We have found that the depression time series cannot
be distinguished from their surrogates generated from a
linear process when comparing the respective test statistics.
These results can mean that either (1) the original
series have linear dynamics, (2) the test statistics for distinguishing
linear from nonlinear behaviour do not have
the power to detect the kind of nonlinearity present or (3)
the process is nonlinear but the sampling is inadequate
or the time series are too short to represent the dynamics.
It is uncertain which hypothesis is to be preferred, but
it would be worthwhile repeating the analysis on longer,
more frequently sampled data. We note that the sample
of eight patients is small, which limits how far any general
inferences about dynamics of mood in bipolar disorder.
On the current evidence, though, there is no reason to
claim for any nonlinearity in mood time series that we
examined.