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).
bioRxiv Subject Collection: Neuroscience