Population receptive fields of human primary visual cortex organised as DC-balanced bandpass filters

The response to visual stimulation of population receptive fields (pRF) in the human visual cortex can be accurately modelled with a Difference of Gaussians model, yet many aspects of their organisation remain poorly understood. Here, we examined the theoretical underpinnings of this model and argue that the DC-balanced Difference of Gaussians (DoG) holds a number of advantages over a DC-biased DoG. Through functional magnetic resonance imaging (fMRI) pRF mapping, we compared performance of DC-balanced and DC-biased models in human primary visual cortex and found that when model complexity is taken into account, the DC-balanced model is preferred. Finally, we present evidence indicating that the BOLD signal DC-offset contains information related to the processing of visual stimuli. Taken together, the results indicate that V1 neurons are at least frequently organised in the exact constellation that allows them to function as bandpass-filters, which allows for the separation of stimulus contrast and luminance. We further speculate that if the DoG models stimulus contrast, the DC-offset may reflect stimulus luminance. These findings suggest that it may be possible to separate contrast and luminance processing in fMRI experiments and this could lead to new insights on the haemodynamic response.