Surround Modulation Measured With Functional MRI in the Human Visual Cortex

Abstract

Visual context profoundly influences 1) the responses of mammalian visual neurons and 2) the perceptual sensitivity of human observers to localized visual stimuli. We present data from functional MRI studies demonstrating contextual modulation in the human visual cortex. Subjects viewed a circular grating patch that was continuously present. A surround grating was added in an ON-OFF block design to reveal its effect on the central region. Stimulus-correlated activation was quantified and visualized on a flattened map of the occipital gray matter. Modulation was measured in a region of interest activated by the central grating alone. The observed effects were predominantly suppressive, consistent with the effects typically found in single neurons and perception. Suppression was greatest when the surround and center had the same orientation and was reduced or absent when it was orthogonal. When spatial phase was manipulated, suppression was greatest for in-phase center/surround gratings and much reduced or reversed (facilitation) for opposite-phase stimuli. With eccentric stimulus presentation, suppression was reduced and facilitation became more common. The findings provide a direct demonstration of the existence of powerful and stimulus-specific surround effects in human visual cortex.

Responses of human visual cortex to uniform surfaces

“…For stimuli varying in brightness contrast, there is a close correlation between fMRI or magnetoencephalography signals in V1 and the perceived contrast of stimuli (12,13,15). Our data show that for the stimuli used in this experiment, responses in V1 and V2͞V3 are scaled in a similar way to the perceived brightness function.…”

“…However, more recent reports suggest that some cells in primary visual cortex do indeed respond to the luminance of uniform surfaces (6-10). Furthermore, in humans there is a close relationship between perceived brightness contrast and responses in primary visual cortex (11)(12)(13)(14)(15), suggesting that other sensations of brightness may also be encoded in primary visual cortex. However, the cortical response function for surfaces of uniform luminance in early visual areas, and its relationship to perceived brightness, has remained uninvestigated.…”

Population receptive field estimates in human visual cortex

“…Additionally, the pRF may be modeled using nonlinear models and sums and differences of Gaussians to test hypotheses about surround suppression. These suppressive effects have been demonstrated using fMRI (Dumoulin and Hess, 2006;Kastner et al, 2001;Williams et al, 2003;Zenger-Landolt and Heeger, 2003), and it has been shown that the strength and spatial extent of these suppressive effects increase in later visual areas (Kastner et al, 2001;Zenger-Landolt and Heeger, 2003). This increase may be related to increases in the RF sizes in these areas (Bles et al, 2006;Kastner et al, 2001).…”

Opposing Effects of Contextual Surround in Human Early Visual Cortex Revealed by Functional Magnetic Resonance Imaging with Continuously Modulated Visual Stimuli

“…In humans, a study using visually evoked potential has implied the existence of contrast-dependent suppression and facilitation (Polat and Norcia, 1996), but precise cortical loci responsible for such contrast-dependent responses are not identified in the study. Although several studies using noninvasive imaging approaches, such as functional magnetic resonance imaging (fMRI) (Williams et al, 2003;Zenger-Landolt and Heeger, 2003) and magnetoencephalography (Ohtani et al, 2002), have documented the suppressive responses as a consequence of the surround modulation in human visual cortex, there has been no evidence for the contrast-dependent switching between suppressive and facilitative responses. Overall, it is difficult to detect these weak modulatory effects, especially when the contrast of the target region is low.…”