senting the orientation of features in the visual image is a fundamental operation of the early cortical visual system. The nature of such representations can be informed by considering anisotropic distributions of response across the range of orientations. Here we used functional MRI to study modulations in the cortical activity elicited by observation of a sinusoidal grating that varied in orientation. We report a significant anisotropy in the measured blood-oxygen leveldependent activity within visual areas V1, V2, V3, and V3A/B in which horizontal orientations evoked a reduced response. These visual areas and hV4 showed a further anisotropy in which increased responses were observed for orientations that were radial to the point of fixation. We speculate that the anisotropies in cortical activity may be related to anisotropies in the prevalence and behavioral relevance of orientations in typical natural environments. Although such studies considered pattern orientation independent of position in the visual field, anisotropic response distributions have been reported across the early human visual system when pattern orientation is specified relative to the local visual field meridian (Fig. 1). Specifically, responses are enhanced when the local pattern orientation is coincident with the angular meridian (radial) compared with when it is tangential (Clifford et al. 2009; Sasaki et al. 2006). An important consequence of this meridian-relative anisotropy is that it confers difficulties in the interpretation of putative anisotropies in field-independent pattern orientation, particularly when the stimulus (Furmanski and Engel 2000; Furmanski et al. 2004) or analysis (Serences et al. 2009; Yacoub et al. 2008) is restricted to a subregion of the visual field. In this circumstance, inferences about anisotropies in field-independent orientation can be confounded with those in meridian-relative orientation; for example, a stimulus restricted to the vertical visual field meridian will produce responses in which only the vertical fieldindependent orientation is also a radial meridian-relative orientation.The aim of this study was to measure the distribution of visual system responses to varied pattern orientation by considering both field-independent and meridian-relative indices. We used fMRI to measure the blood-oxygen level-dependent (BOLD) response to a sinusoidal grating that varied in orientation and to map the preferred visual field meridia from within the early retinotopic regions of human visual cortex. We report the presence of significant anisotropies to both field-independent and meridian-relative orientation from within visual areas V1, V2, V3, and V3A/B. Although the form of meridianrelative anisotropy is consistent with previous reports of a preference for radial orientations, the anisotropy in fieldindependent orientation is characterized by a reduced response to horizontal orientations. M E T H O D S SubjectsFour experienced psychophysical observers participated in this study. Each subject received a re...
Mechanisms of color vision in cortex have not been as well characterized as those in sub-cortical areas, particularly in humans. We used fMRI in conjunction with univariate and multivariate (pattern) analysis to test for the initial transformation of sub-cortical inputs by human visual cortex. Subjects viewed each of two patterns modulating in color between orange-cyan or lime-magenta. We tested for higher order cortical representations of color capable of discriminating these stimuli, which were designed so that they could not be distinguished by the postulated L-M and S-(L + M) sub-cortical opponent channels. We found differences both in the average response and in the pattern of activity evoked by these two types of stimuli, across a range of early visual areas. This result implies that sub-cortical chromatic channels are recombined early in cortical processing to form novel representations of color. Our results also suggest a cortical bias for lime-magenta over orange-cyan stimuli, when they are matched for cone contrast and the response they would elicit in the L-M and S-(L + M) opponent channels.
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