Organization of thalamic and cortical alpha rhythms: Spectra and coherences

Abstract

1. 1. The dog's alpha rhythm recorded from cortex and thalamus has been analysed quantitatively by means of spectral analysis and shown to provide a good model of that of man. 2. 2. The hypothesis that cortical alpha rhythms are paced by thalamic generators has been tested, particularly by studying the relationships between alpha rhythms recorded in the lateral geniculate nucleus and visual cortex, areas where alpha activity predominated. 3. 3. The strength of thalamo-cortical relationships has been measured by means of coherence functions and treated statistically. 4. 4. Significant ( P < 0.05) thalamo-cortical coherences were found. These coherences could be quite specific in a topographic sense. However, cortico-cortical coherences were generally significantly larger than even the largest thalamocortical coherences. The large (> 0.75) corticocortical coherences were found over relatively large areas the diameter of which was often 5 or 6 mm. 5. 5. On the basis of these findings the deterministic pacemaker concept of thalamo-cortical regulation of alpha rhythms was criticized and an alternative stochastic model was proposed. 6. 6. In this model thalamic and cortical alpha rhythms would result from the filter properties of neural networks when submitted to several random inputs. Neural networks with similar design and frequency selectively should exist in different inputs. Neural networks with similar design brain areas where they would give rise to alpha rhythms with the same frequency spectra. The degree of coherence between different areas would depend upon the correlation of the inputs with the different networks. The apparently simultaneous occurrence of bursts of alpha rhythm at different locations would depend on a common modulating influence which would gate the “alpha filters” at the same time.