Model-based Estimation Method of the Cortical Retinotopic Map Using Retinal Polar Coordinate System

The primary visual cortex has a topographic projection from the retina, called the cortical retinotopic map. Since retinotopic maps help evaluate visual responses and estimate retinal defects, they may play an essential role in research on visual reconstruction using animals. So far, several methods have been proposed to estimate the retinotopic map. Since the methods use the synchronous averaging method, it should be assumed that the eye position does not change during measurement. To satisfy this condition, fixation of the eyeball and anesthesia are necessary; however, these are stressful to the animal. Hence, measurements in unanesthetized and rest states are desirable since they are less stressful. However, eye movement occurs in these states. Therefore, we have developed a model-based retinotopic map estimation method without using the synchronous averaging process. This method is robust against changes in eye position during measurement. In the method, we approximated the retina and the shape of the receptive field of the cortex to be plane and Gauss functions, respectively. However, there has been a concern that the error increases in the peripheral part of the retina. This study attempted to overcome this problem by modeling the retina as a sphere and replacing the receptive field function with the Von Mises-Fisher function on the sphere. We compared the horizontal retinotopic maps of unanesthetized and rest mice estimated by Kalatskyʼs method and the proposed method. The single-eye visual field of the azimuth retinotopic map estimated by the proposed method was approximately ± 90°, which was almost the same as that observed in mice. On the other hand, the visual field estimated by Kalatskyʼs method was approximately ± 40°, which was narrower than these. Next, we compared the maps by the proposed method using planar and spherical retinal models. We found that a field of view of about ± 60° was estimated by the planar model, narrower than the spherical model. These results suggest that the proposed method with the spherical retinal model is excellent for estimating the retinotopic maps of mice with eye movement.