Fractal dimension has been proposed as a useful measure for the characterization of electrophysiological time series. This paper investigates what the pointwise dimension of electroencephalographic (EEG) time series can reveal about underlying neuronal generators. The following theoretical assumptions concerning brain function were made (i) within the cortex, strongly coupled neural assemblies exist which oscillate at certain frequencies when they are active, (ii) several such assemblies can oscillate at a time, and (iii) activity flow between assemblies is minimal. If these assumptions are made, cortical activity can be considered as the weighted sum of a finite number of oscillations (plus noise). It is shown that the correlation dimension of finite time series generated by multiple oscillators increases monotonically with the number of oscillators. Furthermore, it is shown that a reliable estimate of the pointwise dimension of the raw EEG signal can be calculated from a time series as short as a few seconds. These results indicate that (i) The pointwise dimension of the EEG allows conclusions regarding the number of independently oscillating networks in the cortex, and (ii) a reliable estimate of the pointwise dimension of the EEG is possible on the basis of short raw signals.
Neurobiological mechanisms underlying language functions can be modeled in the framework of Hebb's cell assembly theory. According to this approach, meaningful words, but not meaningless pseudowords, have a cortical representation in strongly coupled ensembles of neurons distributed over wide cortical areas. Cell assemblies with different cortical topographies can be assumed for different word types. These hypotheses give rise to empirical predictions that can be tested in behavioral and electrophysiological experiments with healthy and neurologically impaired subjects. Results of a series of experiments providing support for the above assumptions are summarized.
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