Beta Gamma Activity of the Neocortex

The identification and characterization of high-frequency rhythms in the neocortex has been concentrated mainly in two neocortical areas—the visual cortex and the somatomotor cortex. Here, some of the properties of these beta/gamma rhythmic activities for these two areas are discussed.

Commonly, the EEG or LFP of the visual cortex is associated with the alpha rhythm, with typical reactivity with closing and opening of the eyes as described previously. However, other types ofrhythmic activities can be present in the same cortical areas, namely within the beta frequency range. In the dog, it was shown that the EEG spectral density was characterized by peaks within the beta/gamma frequency range while the animal was looking attentively at a visual stimulus. These findings in the awake animal are in line with the demonstration in the bulbospinal transected preparation that brain stem electrical stimulation causes not only desynchronization of alpha spindles but also the appearance of fast rhythms in the cortical EEG. Recently, Walter Freeman and Bob van Dijk found in the visual cortex of a rhesus monkey that fast EEG rhythms (spectral peak of 30 7 3.7 Hz) occurred during a conditioned task to a visual stimulus. A possibly related finding is the discovery by the group of Charles Gray and Wolf Singer and by Eckhorn and collaborators of oscillations within the beta/gamma frequency range (most commonly between 30 and 60 Hz) in the firing of individual neurons of the visual cortex in response to moving light bars. Using auto- and cross-correlation analyses, it was demonstrated that neurons tended to fire in synchrony, in an oscillatory mode, within cortical patches that could extend up to distances of about 7 mm. The oscillations in neuronal firing rate were correlated with those of the LFPs. The cortical oscillations are modulated by the activation of the mesencephalic reticular formation (MRF), but the stimulation of the MRF alone does not change the pattern of firing of the cortical neurons. However, MRF stimulation increases the amplitude and coherence of both the LFP and the multiunit responses when applied jointly with a visual stimulus.

In the somatomotor cortex, beta/gamma oscillations of both neuronal firing and LFPs were described in the awake cat by the group of Rougeul-Buser, particularly when the animal was in a state of enhanced vigilance while watching an unreachable mouse. Also, in monkey during a state of enhanced attention, fast oscillations were found in the somatomotor cortex. Oscillations of 25-35 Hz occurred in the sensorimotor cortex of awake behaving monkeys both in LFPs and in single- /multi-unit recordings. They were particularly apparent during the performance of motor tasks that required fine finger movements and focused attention. These oscillations were coherent over cortical patches extending at least up to 14 mm that included the cortical representation of the arm. Synchronous oscillations were also found straddling the central sulcus, so they may reflect the integration of sensory and motor processes. The LFP reversed polarity at about 800 mm under the cortical surface, indicating that the source of the LFP is in the superficial cortical layers. It is noteworthy that at least some of the cortical beta/gamma rhythmic activities appear to depend on projecting dopaminergic fibers arising in the ventral tegmental area, but it is not clear to what extent the beta rhythms of the somatomotor cortex are related to thalamic or other subcortical activities.

It is relevant to correlate the characteristics of EEG beta/gamma activities found in experimental animals with those recorded from the scalp in man. Beta/ gamma activity was reported by DeFrance and Sheer to occur over the parieto-temporo-occipital cortex in man, particularly in relation to the performance of motor tasks.

With respect to the origin of beta/gamma rhythmic activity, several experimental facts have led to the interpretation that these rhythmic activities are primarily generated in the cortex: (i) the fact that oscillations in the beta/gamma frequency range were easily recorded from different cortical sites but not from simultaneously obtained recordings from thala-mic electrodes; (ii) the observation that in the visual cortex there are neurons that show oscillatory firing rates with a phase difference of about one-fourth of a cycle, indicating that a local recurrent feedback circuit can be responsible for the oscillations; and (iii) the finding of intrinsic oscillations in cortical neurons from layer IV of the frontal cortex of guinea pig in vitro. Nevertheless, it is possible that thalamic neuronal networks also contribute to the cortical beta/gamma rhythmic activity since about 40-Hz oscillatory behavior has been observed by Mircea Steriade and collaborators in neurons of the intralaminar centro-lateral nucleus that projects widely to the cerebral cortex. The question cannot be stated as a simple alternative between a cortical versus a thalamic rhythmic process, both considered as exclusive mechanisms. As discussed in relation to other rhythmic activities of the mammalian brain, both network and membrane intrinsic properties cooperate in shaping the behavior of the population, including its rhythmic properties and its capability of synchronizing the neuronal elements.

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