Neurons in the primary visual cortex are selective for the size, orientation and direction of motion of patterns falling within a restricted region of visual space known as the receptive field. The response to stimuli presented within the receptive field can be facilitated or suppressed by other stimuli falling outside the receptive field which, when presented in isolation, fail to activate the cell. Whether this interaction is facilitative or suppressive depends on the relative orientation of pattern elements inside and outside the receptive field. Here we show that neuronal facilitation preferentially occurs when a near-threshold stimulus inside the receptive field is flanked by higher-contrast, collinear elements located in surrounding regions of visual space. Collinear flanks and orthogonally oriented flanks, however, both act to reduce the response to high-contrast stimuli presented within the receptive field. The observed pattern of facilitation and suppression may be the cellular basis for the observation in humans that the detectability of an oriented pattern is enhanced by collinear flanking elements. Modulation of neuronal responses by stimuli falling outside their receptive fields may thus represent an early neural mechanism for encoding objects and enhancing their perceptual saliency.

Immediately after focal retinal lesions, receptive fields (RFs) in primary visual cortex expand considerably, even when the retinal damage is limited to the photoreceptor layer. The time course of these changes suggests that mere lack of stimulation in the vicinity of the RF accompanied by stimulation in the surrounding region causes the RF expansion. While recording from single cells in cat area 17, we simulated this pattern of stimulation with a pattern of moving lines in the visual field, masking out an area covering the RF of the recorded cell, thereby producing an "artificial scotoma. " Over 10 min this masking resulted in a 5-fold average expansion in RF area. Stimulating the RF center caused the field to collapse in size, returning to near its original extent; reconditioning with the masked stimulus led to RF reexpansion. Stimulation in the surrounding region was required for the RF expansion to occur-little expansion was seen during exposure to a blank screen. We propose that the expansion may account for visual illusions, such as perceptual fill-in of stabilized images and illusory contours and may constitute the prodrome of altered cortical topography after retinal lesions. These rin support the idea that even in adult animals RFs are dynamic, capable of being altered by the sensory context.The extent of a receptive field (RF) is usually defined by the visual-field area over which a cell can be activated by a simple stimulus, such as an oriented line segment or edge. It is now evident, however, that the response of a cell can be modulated by stimuli lying outside of the RF (1-7). This principle at the cellular level is accompanied by a corresponding set of observations at the psychophysical level, whereby one's percept of a local attribute is influenced by the context within which a feature is presented (8-10). The basis for transmission of visual information from one part of the visual field to another is seen in the pattern of connections within the visual cortex. Long-range horizontal connections formed by cortical pyramidal cells enable the recipient neurons to integrate information over a large region of cortex and, hence, a larger part of the visual field than that covered by their RFs, as classically defined (11-15).Although the contextual influences on the firing of a cell under ordinary circumstances are subthreshold, certain manipulations of input to visual cortex may elevate these influences to an activating level: focal destruction of the retina causes the cortical area that receives input from the affected retina, over a few months, to reorganize its topography so that cells shift their RFs to the perilesion retina (16)(17)(18). Even in the short term, within minutes after retinal lesions, single-unit RF size in area V1 expands (19). Because effects were observed over such a short time and could be induced by as minor an intervention as destruction of the photoreceptor layer, it was logical to ask whether the effect could be simulated simply by occluding the RF, restricting stimuli...

