Foveal pRF properties in the visual cortex depend on the extent of stimulated visual field

Previous studies demonstrated that alterations in functional MRI derived receptive field (pRF) properties in cortical projection zones of retinal lesions can erroneously be mistaken for cortical large-scale reorganization in response to visual system pathologies. We tested, whether such confounds are also evident in the normal cortical projection zone of the fovea for simulated peripheral visual field defects. We applied fMRI-based visual field mapping of the central visual field at 3 T in eight controls to compare the pRF properties of the central visual field of a reference condition (stimulus radius: 14 degrees) and two conditions with simulated peripheral visual field defect, i.e., with a peripheral gray mask, stimulating only the central 7 degrees or 4 degrees radius. We quantified, for the cortical representation of the actually stimulated visual field, the changes in the position and size of the pRFs associated with reduced peripheral stimulation using conventional and advanced pRF modeling. We found foveal pRF-positions (<= 3 degrees) to be significantly shifted towards the periphery (p < 0.05, corrected). These pRF-shifts were largest for the 4 degrees condition [visual area (mean eccentricity shift): V1 (0.9 degrees), V2 (0.9 degrees), V3 (1.0 degrees)], but also evident for the 7 degrees condition [V1 (0.5 degrees), V2 (0.5 degrees), V3 (0.9 degrees)]. Further, an overall enlargement of pRF-sizes was observed. These findings indicate the dependence of foveal pRF parameters on the spatial extent of the stimulated visual field and are likely associated with methodological biases and/or physiological mechanisms. Consequently, our results imply that, previously reported similar findings in patients with actual peripheral scotomas need to be interpreted with caution and indicate the need for adequate control conditions in investigations of visual cortex reorganization.