Modulating the global orientation bias of the visual system changes population receptive field elongations

Abstract

The topographical structure of the visual system in individual subjects can be visualized using fMRI. Recently, a radial bias for the long axis of population receptive fields (pRF) has been shown using fMRI. It has been theorized that the elongation of receptive fields pointing toward the fovea results from horizontal local connections bundling orientation selective units mostly parallel to their polar position within the visual field. In order to investigate whether there is a causal relationship between orientation selectivity and pRF elongation the current study employed a global orientation adapter to modulate the orientation bias for the visual system while measuring spatial pRF characteristics. The hypothesis was that the orientation tuning change of neural populations would alter pRF elongations toward the fovea particularly at axial positions parallel and orthogonal to the affected orientation. The results indeed show a different amount of elongation of pRF units and their orientation at parallel and orthogonal axial positions relative to the adapter orientation. Within the lower left hemifield, pRF radial bias and elongation showed an increase during adaptation to a 135° grating while both parameters decreased during the presentation of a 45° adapter stimulus. The lower right visual field showed the reverse pattern. No modulation of the pRF topographies were observed in the upper visual field probably due to a vertical visual field asymmetry of sensitivity toward the low contrast spatial frequency pattern of the adapter stimulus. These data suggest a direct relationship between orientation selectivity and elongation of population units within the visual cortex.

Population receptive field shapes in early visual cortex are nearly circular

“…Through simulation we identify methods for improving sensitivity that may estimate ellipses with smaller aspect ratios. The results we present are quantitatively consistent with assessments using several other methodologies (Greene et al, 2014; Zeidman et al, 2018; Merkel et al, 2020).…”

“…For example, Merkel et al (2018; 2020) estimated the aspect ratios of voxels in early visual cortex and reported ellipticity (the inverse of aspect ratio) ranging between 0.6 and 1, which corresponds to aspect ratios of 1 to 1.67 (Figure 7A; Merkel et al, 2020). Using tomographic methods to estimate pRF shapes (Greene et al, 2014) also estimated aspect ratios (Figure 7B).…”

“…Direct comparisons of standard pRF models suggest that circular receptive field models provide the best fits (Zeidman et al, 2018 ; Figure 10). Using novel measurement approaches, investigators report that individual fMRI voxels may have some orientation preference (Greene et al, 2014;Merkel et al, 2018Merkel et al, , 2020 .…”

“…The distribution they report had 11% of the aspect ratios greater than 2 which is close to the expected amount based on our simulations with synthetic data assuming circular pRFs (14%) and based on analysis with the HCP 7T (8%) data. (Merkel et al, 2020; adapted from Figure 2b) . (B) Aspect ratio reported in (Greene et al, 2014; adapted with permission from Figure 10F) .…”

“…On the other hand, Silson et al (2018) report that pRFs are significantly elongated, often with an aspect ratio (ratio of long to short axis) of 2.5 or greater. Groups using a broader class of allowable shapes have reported inconclusive results, for example finding most pRFs in V1-V3 to be nearly circular but a small percentage to be quite elongated (Greene et al, 2014) , or finding many voxels to be slightly elongated (Merkel et al, 2018(Merkel et al, , 2020 . We set out to investigate the discrepancy between the high ellipticity report and the more common assumption of near circularity by replicating their findings.…”