Nick J. Davis

The salience network is responsible for switching between the default mode network and the central executive network: Replication from DCM

With the advent of new analysis methods in neuroimaging that involve independent component analysis (ICA) and dynamic causal modelling (DCM), investigations have focused on measuring both the activity and connectivity of specific brain networks. In this study we combined DCM with spatial ICA to investigate network switching in the brain. Using time courses determined by ICA in our dynamic causal models, we focused on the dynamics of switching between the default mode network (DMN), the network which is active when the brain is not engaging in a specific task, and the central executive network (CEN), which is active when the brain is engaging in a task requiring attention. Previous work using Granger causality methods has shown that regions of the brain which respond to the degree of subjective salience of a stimulus, the salience network, are responsible for switching between the DMN and the CEN (Sridharan et al., 2008). In this work we apply DCM to ICA time courses representing these networks in resting state data. In order to test the repeatability of our work we applied this to two independent datasets. This work confirms that the salience network drives the switching between default mode and central executive networks and that our novel technique is repeatable.

Cerebellar Transcranial Direct Current Stimulation (ctDCS) A Novel Approach to Understanding Cerebellar Function in Health and Disease

The cerebellum is critical for both motor and cognitive control. Dysfunction of the cerebellum is a component of multiple neurological disorders. In recent years, interventions have been developed that aim to excite or inhibit the activity and function of the human cerebellum. Transcranial direct current stimulation of the cerebellum (ctDCS) promises to be a powerful tool for the modulation of cerebellar excitability. This technique has gained popularity in recent years as it can be used to investigate human cerebellar function, is easily delivered, is well tolerated, and has not shown serious adverse effects. Importantly, the ability of ctDCS to modify behavior makes it an interesting approach with a potential therapeutic role for neurological patients. Through both electrical and non-electrical effects (vascular, metabolic) ctDCS is thought to modify the activity of the cerebellum and alter the output from cerebellar nuclei. Physiological studies have shown a polarity-specific effect on the modulation of cerebellar–motor cortex connectivity, likely via cerebellar–thalamocortical pathways. Modeling studies that have assessed commonly used electrode montages have shown that the ctDCS-generated electric field reaches the human cerebellum with little diffusion to neighboring structures. The posterior and inferior parts of the cerebellum (i.e., lobules VI-VIII) seem particularly susceptible to modulation by ctDCS. Numerous studies have shown to date that ctDCS can modulate motor learning, and affect cognitive and emotional processes. Importantly, this intervention has a good safety profile; similar to when applied over cerebral areas. Thus, investigations have begun exploring ctDCS as a viable intervention for patients with neurological conditions.

Transcranial stimulation of the developing brain: A plea for extreme caution

Transcranial stimulation can be used to modulate the activity of the brain. Recent developments in our understanding of technologies such as transcranial magnetic or electrical stimulation have afforded reasonable grounds for optimism that techniques such as TMS or tDCS might be effective treatments for neurally-mediated disorders. Researchers have demonstrated encouraging benefits of TMS and tDCS in treating conditions such as tinnitus (Burger et al., 2011), depression (Arul-Anandam and Loo, 2009), and stroke (Nowak et al., 2010). Collectively these techniques are often referred to as “non-invasive brain stimulation” (NIBS), although I would argue that this term is not appropriate since in all cases energy is being transferred across the skull (Davis and van Koningsbruggen, 2013), and the use of this term may be misleading to the general public who are not aware of the documented risks associated with these procedures. More recently it has been suggested that brain stimulation be used to treat neurological disorders in pediatric cases. A recent review by Vicario and Nitsche (2013a) identified a number of opportunities and challenges for the use of brain stimulation in children. Here I offer a plea for calm and for caution. The ethical stakes in clinical and research procedures with children are high enough that a conservative approach is warranted. Many of the ethical issues, relevant both to adult and child participants, have been touched on by other authors (e.g., Cohen Kadosh et al., 2012; Krause and Cohen Kadosh, 2013); however this paper will focus on the gaps in our knowledge that affect our ability to assess risk in translating brain stimulation procedures to pediatric cases. There are a number of known risks associated with brain stimulation. Mild side-effects may include scalp tenderness, headache or dizziness, which are typically associated with the mechanism of delivery or with being immobilized in a chair or frame, and which may be under-reported (Brunoni et al., 2011). More serious effects may include seizure, mood changes or induction of hyper- or hypo-mania. However, the risk of seizure is low, at around 0.1% of adult cases and around 0.2% of pediatric reports, although these figures may not reflect unreported off-label use of the techniques (Rossi et al., 2009). These more serious symptoms are largely associated with people who already possess a degree of susceptibility, such as people with a history of epilepsy (Davis et al., 2013). Adult brain stimulation is thought be reasonably safe when used within defined limits (see below), however here I wish to focus on a number of factors that complicate the translation of TMS and tDCS protocols to pediatric cases. I will focus on the key unknowns in brain stimulation research: 1. The unknown effects of stimulation; 2. The unknown side-effects of stimulation; 3. The lack of clear dosing guidelines; 4. The lack of translational studies from adults to children. I will set out these “known unknowns” in translating our knowledge about TMS and tDCS effects to clinical pediatric applications, and touch on the practical and ethical barriers to their widespread usage.

