Is neuroenhancement by noninvasive brain stimulation a net zero-sum proposition?

Abstract

In the past several years, the number of studies investigating enhancement of cognitive functions through noninvasive brain stimulation (NBS) has increased considerably. NBS techniques, such as transcranial magnetic stimulation and transcranial current stimulation, seem capable of enhancing cognitive functions in patients and in healthy humans, particularly when combined with other interventions, including pharmacologic, behavioral and cognitive therapies. The "net zero-sum model", based on the assumption that brain resources are subjected to the physical principle of conservation of energy, is one of the theoretical frameworks proposed to account for such enhancement of function and its potential cost. We argue that to guide future neuroenhancement studies, the net-zero sum concept is helpful, but only if its limits are tightly defined.

Fig. 1

Factors that may contribute to enhancement and cost effects.

Fig. 2

A) Processing power is distributed through a central processor across functionally relevant networks (top-down modulation). B) Dynamic interactions between network A and B might not be fully dependent on top-down control. Neural elements and networks can be implied in more than one higher-level network and could thus serve as trade-off switches. Note also that in the figure, networks are meant to possibly represent ensembles of neurons within or across columns, cortico-subcortical networks, or even large-scale bi-hemispheric networks.

Fig. 3

Example of decision-making involving a low (A) or high (B) level speed–accuracy trade-off. Zero-sum refers to 100% of processing power available at any moment. According to internal and external needs, processing power is distributed to fulfill functional demands. A variable part of processing power is lost through interference. Cognitive enhancement within this construct may be achieved through an impact on power distribution, reduction of interference, and impact on the speed by which power distribution is achieved. Respective interference levels are either low or high resulting in a further loss of processing power. Real trade-off processes therefore only account for a small number of situations implying a competing environment.

Fig. 4

Zero-sum enhancement could be achieved through different ways (change of power distribution, interference reduction, increase of allotting speed).

Fig. 5

Schematic showing purported interactions between externally-focused task-positive network (+) and internally-focused task-negative (−) network.

Fig. 6

Example of “natural” paradoxical facilitation (Vuilleumier et al., 1996). After a first stroke (lesion 1) the patient suffered from a left visuospatial neglect and hemianopia which disappeared after a second stroke that affected the frontal eye fields (lesion 2). The second stroke had a paradoxical facilitatory effect on visuospatial attention, while the patient newly developed aphasic symptoms. While the first lesions shifted and increased processing power availability towards the left hemisphere (over-excitability) hereby increasing attention towards the right visual hemifield, the second lesion re-shifted and therefore normalized attention allocation while losing overall processing power.