Population receptive field shapes in early visual cortex are nearly circular

Abstract

The visual field region where a stimulus evokes a neural response is called the receptive field (RF). Analytical tools combined with functional MRI can estimate the receptive field of the population of neurons within a voxel. These population RF (pRF) methods accurately specify the central position of the pRF and provide some information about the spatial extent (size) of the receptive field. A number of investigators developed methods to further estimate the shape of the pRF, for example whether the shape is more circular or elliptical. This journal published a report that there are many pRFs with highly elliptical pRFs in early visual cortex (V1-V3; Silson et al., 2018). Such a large aspect ratio is difficult to reconcile with the spatial scale of orientation columns or visual field maps in early visual cortex. We started to replicate the experiments using their methods and found that the software used in the publication does not accurately estimate RF shape: it produces elliptical fits to circular ground-truth data. We analyzed an independent data set with a different software package that was validated over a specific range of measurement conditions. Our analysis of pRF elliptical fits show that in early visual cortex the aspect ratios are less than 2 and potentially consistent with nearly circular shapes. Furthermore, current empirical and theoretical methods do not have enough precision to discriminate ellipses with aspect ratios of 1.5 from circles (aspect ratio of 1). 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).

Extremely Fast pRF Mapping for Real-Time Applications

“…The authors argue that when a receptive field partially lies outside the stimulus space, the part of the receptive field that lies inside may be incorrectly identified as having an oval shape. This would be in line with recent studies arguing that receptive fields are generally circular in shape (Lerma-Usabiaga et al, 2020). Other studies have suggested that receptive fields do become increasingly elongated at higher eccentricities.…”

Divisive normalization unifies disparate response signatures throughout the human visual hierarchy

“…In particular, exponentiation may be a useful computational component to build back in the model if attempting to capture richer datasets. With regard to the ongoing debate concerning the isotropy of pRF shapes (51)(52)(53)(54)(55)(56), we note that the shape of the activation and normalization pRFs could Modeling brain responses to naturalistic stimuli is a crucial goal of computational neuroscience (57) and is likely to involve DN computations (58). Some authors have already proposed models that effectively capture BOLD responses to naturalistic visual stimuli (58,59).…”

Mismatch between human early visual cortex and perception in spatial extent representation: Radial bias shapes cortical representation while co-axial bias shapes perception

“…(2) The elongation axis of an ellipsoid is constrained to the radial or tangential axis. (3) The aspect ratio parameter was constrained so that the absolute log aspect ratio is smaller than 2, based on previous studies reported that voxels in early visual areas slightly elongated with an aspect ratio ranging between 1-1.67 (Merkel et al, 2018(Merkel et al, , 2020Lerma-Usabiaga et al, 2021). Constraints with log aspect ratio < 3 do not affect our results (data are not shown).…”

“…We would like to explain why we opted to quantify the degree of pRF elongation based on subtractive contrast, unlike the previous fMRI studies which quantified pRF anisotropy based on divisive contrast (e.g., Merkel et al, 2018Merkel et al, , 2020Silva et al, 2018;Lerma-Usabiaga et al, 2021). As stated clearly in Introduction and the previous section of Results, the purpose of the current work is to determine the type of the systematic bias in neural properties that governs the spatial representation of EVC.…”