallel arrangements (Ohla, Busch, Markus, & Herrmann, 2005;Ostwald, Lam, Li, & Kourtzi, 2008;Pei, Pettet, Vildavski, & Norcia, 2005;Swettenham, Anderson, & Thai, 2010). It is therefore likely that global orientation coherence in Glass pattern stimuli affects the N1 component despite a lack of connected or continuous contours.…”