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Jennifer F. Schumacher, Cheryl A. Olman; BOLD fMRI response to local neural inhibition. Journal of Vision 2007;7(9):80. doi: 10.1167/7.9.80.
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© ARVO (1962-2015); The Authors (2016-present)
Lateral inhibition is an important neural process for vision, however the BOLD fMRI response to this type of neural activity is not thoroughly understood. Primary visual cortex (V1) neural activity to a parafoveal target Gabor patch is inhibited when neighboring Gabor patches of the same orientation surround the target, while the activity is facilitated by Gabor patch flankers oriented perpendicular to the target Gabor patch. Psychophysical measurements confirm the inhibitory and facilitatory relationships between flanker and target via determination of contrast response functions inferred from the contrast discrimination thresholds. The inferred contrast response functions for target Gabor stimuli predict that local inhibitory processes decrease the V1 neural response to a 35% contrast target by approximately 30% when high contrast parallel flankers are present. The functional imaging data show that the 3T GE BOLD response to the targets is not reduced in the presence of parallel flankers. Furthermore, principal components analysis of the response to the target stimuli alone (with neither flanking nor facilitatory surround) reveals a heterogeneity in the hemodynamic response that has not been observed before. The close juxtaposition of target and flanker cortical representations presents a significant challenge for BOLD fMRI studies of contextual modulation. In this case, the hemodynamic responses to target and flanker stimuli overlap, and the failure of BOLD to accurately reflect local neural inhibition may simply be a consequence of hemodynamic blurring, rather than an indication of a more basic aspect of neuro-hemodynamic coupling. Further experiments at high field using perfusion and spin-echo fMRI will reveal whether, in the absence of hemodynamic blurring, the BOLD response is reduced by local neural inhibition.
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