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Allison B. Sekuler, Lisa R. Betts, Eugenie Roudaia, Yaroslav Konar, Patrick J. Bennett; Surround suppression in visual cortex: Effects of spatial frequency. Journal of Vision 2009;9(8):977. doi: 10.1167/9.8.977.
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© ARVO (1962-2015); The Authors (2016-present)
Surround suppression, the phenomenon in which the perception and/or neural response to a target is reduced by the presence of a surrounding annulus, was previously demonstrated in early visual cortex with fMRI (e.g., Williams, Singh, & Smith, 2003). Here, we investigated whether the surround suppression of fMRI responses is influenced by stimulus spatial frequency, an effect that has been shown psychophysically (Betts, Sekuler, & Bennett, 2008). The central region of interest (ROI) was defined in four observers (mean age: 28.3 years) as the cortical area exhibiting greater responses to a 3 deg circular checkerboard than to an annulus (inner diameter = 3 deg, outer diameter = 9 deg). Activation of this central ROI was measured during two types of scans: 1) the Center-Alone (CA) scan, in which the presentation of a 3 deg circular grating alternated with a uniform field, and 2) the Center + Surround (CS) scan, in which the 3 deg circular grating was always present and a surrounding annulus grating alternated on and off. CA and CS scans used horizontal gratings with spatial frequencies of 0.33 and 1 c/deg that flickered at 5 Hz. As expected, ROI activation was positively correlated with the presence of the circular grating in the CA condition. ROI activation was negatively correlated with the presence of the annulus in the CS condition, even though the central stimulus was always present. This result is consistent with surround suppression. The ratio of CS to CA activation in the 25% most responsive voxels was significantly greater in the 0.33 c/deg condition in all observers. This result is consistent with psychophysical results showing greater surround suppression for 0.5 c/deg than for 1 c/deg. Future research will extend the paradigm to older adults to examine the hypothesis that aging reduces surround suppression in early visual cortex.
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