I am trying to understand the process of creating Visual Field Maps (VFM, also knows and Retinotopy) using fMRI with respect to the two orthogonal dimensions in visual space: eccentricity and polar angle. I am reading a chapter of an open access book called Visual Field Map Organization in Human Visual Cortex.
As per my understanding, eccentricity represents how circular a curve is. Higher the circularity, lower would be the eccentricity. And, polar angle represents the angle of a point w.r.t the x-axis in case of Polar coordinate system.
Question: Taking the above facts into consideration, why has it been said in the book that the Eccentricity and Polar Angles are two orthogonal dimensions?