Depressed People Move in a Mathematically Different Way

In this week's Physical Review Letters, Yoshiharu Yamamoto of the University of Tokyo and his colleagues explain how the movements of people suffering from clinical depression can be described by a power law—and how this law is so different from that of healthy people that it looks truly diagnostic.

Like many mathematical functions, a power law plots two variables against each other to form a characteristic curve on a graph. Dr Yamamoto collected the data for his own particular power-law curves by fitting his experimental subjects—about half of whom were healthy, and half of whom had been diagnosed as having clinical (or “major”) depression—with accelerometers. These devices measure how often someone changes his rate of movement by recording each time his acceleration exceeds a certain threshold.

The basic results confirmed a known feature of depressed people. The normal daily rhythm that would lead to a high, steady number of counts during daylight hours and low counts during the night was replaced by occasional bursts of activity. The surprise came when the team started plotting their results out on graphs.

The curves produced by plotting the lengths of low-activity periods against their frequency were strikingly different in healthy and depressed people. This reflects not inactivity by the depressed (though they were, indeed, less active) but a difference in the way that the healthy and the depressed spread their resting periods over the day. Depressed people experience longer resting periods more frequently and shorter ones less frequently than healthy people do.

Interesting. Raises the possibility of creating a shirt or a watch with a built in accelerometer that could diagnose depression right as it begins.

via The Economist