Journal of Neuroscience, Vol 14, 4731-4739, Copyright © 1994 by Society for Neuroscience
The algorithmic complexity of neural spike trains increases during focal seizures
PE Rapp, ID Zimmerman, EP Vining, N Cohen, AM Albano and MA Jimenez-Montano
Department of Physiology, Medical College of Pennsylvania, Philadelphia 19129.
The interspike interval spike trains of spontaneously active cortical
neurons can display nonrandom internal structure. The degree of nonrandom
structure can be quantified and was found to decrease during focal
epileptic seizures. Greater statistical discrimination between the two
physiological conditions (normal vs seizure) was obtained with measurements
of context-free grammar complexity than by measures of the distribution of
the interspike intervals such as the mean interval, its standard deviation,
skewness, or kurtosis. An examination of fixed epoch data sets showed that
two factors contribute to the complexity: the firing rate and the internal
structure of the spike train. However, calculations with randomly shuffled
surrogates of the original data sets showed that the complexity is not
completely determined by the firing rate. The sequence-sensitive structure
of the spike train is a significant contributor. By combining complexity
measurements with statistically related surrogate data sets, it is possible
to classify neurons according to the dynamical structure of their spike
trains. This classification could not have been made on the basis of
conventional distribution-determined measures. Computations with more
sophisticated kinds of surrogate data show that the structure observed
using complexity measures cannot be attributed to linearly correlated noise
or to linearly correlated noise transformed by a static monotonic
nonlinearity. The patterns in spike trains appear to reflect genuine
nonlinear structure. The limitations of these results are also discussed.
The results presented in this article do not, of themselves, establish the
presence of a fine-structure encoding of neural information.