Symmetry is an important cue in face perception. We manipulated symmetry and other configurational variables to study their role in face processing in the human brain. We employed 2 types of symmetry: image symmetry (where one part of the image is defined as the mirrored transform of the other part about an axis) and object symmetry (where the spatial relationships among the image components are interpreted as parts of a symmetric 3-dimensional object). We compared blood oxygenation level dependent responses in healthy human observers for upright front-view faces with responses to different symmetry-controlled images. The cortical areas activated by the face images, relative to Fourier-matched scrambled images, were the fusiform (FFA) and occipital (OFA) face areas, the middle occipital gyri (MOG), and areas around the superior temporal and intraoccipital sulci (IOS). Contrasting faces and their image-symmetric scrambled versions showed a similar activation pattern except in the right OFA, suggesting an involvement in facial symmetry processing. The upright versus inverted faces (with the same image symmetry but unfamiliar object identity) showed robust differential activation in the FFA, OFA, MOG, IOS, and precuneus. The response to frontal-view versus 3/4-view faces (having the same object symmetry but disrupted image symmetry) showed little differential activation in the FFA or the OFA but strong responses in the MOG and IOS, suggesting that face processing in the FFA and the OFA is holistic and viewpoint invariant.

Single-cell responses in visual cortex to a target falling within their receptive field can be modified by collinear flanking stimuli concurrently presented outside the receptive field. Here, we report the presence of four types of contrast-dependent lateral effects: (1) facilitation at low target contrasts and suppression at high contrasts, (2) facilitation that increases with contrast, (3) suppression that increases with contrast, and (4) suppression at low contrasts with facilitation at high contrasts. We propose a sensitivity modulation model that accounts for all the four types of lateral effects by changes in two parameters. In this model, activation of neighboring neurons changes the sensitivities of the target neuron to both the direct feedforward input and inhibitory, divisive feedback from neighboring neurons.

We used a dual–masking paradigm to study how contrast discrimination can be influenced by the presence of adjacent stimuli. The task of the observer was to detect a target superimposed on a pedestal in the presence of flankers. The flankers (i) reduce the target threshold at zero pedestal contrast, (ii) shift the target threshold versus pedestal contrast (TvC) function horizontally to the left on a log–log plot at high pedestal contrasts, and (iii) reduce the size of pedestal facilitation at low pedestal contrasts. The horizontal shift at high pedestal contrasts suggests that the flanker effect is a multiplicative factor that cannot be explained by previous models of contrast discrimination. We extend the divisive inhibition model of contrast discrimination by implementing the flanker effect as a lateral multiplicative sensitivity modulation. This extended model provides a good account of the data.