Enhancement of Normal Cognitive Abilities through Noninvasive Brain Stimulation: A Look at Possible Mechanisms of Action
The last decade has seen a dramatic influx in research exploring the cosmetic utilization of noninvasive brain stimulation (NBS) techniques – such as transcranial magnetic stimulation and transcranial direct current stimulation – to augment neural processes. Termed neuroenhancement, this emerging field has generated over 100 scientific articles and numerous popular press articles demonstrating the capacity of NBS devices to improve various cognitive and physical abilities. Despite this mounting evidence of enhancement efficacy, very little has been suggested in regards to enhancement mechanisms.
The purpose of this article is to construct a conceptual framework for NBS neuroenhancement and suggest four possible mechanisms-of-action. The first and second, Zero-Sum and Paradoxical Facilitation, posit NBS stimulation serves to ‘shift’ and reallocate neural processing in a manner that leads to enhancement of certain neural functions and concurrent detriments of other, functionally related functions. The third, Stochastic Resonance, posits NBS serves to add noise into the neural processing system thereby enhancing signal salience and decreasing sensorial threshold levels. The fourth, Entrainment, posits NBS serves to ‘jump-start’ particular neural patterns linked to specific tasks thereby extending the functional duration of said neural pattern resulting in enhanced behavioral effects.
Throughout this exploration, we present a number of ways in which enhancement results can be interpreted. Furthermore, we suggest a number of ways researchers can potentially determine and elucidate which mechanism/s are at play within their own enhancement studies.
For centuries, human beings have endeavored to transcend the physical and mental limitations of the body. Consider the axe: a device that increases and concentrates the force man can exert upon a given target. Or the bicycle: a device that increases the speed with which man can travel and work. These ubiquitous tools have long allowed us to achieve feats beyond our natural ability and, accordingly, can be understood as enhancement technologies.
Today, the field of enhancement has shifted from external aid to internal intervention. Tools once thought to be the exclusive purview of clinical remediation – genetic engineering, pharmaceutical development, synthetic biocultivation – are being increasingly considered for transcendent purposes. One specific example of this new approach is Neuroenhancement: the betterment of human cognition and behavior via the direct manipulation of neural processes at the chemical and/or cellular level. The emergence of this unique field of study has likely been encouraged by the realization that the human brain optimizes not for rigid activity but for behavioral flexibility (Kapur et al 2011) and the ever increasing social desire (and pressure) to increase human potential (Farah et al 2004; Chatterjee 2004, 2007).
Although varied tools and devices can be used to effect neuroenhancement (for review: Farah et al 2004), in this review we focus on noninvasive brain stimulation (NBS) devices. Recent decades have seen the emergence of several promising NBS devices, including Transcranial Magnetic Stimulation (TMS) and Transcranial Direct Current Stimulation (tDCS). Despite originally being developed to aid in the characterization of human brain physiology, these devices are currently being utilized to enhance physical and cognitive performance by people ranging from at-home engineers to members of the US Armed Forces (Fox 2011). In fact, over 100 published articles demonstrating the capacity of NBS devices to improve various cognitive and physical abilities in healthy participants have appeared in the scientific literature (for review: Pascual-Leone et al 2013).
Although uncertainties regarding effect size and duration still exist, it certainly appears the evidence is mounting in favor of the enhancement capabilities of NBS devices, As such, we feel it is time to move beyond the purely descriptive and begin exploring the mechanistic basis for these effects. The delineation of these mechanisms of action, beyond guiding and improving future healthy enhancement research, will almost certainly reveal certain fundamental aspects of brain function which may, in turn, aid in non-enhancement endeavors (eg: treatment and rehabilitation). That is our goal with this review article: to begin the process of mechanistic delineation and characterization with the aim of exploring what this might reveal about more general brain function and refining future exploratory protocols, both in healthy and non-healthy populations.
NONINVASIVE BRAIN STIMULATION ENHANCEMENT: NETWORK EFFECTS
Although early research assumed that NBS was focal and affected only localized brain regions, it is now accepted that stimulation dynamically modulates activity across distributed neural networks (for review: Shafi et al. 2012). With a focus on these variable network effects (Figures 1 & 2), one may conceive of four broad mechanisms by which NBS might enhance behavioral performance: zero-sum, paradoxical facilitation, stochastic resonance, and entrainment enhancement. As we outline these concepts below, it will be important to keep in mind that these four mechanisms may not be mutually exclusive. This is a matter we will return to later.
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Figure 1: NBS stimulation can serve to transiently modulate the activity of a discrete brain region which, via trans-synaptic effects, can further impact any diffuse neural network with which the target interacts. These distributed effects can vary according to stimulation parameters and functional neural relationships. For instance, if the target node sends excitatory signals to a secondary node, then excitatory stimulation will increase these afferences effectively enhancing activity in both nodes and enhancing network activation (A). Conversely, if the target node sends inhibitory signals to a secondary node, then excitatory stimulation will increase these inhibitory afferences effectively suppressing activity in later node and shifting network activation in a more complex manner (B). If the target node sends excitatory signals to a secondary node, then inhibitory stimulation will decrease these afferences effectively inhibiting activity in both nodes (C). Interestingly, if the target node sends inhibitory signals to a secondary node, then inhibitory stimulation will decrease these afferences effectively enhancing activity in the later node (D). This form of enhanced distal activity via local inhibitory stimulation is known as paradoxical facilitation.
Figure 2: Neuronal network interactions can change and shift across time and circumstance in a state dependent manner (Silvanto & Pascual-Leone 2008). These variations will alter the effects and behavioral consequences of NBS stimulation and are, accordingly, an important variable when considering neural function and enhancement capabilities.
Figure 3: Zero-Sum enhancement posits a finite but shifting amount of neural processing power. Any power gain to a discrete cerebral region will be matched by an equal power loss to one or several cerebral regions.