Systematic rules underlying spectral pattern variability: experimental results and a review of the evidence.

On the basis of three different experiments: oddball task (visual, auditory, and audio-visual stimuli), modified Sternberg's, and multistage memory tasks, it was shown that: a) there was not a single typical spectral pattern type that would characterize the majority of the trials; b) the total number of the different spectral pattern types was limited; c) different spectral pattern types had different importance to the brain--their occurrence was more or less probable; d) the total number and the number of the most probable spectral pattern types were dependent on the functional brain state; e) actual spectral pattern of variability during rest with closed eyes was relatively high (around 65% from the maximum possible rate), but significantly less than stochastic spectral pattern variability. It is suggested that identical sensory events can potentially trigger a limited number of several different alternative reaction patterns in EEG/MEG, depending on the situational context.