Comparing population receptive fields in human and macaque visual cortex

Abstract

The population receptive field (pRF) modelling approach has been applied widely in human fMRI to inform the organization of both early and late stages of the cortical visual hierarchy. Recently, a pRF implementation was developed that allows for oriented and elliptical pRFs and demonstrated that dorsal and ventral divisions of early visual cortex (EVC) differentially sample visual space. Thus, differences between the larger dorsal and ventral visual processing streams may be set early on in the visual hierarchy. However, despite its common use, we do not yet understand how pRFs relate to the receptive fields of single neurons. Therefore, cross-species validation by means of comparative fMRI would go a long way to bridging the gap between single-unit recordings in monkeys and fMRI in humans. Here, we compare pRFs from dorsal and ventral divisions of EVC in both humans (n=12) and macaques (Macaca mulatta; n=2). Participants (both humans and macaques) fixated centrally while a bar aperture traversed gradually across the visual field revealing brief scene fragments. Data from both study populations were analyzed in AFNI using both standard (circular) and elliptical pRF models. Cross-validation analyses revealed that, similar to the human, the elliptical pRF model captured more variance in the time-course of macaque data than a circular pRF implementation. Further, pRF eccentricity was positively correlated with both pRF aspect ratio and pRF area, with the strength of correlation increasing generally as a function of position in the visual hierarchy. Also consistent with our human data, the average pRF angles of dorsal and ventral divisions of EVC are oriented differentially. Finally, we found that on average pRF size estimates in macaques tended to be larger than in humans, despite the overall similarity. Overall, these results reveal a very similar pattern of pRF properties between macaque and human visual cortex