Human visual cortex does not represent the whole visual field with the same detail.
Changes in receptive field size, population receptive field (pRF) size and cortical
magnification factor (CMF) with eccentricity are well established, and associated
with changes in visual acuity with eccentricity. Visual acuity also changes across
polar angle. However, it remains unclear how RF size, pRF size and CMF change across
polar angle. Here, we examine differences in pRF size and CMF across polar angle in
V1, V2 and V3 using pRF modeling of human fMRI data. In these visual field maps, we
find smaller pRFs and larger CMFs in horizontal (left and right) than vertical (upper
and lower) visual field quadrants. Differences increase with eccentricity, approximately
in proportion to average pRF size and CMF. Similarly, we find larger CMFs in the lower
than upper quadrant, and again differences increase with eccentricity. However, pRF
size differences between lower and upper quadrants change direction with eccentricity.
Finally, we find slightly smaller pRFs in the left than right quadrants of V2 and
V3, though this difference is very small, and we find no differences in V1 and no
differences in CMF. Moreover, differences in pRF size and CMF vary gradually with
polar angle and are not limited to the meridians or visual field map discontinuities.
PRF size and CMF differences do not consistently follow patterns of cortical curvature,
despite the link between cortical curvature and polar angle in V1. Thus, the early
human visual cortex has a radially asymmetric representation of the visual field.
These asymmetries may underlie consistent reports of asymmetries in perceptual abilities.