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Jonas Larsson, Michael S. Landy, David J. Heeger; Orientation-selective adaptation to first- and second-order stimuli in human visual cortex measured with fMRI. Journal of Vision 2004;4(8):543. doi: 10.1167/4.8.543.
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PURPOSE We used FMRI to identify neuronal populations selective for the orientation of first- and second-order grating patterns to test the hypothesis that neurons selective for first- and second-order stimuli are differentially distributed in retinotopic visual areas. METHODS Stimuli were 0.75 cpd sine wave gratings at 4 orientations (0, 45, 90, 135 deg), defined by luminance or contrast modulated noise, and restricted to an annulus (1.5–5 deg eccentricity) around fixation. 4 subjects were scanned while performing a spatial frequency discrimination task to control attention. Each scan consisted of 6 adaptation and 6 non-adaptation blocks. In adaptation blocks, a single orientation was presented repeatedly. In non-adaptation blocks, the orientation changed every trial between the 3 non-adapted orientations. The adaptation orientation was vertical in half of the sessions and 45 deg in the remaining sessions. Data were analyzed by fitting a sinusoid at the block alternation frequency to the FMRI responses within each visual area, defined in separate sessions by standard retinotopic mapping methods. RESULTS Orientation-selective adaptation to both first- and second-order gratings was found in several extrastriate retinotopic areas, but was weak in V1 and V2. The strongest adaptation was observed in area V3A. Other areas showed intermediate degrees of adaptation. The relative degree of adaptation across visual areas was similar for first- and second-order stimuli. We also found an oblique effect only in area V3A: the magnitude of adaptation in this area was stronger for vertical than for 45 deg adapter orientations. CONCLUSION The distribution of orientation-tuned neurons across visual areas is similar for first- and second-order stimuli. Although psychophysics suggests that separate mechanisms underlie first- and second-order static pattern perception, the results of this study show that these mechanisms are not anatomically segregated at the scale observable by FMRI.
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