Consequences of polar form coherence for fMRI responses in human visual cortex

Abstract

Relevant features in the visual image are often spatially extensive and have complex orientation structure. Our perceptual sensitivity to such spatial form is demonstrated by polar Glass patterns, in which an array of randomly-positioned dot pairs that are each aligned with a particular polar displacement (rotation, for example) yield a salient impression of spatial structure. Such patterns are typically considered to be processed in two main stages: local spatial filtering in low-level visual cortex followed by spatial pooling and complex form selectivity in mid-level visual cortex. However, it remains unclear both whether reciprocal interactions within the cortical hierarchy are involved in polar Glass pattern processing and which mid-level areas identify and communicate polar Glass pattern structure. Here, we used functional magnetic resonance imaging (fMRI) at 7T to infer the magnitude of neural response within human low-level and mid-level visual cortex to polar Glass patterns of varying coherence (proportion of signal elements). The activity within low-level visual areas V1 and V2 was not significantly modulated by polar Glass pattern coherence, while the low-level area V3, dorsal and ventral mid-level areas, and the human MT complex each showed a positive linear coherence response functions. The cortical processing of polar Glass patterns thus appears to involve primarily feedforward communication of local signals from V1 and V2, with initial polar form selectivity reached in V3 and distributed to multiple pathways in mid-level visual cortex.

Regions of Mid-level Human Visual Cortex Sensitive to the Global Coherence of Local Image Patches

“…A. Cox et al, 2013), including amodal illusory surfaces. The above-chance classification performance observed in hV4 may also relate to the capacity for contour completion and complex feature selectivity afforded by the coherence between image patches, given that ventral regions of mid-level visual cortex, including visual area hV4, are sensitive to isolated global form (Mannion et al, 2013; Ostwald, Lam, Li, & Kourtzi, 2008; Wilkinson et al, 2000). …”

“…Conventional retinotopic mapping and visual area localisation acquisition and analysis procedures, implemented as detailed in Mannion et al (2013), were performed on each participant’s standardised surface space. These surface datasets were combined across participants at each node on the surface, and the resulting maps of angular and eccentric visual field preference were used to assign likely visual area labels to low and mid-level visual cortex, as shown in Figure 2, to provide a framework for interpreting the location of regional activation in the main experiment.…”

“…This often involves identifying and isolating aspects of local stimulation that are considered to be candidates for global integration. For example, the integration of local edges into global shapes can be probed by using a spatial array of oriented elements in which the global arrangement of edge orientations either do or do not cohere into a perception of global form (Altmann, Bülthoff, & Kourtzi, 2003; Kourtzi, Tolias, Altmann, Augath, & Logothetis, 2003; Mannion, Kersten, & Olman, 2013). However, a potentially fruitful complementary strategy is to assess global integration in naturalistic environments, which contain a complex and rich spatial structure (Simoncelli & Olshausen, 2001) with myriad components that could potentially be targeted by global processes.…”

Scene coherence can affect the local response to natural images in human V1

“…Participants observed four runs of a clockwise/anticlockwise rotating wedge stimulus and two runs of an expanding/contracting ring stimulus (Sereno et al ., ; DeYoe et al ., ; Engel et al ., ; Larsson & Heeger, ; Hansen et al ., ; Schira et al ., ), and the data were analysed via phase‐encoding methods (Engel, ) to establish visual field preferences over the cortical surface; see Mannion et al . () for details. The angular and eccentricity phase maps were used to manually define each participant's V1.…”

“…The V1 region of occipital cortex was defined based on analysis of functional acquisitions, obtained in a separate scanning session (using the same 7-T scanner as in the main experiment), that followed standard procedures for the delineation of retinotopic regions in human visual cortex. Participants observed four runs of a clockwise/ anticlockwise rotating wedge stimulus and two runs of an expanding/contracting ring stimulus (Sereno et al, 1995;DeYoe et al, 1996;Engel et al, 1997;Larsson & Heeger, 2006;Hansen et al, 2007;Schira et al, 2007), and the data were analysed via phaseencoding methods (Engel, 2012) to establish visual field preferences over the cortical surface; see Mannion et al (2013) for details. The angular and eccentricity phase maps were used to manually define each participant's V1.…”