For a given spot at row i and column j, its normalized signal is compared to surrounding spots in a nine by nine window W ij (4) . Signals within this window are fit to a normal distribution. The mean μ i,j and standard deviation σ i,j will be calculated and the default threshold is set at two standard deviations above the signal mean. A spot (i, j) will be called positive only if its signal is above the threshold: