Changes in population receptive fields following artificial scotoma; remapping or nonlinear responses?

There is considerable debate regarding the extent of plasticity in the adult visual cortex, following retinal lesions. Early electrophysiological studies in animal models provided evidence that receptive fields (RF) inside the lesion projection zone adaptively change position preference towards spared portions of the visual field. However, subsequent multimodal studies did not find evidence of RF position change. Changes in population RFs (pRF) have also been observed in healthy controls following simulated scotoma. Importantly, changes in pRF locations are not limited to the simulated lesion projection zone, as pRFs around the visual field display change their apparent position. This suggests that changes in pRFs around scotoma, either simulated or real, are not necessarily due to plasticity. Previous studies generally used a linear, single gaussian pRF model. However, the visual cortex responds non-linearly to stimuli, hence it is possible that apparent position changes following scotoma are driven by non-linear responses. To test this hypothesis, five participants were shown a standard retinotopic mapping stimulus, with and without simulated scotoma (an unstimulated mean-luminance patch on the screen). We modelled the pRF properties using both a linear single gaussian and a pRF model based on divisive normalization (DN). The DN pRF model captures non-linear responses, such as surround suppression, compression and oversaturation. In line with previous studies, we find changes in preferred position using the linear single gaussian pRF model. When fitting with the DN pRF model, the results also displayed position changes, however, these were significantly smaller. Our results suggest that at least a portion of pRF preferred position changes can be captured by non-linear responses. Thus, non-linear responses may be misinterpreted as signs of plasticity, and we propose that the studies of cortical plasticity and stability must consider non-linear responses of visual cortex which are part of normal cortical dynamics.