Oscillatory dynamics coordinating human frontal networks in support of goal maintenance :: Bradley Voytek et al :: www.nature.com/neuro
Summary and review of the above paper
Brain oscillations – Humans can perform short-term actions while remaining aware of longer-term goals. Evidence suggests that prefrontal areas coordinate motor activity when such goals are being aimed at. The more that abstract rules are involved, the more there is a combination of theta (4-8 Hz) and high gamma (80-150 Hz) phase together with inter-regional information encoding in the frontal cortex.
Higher activity in prefrontal
In the case of abstract rules, there is a higher level of activity in the prefrontal cortex than in the primary motor and premotor cortices. There is considered to be cognitive/prefrontal control over these motor areas. The prefrontal cortex is seen as being capable of processing information at different timescales and different levels of abstraction.
Electrocorticographic (ECoG) recording enables the tracking of high gamma phase that correlates with both blood oxygen levels and local neuronal activity. High gamma links single neuron activity to large scale oscillations. Theta and high gamma activity are connected with states in which local neuron spiking is biased according to the local oscillatory phase; this is referred to as spike and phase coupling. In addition to this, there is now evidence that theta oscillation may have a role in coordinating neuronal assemblies involved in higher cognition. Midline frontal theta oscillation is observed in decision-making, but is not observed during the presentation of the stimuli. Theta phase in the hippocampus has been shown to reduce task-irrelevant activity and thus improve behavioural outcomes.
Testing a model
The authors have tested a model in which theta phase coordinates information transmission between frontal regions. This is seen as being linked by high gamma oscillation across different sites, and this allows more than one goal of behaviour to be sustained at the same time. As the task involved in this model continued, progressively more anterior parts of the prefrontal cortex were involved in the neural processing. Prior research had shown that rule abstraction involved more anterior frontal regions. Lower order stimulus-response activity could be dealt with by the more posterior frontal regions, with anterior regions becoming involved in higher cognitive activity. Such activity was also seen as involving more higher gamma and theta phase activity. This finding appears to suggest that with more deliberative processing activity peaks in the prefrontal before it peaks in the motor cortex. This appears to discredit the continuing Libet orthodoxy.
This study is stated to provide evidence for frontal cortex communication during cognitive control. The changes in high gamma amplitude vary between areas in the frontal cortex, and this correlates to the degree of rule abstraction being dealt with. The strength and timing of high gamma and inter-regional theta are indicative of the role of large scale frontal networks in coordinating behaviour.
High gamma peaked in the frontal area later than in the motor cortex for simple behaviours that did not involve the prefrontal, but for more abstract activities involving the prefrontal, higher gamma peaked there first. The theta/gamma phase amplitude coupling is seen as a possible mechanism for allowing multiple behaviours to be handled at the same time. This may involve both large scale oscillation and temporally precise spiking of neurons.