Lateral occipital cortical areas are involved in the perception of objects, but it is not clear how these areas interact with first tier visual areas. Using synthetic images portraying a simple texture-defined figure and an electrophysiological paradigm that allows us to monitor cortical responses to figure and background regions separately, we found distinct neuronal networks responsible for the processing of each region. The figure region of our displays was tagged with one temporal frequency (3.0 Hz) and the background region with another (3.6 Hz). Spectral analysis was used to separate the responses to the two regions during their simultaneous presentation. Distributed source reconstructions were made by using the minimum norm method, and cortical current density was measured in a set of visual areas defined on retinotopic and functional criteria with the use of functional magnetic resonance imaging. The results of the main experiments, combined with a set of control experiments, indicate that the figure region, but not the background, was routed preferentially to lateral cortex. A separate network extending from first tier through more dorsal areas responded preferentially to the background region. The figure-related responses were mostly invariant with respect to the texture types used to define the figure, did not depend on its spatial location or size, and mostly were unaffected by attentional instructions. Because of the emergent nature of a segmented figure in our displays, feedback from higher cortical areas is a likely candidate for the selection mechanism by which the figure region is routed to lateral occipital cortex.Key words: visual cortex; object processing; figure/ground; cue invariance; lateral occipital complex; source imaging IntroductionObject recognition mechanisms must be able to extract shape independently of the surface cues that are present. Local estimates of surface cues such as texture grain or orientation, although necessary as inputs to the recognition process, convey little sense of object shape. Rather it is the pattern of cue similarity across regions and cue discontinuity across borders that must be integrated to recover object shape.The process of cue-invariant shape processing begins at an early stage of visual cortex and extends deep into extrastriate cortex and the temporal lobe. Cue invariance first is seen as early as V2, where some cells have a similar orientation or direction tuning for borders defined by different feature discontinuities (Leventhal et al., 1998;Marcar et al., 2000;Ramsden et al., 2001;Zhan and Baker, 2006). At higher levels of the visual system, such as inferotemporal cortex (Sary et al., 1993) and medial superior temporal area (Geesaman and Andersen, 1996), cells show shape selectivity that is mostly independent of the defining cues and spatial position. Functional magnetic resonance imaging (fMRI) studies in humans have implicated homologous extrastriate regions, in particular the lateral occipital complex (LOC), as sites of category-specific, cue-...

Visual stimulation of a region outside the receptive field of single cells in visual cortex often results in the modulation of their responses. The modulatory effects are thought to be mediated through lateral connections within visual cortex. Research on lateral interactions commonly shows suppression. There has been no systematic study of the optimal conditions for facilitation. Here we have studied the nature of the modulation using a new type of compound stimulus: contrast reversal of pattern stimuli made of three discrete grating patches. The middle patch, optimally fitted to the receptive field in orientation, size, and spatial as well as temporal frequencies, was flanked by two similar patches presented well outside the receptive field. We found that (1) both facilitation and suppression occurred often in the same cells, when orientations of the target and flankers matched the receptive-field's optimal orientation; (2) facilitation with collinear flankers occurred most frequently at target contrasts just above the cell's firing threshold and suppression prevailed at high contrasts; (3) facilitative or suppressive modulation was obtained with target-flankers separation of up to 12 deg or more; (4) collinear facilitation was lost when flankers' orientation was rotated by 90 deg, while keeping all other parameters the same; and (5) neither the modulation mode nor the proportion of modulated cells was related to the cell types (simple vs. complex cells) and cells' laminar locations. Here we have provided physiological evidence for contrast-dependent, collinear facilitation probably underlying perceptual grouping in humans.

Contour detection may be mediated by lateral interactions between neighboring cortical neurons whose receptive fields have collinear axes of preferred orientation. This hypothesis was tested in psychophysical experiments and computer simulations using a contour detection task in which observers searched for groups of Gabor patches that followed spatially extended contour paths embedded in noise consisting of several hundred Gabor patches with random positions and orientations. The orientation-selective units in the simulated neural network were linked by facilitatory interconnections whose strength depended on the geometry (distance, curvature, change in curvature) of smooth curves connecting the orientation axes of units in a pairwise fashion. Psychophysical detection performance was much higher for contour signal groups that followed closed rather than open-ended paths. However, just two sudden changes in orientation of neighboring Gabor patch elements in closed-path contours reduced detection performance to the same levels obtained with open-ended contours. These psychophysical data agreed with the results of the neural network simulations. Furthermore, the simulations also accounted for previous findings that removal of a single Gabor patch element from a closed-path contour group significantly degraded detection performance. We conclude that closure alone is not sufficient to enhance the visibility of a contour. However, if a closed contour meets certain geometric constraints, then lateral interactions based on these constraints can generate facilitation that reverberates around the closed path, thereby enhancing the contour's visibility.