On the visualisation of nonstationarities in point processes

A method is described for graphically presenting interval data, such as neural interspike intervals or electrocardiographic R-R intervals, in a form that facilitates the identification of nonstationaries. The method is essentially a plot of isoprobability contours of the cumulative interval histogram, as functions of time. A sequential algorithm is used for updating the contour-line positions. This display is used in an interactive system for visually identifying nonstationarities, and for subsequently comparing selected segments of the data quantitatively using the Kolmogorov-Smirnov test.