(A) The 1D tuning curve . (B) The 1D tuning curve Fourier transform . The black circles indicate k-lattice points. The PCA solution, , is given by the circles closest to , the peak of (red cross). (C) A contour plot of the 2D tuning curve Fourier transform . In 2D k-space the peak of becomes a circle (red), and the k-lattice is a square lattice (black circles). The lattice point can be partitioned into equivalent groups. Several such groups are marked in blue on the lattice. For example, the PCA solution Fourier components lie on the four lattice points closest to the circle, denoted A1-4. Note the grouping of A,B,C & D (4,8,4 and 4, respectively) corresponds to the grouping of the 20 highest principal components in Figure 4. Parameters: .