Challenges of proper placebo control for non-invasive brain stimulation in clinical and experimental applications

A range of techniques are now available for modulating the activity of the brain in healthy people and people with neurological conditions. These techniques, including transcranial magnetic stimulation (TMS) and transcranial current stimulation (tCS, which includes direct and alternating current), create magnetic or electrical fields that cross the intact skull and affect neural processing in brain areas near to the scalp location where the stimulation is delivered. TMS and tCS have proved to be valuable tools in behavioural neuroscience laboratories, where causal involvement of specific brain areas in specific tasks can be shown. In clinical neuroscience, the techniques offer the promise of correcting abnormal activity, such as when a stroke leaves a brain area underactive. As the use of brain stimulation becomes more commonplace in laboratories and clinics, we discuss the safety and ethical issues inherent in using the techniques with human participants, and we suggest how to balance scientific integrity with the safety of the participant.

“Non-invasive” brain stimulation is not non-invasive

The functions of the healthy brain can be studied in two main ways. Firstly, the changes in the brains state can be measured using techniques such as EEG or functional MRI. Secondly, the activity of the brain can be disrupted through the use of brain stimulation. The famous experiments of Wilder Penfield and colleagues in the 1950s showed the power of brain stimulation in people whose brain was exposed in surgery, and highlighted the possibility of inducing changes in the brains state to demonstrate the involvement of specific brain areas in particular functions (Jasper and Penfield, 1954). Two main techniques are available for human brain stimulation: transcranial magnetic stimulation (TMS) and transcranial current stimulation (tCS). More recently, it has been suggested that TMS and tCS might be used to enhance brain function, as well as to disrupt activity. These techniques have collectively become known as “non-invasive brain stimulation.” We argue that this term is inappropriate and perhaps oxymoronic, as it obscures both the possibility of side-effects from the stimulation, and the longer-term effects (both adverse and desirable) that may result from brain stimulation. We also argue that the established tendency for the effects of TMS and tCS to spread from the target brain area to neighboring areas is in itself contrary to the definition of non-invasiveness. We argue that the traditional definition of an invasive procedure, one which requires an incision or insertion in the body, should be re-examined, and we propose that it be widened to include targeted transcutaneous interventions.

The Role of Beta-Frequency Neural Oscillations in Motor Control

Human sensorimotor and cognitive behavior is associated with changes in the oscillatory activity of the brain. For example, the integration of diverse aspects of a stimulus into a unitary percept is related to synchronized oscillations in the gamma range (30–100 Hz), while power in the alpha band

Brain stimulation studies of non-motor cerebellar function: A systematic review

Evidence for a cerebellar role in non-motor functions has been demonstrated by clinical and neuroimaging research. These approaches do not allow causal relationships to be inferred though the experimental manipulation of the cerebellum. Transcranial magnetic and current stimulation may allow better understanding of the cerebellum via the temporary alteration of its operation in healthy volunteers. This review examined all studies of the cerebellar role in non-motor functions using non-invasive brain stimulation. Of 7585 papers captured by an initial search, 26 met specific selection criteria. Analysis revealed behavioural effects across learning, memory, cognition, emotional processing, perception and timing, though the results were not sufficiently similar as to offer a definitive statement of the cerebellums role. The non-invasive application of stimulation to the cerebellum presents challenges due to surrounding anatomy and the relatively small target areas involved. This review analysed the methods used to address these challenges with a view to suggesting methodological improvements for the establishment of standards for the location of cerebellar stimulation targets and appropriate levels of stimulation.

Neurodoping: Brain Stimulation as a Performance-Enhancing Measure

Doping may be defined, broadly, as the use of unauthorised means to increase performance in sport. Doping is most commonly associated with the use of drugs. In this paper, I discuss the use of emerging techniques for the modulation of brain activity in healthy people using electric or magnetic fields, and suggest how they might be used to enhance physical and mental performance in sports. I will suggest that neurodoping may have different uses in different sports, and I argue that each sport must determine whether neurodoping should be considered as cheating, or should be considered a legitimate aid to training or performance.

Autonomous Sensory Meridian Response (ASMR): a flow-like mental state

Autonomous Sensory Meridian Response (ASMR) is a previously unstudied sensory phenomenon, in which individuals experience a tingling, static-like sensation across the scalp, back of the neck and at times further areas in response to specific triggering audio and visual stimuli. This sensation is widely reported to be accompanied by feelings of relaxation and well-being. The current study identifies several common triggers used to achieve ASMR, including whispering, personal attention, crisp sounds and slow movements. Data obtained also illustrates temporary improvements in symptoms of depression and chronic pain in those who engage in ASMR. A high prevalence of synaesthesia (5.9%) within the sample suggests a possible link between ASMR and synaesthesia, similar to that of misophonia. Links between number of effective triggers and heightened flow state suggest that flow may be necessary to achieve sensations associated with ASMR.

Memory and coordination in bimanual isometric finger force production

Isometric force output of the dominant hand has previously been shown to decline when feedback of that output is withdrawn. This effect is more pronounced for higher levels of force output, and appears to rely upon visuomotor memory processes. In the present study these existing findings are extended to a task where subjects produced force output with both the dominant and non-dominant hand, and with both hands together. The results suggested that force change following the withdrawal of feedback follows the same pattern in bimanual conditions as it does in unimanual conditions. In addition it was found that the proportion of the total force contributed by each hand in the bimanual condition varied through a trial, which was achieved without a corresponding drop in force output when feedback was available. Taken together, the results support the idea of a central representation for target force level, which, when available, makes use of visual information to control a mutually redundant pair of effectors.