Extended Data 2. Detection of spontaneous traveling waves.

(a) The method for detecting spontaneous waves from the Generalized Phase. First, the detection algorithm finds the most likely starting point for a putative wave as the point that maximizes the divergence of the phase gradient (step 1). (b) With this source point found, the algorithm then quantifies the spatiotemporal organization about this point from the circular-linear correlation of phase with distance across the whole array (step 2). With this approach, the algorithm can robustly detect arbitrarily shaped wavefronts in the array data. (c) The average power spectrum for waves (N = 215) has significantly less power in low frequencies (<12Hz) as compared to non-wave fluctuations (N = 524). Dotted bounds represent SEM. Asterisk: p < 1 × 10⁻⁵, two-tailed Wilcoxon rank-sum test. (d) Detected waves in both monkeys predominantly travel at speeds consistent with the conduction velocity of unmyelinated horizontal axons (0.1–0.6 m/s, red dashed lines; monkey W, 5571 waves, blue line; monkey T, 9285 waves, red line). (e) There is no difference in the amplitude of fluctuations that are detected as waves (blue line; N = 696 waves) or rejected (non-wave, gray line; N = 565 non-wave fluctuations; example monkey T session).