The algorithmic complexity of neural spike trains increases during focal seizures.

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.

Saltatory transitions are a naturally occurring property of evolving systems.

On the basis of paleological evidence, it has been suggested that biological evolution need not necessarily be characterized by gradual change. Rather, evolutionary history may display saltatory periods of rapid speciation alternating with periods of relative quiescence, the whole dynamic being called punctuated equilibria. The empirical evidence that has been presented in support of this hypothesis has been the object of a vigorous dispute. Mathematical investigations of complex models of biological evolution that contain random elements have demonstrated that these systems can display saltatory behavior. In this paper we address a more abstract question: can saltations occur in the evolution of very simple, deterministic mathematical systems that function in a constant environment? The answer appears to be yes. Saltations appear as a natural dynamical behavior in the evolution of simplistic information processing networks. We stress that these networks do not constitute a model of biological evolution. However, the appearance of saltations in such simple systems suggests that their appearance in a process as complex as biological evolution is not surprising.

Dynamics of brain electrical activity.

In addition to providing important theoretical insights into chaotic deterministic systems, dynamical systems theory has provided techniques for analyzing experimental data. These methods have been applied to a variety of physical and chemical systems. More recently, biological applications have become important. In this paper, we report applications of one of these techniques, estimation of a signal's correlation dimension, to the characterization of human electroencephalographic (EEG) signals and event-related brain potentials (ERPs). These calculations demonstrate that the magnitude of the technical difficulties encountered when attempting to estimate dimensions from noisy biological signals are substantial. However, these results also suggest that this procedure can provide a partial characterization of changes in cerebral electrical activity associated with changes in cognitive behavior that complements classical analytic procedures.

An effect of age on the phosphodiesterase activator protein of rat cerebral cortex. A brief note.

Measurements were made of the effectiveness of phosphodiesterase activator protein isolated from the cerebral cortex of rats of various ages in stimulating the phosphodiesterase-catalyzed conversion of cyclic-AMP into 5'-AMP. An age dependent increase in the potency of preparations of activator protein was observed, with the activator isolated from animals six months of age and older being generally more efficacious than that from younger animals.

Effects of electrical stimulation and norepinephrine on cyclic-AMP levels in the cerebral cortex of the aging rat.

One to 24 month old rats were anesthetized with alpha-chloralose, their cerebral cortices exposed surgically and subjected in vivo to stimulation by norepinephrine (1.0mM for 5 minutes) or electrical pulses (100 pulses/s for 10 s). After microwave fixation, cortical samples were analyzed for cyclic-AMP by the Gilman method. In similarly prepared but unstimulated control animals, cyclic-AMP levels were found to be three to four-fold lower in older than in younger rats, a pattern previously reported for unanesthetized rats. Both norepinephrine and electrical pulses led to cyclic AMP increases. With increasing age the cyclic-AMP levels attained under both stimulus conditions declined, with the levels achieved in the two cases becoming increasingly divergent.

A possible effect of testosterone on the adenosine 3'-5' cyclic monophosphate levels in rat cerebral cortex. A brief note.

Based on a correlation of serum testosterone levels and cortical cyclic-AMP concentrations in a normal aging population of male CD Fischer rats and a demonstration of decreased cortical cyclic-AMP subsequent to injection of testosterone, it was concluded that a cause and effect relation between hormone concentrations and brain cyclic nucleotide levels is tenable.

Phosphodiesterase and adenyl-cyclase activities in the cerebral cotex of the aging rat.

In an effort to determine the factors responsible for the four-fold decrease in cyclic-AMP content of the rat cerebral cortex, observed to occur between the ages of 3 and 6 months, studies were performed on adenyl-cyclase and phosphodiesterase, the cyclic-AMP synthesizing and hydrolyzing enzymes. The activities and kinetic characteristics were determined for both enzymes as obtained from the cerebral cortex of rats ranging in age from one to 24 months. No age dependence was observed either in adenyl-cyclase activity, assayed with or without fluoride ion, or in phosphodiesterase activity. It was concluded that age related changes in factors other than the direc levels of these enzymes underlie the age related decline in cortical cyclic-AMP levels.