Vascular effects on the BOLD response and the retinotopic mapping of hV4

Abstract

Despite general acceptance that the retinotopic organisation of human V4 (hV4) takes the form of a single, uninterrupted ventral hemifield, measured retinotopic maps of this visual area are often incomplete. Here, we test hypotheses that artefact from draining veins close to hV4 cause inverted BOLD responses that may serve to obscure a portion of the lower visual quarterfield -including the lower vertical meridian -in some hemispheres. We further test whether correcting such responses can restore the 'missing' retinotopic coverage in hV4. Subjects (N=11) viewed bowtie, ring, drifting bar and full field flash stimuli. Functional EPIs were acquired over approximately 1.5h and analysed to reveal retinotopic maps of early visual cortex, including hV4. Normalised mean maps (which show the average EPI signal amplitude) were constructed by voxel-wise averaging of the EPI time course and used to locate venous eclipses, which can be identified by a decrease in the EPI signal caused by deoxygenated blood. Inverted responses are shown to cluster in these regions, and correcting these responses improves maps of hV4 in some hemispheres, including restoring a complete hemifield map in one. A leftwards bias was found in which 11/11 hV4 maps in the left hemisphere were classified as incomplete, while this was the case in only 3/11 right hemisphere maps. Incomplete hV4 maps did not correspond with venous artefact in many instances, with incomplete maps being present in the absence of a venous eclipse and complete maps coexisting with a proximate venous eclipse. We also show that mean maps of upper surfaces (near the boundary between cortical grey matter and CSF) provide highly detailed maps of veins on the cortical surface. Results suggest that venous eclipses and inverted voxels can explain some incomplete hV4 maps, but cannot explain the remainder nor the leftwards bias in hV4 coverage reported here. Introduction 1 Human visual cortex is comprised of multiple orderly visual areas which are functionally 2 and anatomically distinct from each other [1-4]. Many of these areas are organised 3 according to the principle of retinotopy, which is to say they are topographically 4 organised such that the spatial organisation of the retina, and therefore the visual field, 5 is mapped to the neurons of each area [5-7]. These retinotopic maps are highly 6 preserved across individuals and species, and hence the cortical organisation of visual 7 areas is unlikely to be random, but an integral part of visuo-cortical function [8-10]. It 8 PLOS 1/20is therefore important to determine the organisation and position of visual areas, as 9 these factors are central to our understanding of visual information processing. 10Functional magnetic resonance imaging (fMRI) is currently one of the most 11 well-suited and popular tools for studying visual areas in human visual cortex, due to its 12 safe and non-invasive nature; however, it is not without issue. The impact of veins on 13 the measured blood-oxygen-level dependent (BOLD) response in fMRI has been...

Variability of visual field maps in human early extrastriate cortex challenges the canonical model of organization of V2 and V3

“…Additionally, to determine if our findings were not merely due to other covariates, we included the normalized mean BOLD signal and intra-individual variability in pRF estimates as additional covariates. The former is an important proxy for the location of large veins (Boyd Taylor et al, 2019; Kurzawski et al, 2022), which are known to affect pRF estimates systematically (Boyd Taylor et al, 2019; Winawer et al, 2010); the latter is a proxy for the reliability of individuals’ retinotopic maps, which could vary across visual areas. While we found a significant effect of intra-individual variability, the main effects of all factors (hemispheres, visual areas, and portions) on individual variability of polar angle maps persisted.…”

“…The mean pre-processed BOLD signal is used as a proxy for the location of large veins (Boyd Taylor et al, 2019; Kurzawski et al, 2022), which are known to affect pRF estimates. Voxels near large veins show lower mean BOLD signal, a phenomenon known as the venous eclipse, which systematically affects pRF estimates, for example, in area hV4 (Boyd Taylor et al, 2019; Winawer et al, 2010). As such, deviations in the expected mean BOLD signal could lead to deviations from the expected retinotopic organization.…”

“…Therefore, we considered individual variability in the mean BOLD signal as another covariate. To do so, we first normalized the mean BOLD signal (over the time course) by dividing the value of each vertex by the mean maximum intensity (Boyd Taylor et al, 2019). Then, we computed the individual variability as previously described for curvature maps.…”