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Kuwook Cha, Sang-Hun Lee; Anisotropic representation of oriented bars in human visual cortex is revealed by fMRI projective and receptive field mapping techniques. Journal of Vision 2007;7(9):597. doi: 10.1167/7.9.597.
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© ARVO (1962-2015); The Authors (2016-present)
Optical imaging studies in primates demonstrated that neural responses to an oriented stimulus are distributed over a substantial region in early visual cortex (Grinvald et al., 1994), with the spreading pattern being elongated along the axis of the orientation of the stimulus (Bosking et al., 1997). Here we report that this anisotropy exists also in human visual cortex using two converging paradigms of fMRI, ‘projective field mapping (PFM)’ and ‘receptive field mapping (RFM)’ techniques.
In the PFM experiment, observers fixated the center of a screen, where a punctual (800ms), small (s=.33°) Gabor patch (sf=1.1∼4.4 cyc/°) appeared repeatedly every 13.2 secs, with its orientation alternating randomly between 45° and 135° across trials. We estimated the spatial extension of cortical activity by measuring fMRI responses from cortical subregions which represent 1.16°-width annuli at different eccentricities (0.58°∼5.22°). In the RFM experiment, observers viewed an 8 × 8 array of grids inside each of which a Gobor patch intermittently appeared according to m-sequence. For each voxel of early visual cortex, we derived a field map of stimuli that evoked activity of that voxel using the spectral reverse correlation technique (Nishimoto et al., 2006).
In the PFM experiment, cortical activity spread farther to the periphery along the collinear than orthogonal axis of the foveal Gabor stimulus, resulting in an elliptical ‘projective field’ map whose major axis matched the orientation of the Gabor patch. An analogous ‘receptive field’ map was obtained from the RFM experiment: a set of Gabor elements that activated a given voxel were distributed anisotropically, such that the elongated axis of the distribution matched the orientation of stimuli composing the array. These two maps, taken together, lend converging supports to the presence of anisotropic representations of oriented stimuli in human visual cortex, which may contribute to contour integration in form perception.
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