One unique characteristic of humans that distinguishes us from other animals is that each individual of us has a mind of our own. This means that we have the freedom to act according to our own will and, more importantly, break the connections between stimulus and response and suppress any undesirable thoughts and behaviours. The importance of this high-level cognitive control in humans can be reflected in the protracted period of development of the human prefrontal cortex, a critical brain region involved in the implementation of this ability. In contrast to the sensorimotor and association cortices of the brain which matures early in life, the prefrontal cortex is not fully developed until adolescence. The maturation of cognitive control ability that accompanies this extended development of the prefrontal cortex highlights that both structural and functional organisation of the brain and life experiences are at play in their development. This explains why children exhibit greater difficulty in inhibiting a motor response or in resisting interference when compared with adults. Cognitive control also has long-term importance, as individual differences in childhood self-control have been shown to predict adult well-being such as health and wealth three decades later. Impaired ability to exert this top-down, voluntary control is common not only in patients with prefrontal cortex damage or neuropsychiatric conditions such as obsessive-compulsive disorder but also in healthy people who are under the influence of alcohol or drug.
Advances in neurophysiological recording, neuroimaging and non-invasive brain stimulation techniques have made it possible for researchers to investigate the functioning of the human brain without having to open the skull. These techniques enable scientists to study how neural activity changes in response to the requirements of a cognitive task or how behavioural performance is affected when the activity of a particular brain region is temporarily interrupted.
A research team led by Professor Chi-Hung Juan from the Institute of Cognitive Neuroscience of National Central University – a partner university of the University System of Taiwan – have successfully employed these techniques and advanced computational methods to investigate the neural mechanisms underlying the development of cognitive control in preschool children and the neuromodulation of pathways associated with cognitive control in adults.
By decomposing brain waves into magnitude and phase information for each frequency band of electroencephalography (EEG), they found that 6-year-old children show significantly greater beta power especially in the frontal regions when compared to 5-year-olds. This change in beta band activity may explain why they are more successful in stopping their motor responses relative to the younger peers. These results suggest that frontal beta band activity may serve as a potential electrophysiological index of inhibitory control in preschool children.
In adults, they used a temporally precise method called transcranial magnetic stimulation (TMS) to demonstrate that the pre-supplementary motor area (pre-SMA) in the dorsomedial prefrontal cortex is causally linked to successful inhibition of an ongoing motor response. Furthermore, when the neural activity of this area is selectively excited by applying transcranial direct current stimulation (tDCS) over the scalp, efficiency of inhibitory control will be improved.
In order to explain for this cognitive improvement, Professor Juan and his team were the first to employ multiscale entropy (MSE), which is an algorithm that quantifies the ‘‘complexity’’ of physiologic signals commonly used in the study of health and disease, to investigate the neural dynamics underlying cognitive processes. Higher complexity of a biological system can be seen as showing greater adaptability to the environment and greater brain plasticity. They found that improvement in response inhibition after tDCS can be traced back to a person’s complexity within their neural signals, because tDCS can effectively increase the complexity of these signals in the frontal lobe.
The use of non-invasive brain stimulation techniques and advanced computational methods has provided a new avenue for understanding the causal relationship between brain and behaviour and how the brain controls behaviour. Professor Juan believes that these research results may offer a theoretical basis for clinical intervention via brain stimulation such as TMS and tDCS for individuals exhibiting difficulties in cognitive control.