Abstract
Determining whether a region belongs to the interior or exterior of a shape (figure-ground segregation) is a core competency of the primate brain, yet the underlying mechanisms are not well understood. Many models assume that figure-ground segregation occurs by assembling progressively more complex representations through feedforward connections, with feedback playing only a modulatory role. We present a dynamical model of figure-ground segregation in the primate ventral stream wherein feedback plays a crucial role in disambiguating a figure's interior and exterior. We introduce a processing strategy whereby jitter in RF center locations and variation in RF sizes is exploited to enhance and suppress neural activity inside and outside of figures, respectively. Feedforward projections emanate from units that model cells in V4 known to respond to the curvature of boundary contours (curved contour cells), and feedback projections from units predicted to exist in IT that strategically group neurons with different RF sizes and RF center locations (teardrop cells). Neurons (convex cells) that preferentially respond when centered on a figure dynamically balance feedforward (bottom-up) information and feedback from higher visual areas. The activation is enhanced when an interior portion of a figure is in the RF via feedback from units that detect closure in the boundary contours of a figure. Our model produces maximal activity along the medial axis of well-known figures with and without concavities, and inside algorithmically generated shapes. Our results suggest that the dynamic balancing of feedforward signals with the specific feedback mechanisms proposed by the model is crucial for figure-ground segregation.
Highlights
Figure-ground segregation refers to the process by which the visual system parses the complex array of luminance that appears on the retina into perceptually grouped foreground objects and backgrounds
If higher visual areas mediate the effect, what are neurons with limited receptive field (RF) sizes in early visual cortex that demonstrate interior enhancement signaling about the interior of a figure? We propose that interior enhancement is a means to code the figure with respect to its medial axis (Burbeck and Pizer, 1995; Kovács et al, 1998; Pizer et al, 1998)
In Section Spatio-Temporal Dynamics, we focus on the spatio-temporal response of convex cells to show that these units do exhibit interior enhancement, similar to units in primary visual cortex
Summary
Figure-ground segregation refers to the process by which the visual system parses the complex array of luminance that appears on the retina into perceptually grouped foreground objects (figures) and backgrounds (ground). Populations of edge-sensitive neurons in primate visual areas V1, V2, and V4 have been shown to exhibit sensitivity to border-ownership: neurons respond with a higher firing rate when the figure to which the edge in the receptive field (RF) is attached appears on the preferred side (Zhou et al, 2000). Neural models have suggested that border-ownership selectivity may arise through feedback from neurons with larger RFs in higher visual areas (Kelly and Grossberg, 2000; Craft et al, 2007; Jehee et al, 2007; Layton et al, 2012), through feedforward processing alone (Supèr et al, 2010), or through horizontal connections within V2 (Zhaoping, 2005). Feedback connections are likely involved in borderownership because they span large cortical areas with minimal delay, unlike horizontal connections
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