Image cross-correlation analysis reveals the emergence of a dynamic steady state actin distribution in the minimal cortex. Image cross-correlation analysis reveals the emergence of a dynamic steady state actin distribution in the minimal cortex. (A) Correlation coefficient calculated by cross-correlating the actin fluorescence signal from each frame with the first frame of the time series, i.e., with the actin distribution before myosin was added (increasing time window, fixed reference frame). A correlation coefficient close to 1 indicates a high similarity of the frame with the initial image; comparing two random images would give a correlation coefficient of 0. Three samples each show coarsening (purple) or homogeneous contraction (gray). The region between 15 and 60 min is shaded gray to denote the period in which the correlation coefficient is stable at approximately 0.8 (i.e., the steady state). (B) Correlation coefficient along the time series for the correlation of each frame with the frame 25 s ahead (constant time window, moving reference frame), calculated for a coarsening sample. This correlation coefficient indicates the self-similarity of the actin distribution with a time window of 25 s. The inset shows a plot of the mean of the correlation coefficient over 10 min during the steady state for different time windows. The period between 40 and 50 min was selected as a representative time period within the steady state regime. Three time points (a, b, c) were selected for further analysis (shown in C). Movie 7 shows the actin dynamics observed during the steady state. (C) Panels show the actin distribution (upper row) before the addition of myosin (a), at the peak of coarsening (b) and during the dynamic steady state (c). All other rows show analysis of actin flows at the same time points. The second row from the top shows a vector map of the spatiotemporal gradient of intensity calculated for a time window of 25 s (five frames). The arrows are color-coded for the absolute value of the spatiotemporal gradient of intensity. The third row shows an overlay of the spatiotemporal gradient map on the actin intensity image. The bottom row visualizes the pixel-wise temporal gradient of the intensity, which is simply given by the difference of the image at the selected time point and the image 25 s ahead. Positive values indicate an increase in intensity and negative a decrease. Data is representative of the analysis from three coarsening samples. Sonal et al. J Cell Sci 2019;132:jcs219899 © 2018. Published by The Company of Biologists Ltd