The connectivity of cortical and subcortical brain tissue is often too intricate to parse information from a single DBS electrode. Generation of meaningful visual perception by stimulation of the visual cortex for example requires a large number of much smaller electrodes (Schmidt et al, 1996). DBS is therefore used merely to suppress or normalise electrical activity in dysfunctional brain regions (hence the nickname 'brain pacemaker').
However, many monoaminergic neurons in the midbrain and brainstem fire in unison and project widely throughout the brain: by stimulating neurons that produce dopamine for instance, researchers can directly modulate dopamine concentrations in diverse cortical and subcortical regions (Hernandez et al, 2006; Garris et al, 1997; Fiorino et al, 1993; Bean & Roth, 1991). Stimulation of dopaminergic neurons, serotonergic neurons or the nerve bundles that carry their axons to cortical and subcortical targets is highly rewarding and is referred to as brain stimulation reward (BSR) in the literature.
BSR motivates animals to perform behaviors with which it is repeatedly associated and has been used to drive behaviors such as heavy physical exercise and learning (Burgess et al, 1991; Garner et al, 1991; Hermer-Vazquez et al, 2005). It seems likely that DBS of monoaminergic regions would support BSR also in humans and that it could be used to motivate behaviors such as physical exercise and learning.
There are two reasons to develop monoamine modulation by DBS (MMDBS) for human use:
First, MMDBS could have a devastating impact on humanity if developed under autocratic regimes without public oversight and vigorous debate. It could for instance be used to create large factories of workers addicted to manual labour. Even if developed in a democracy there are numerous human rights concerns associated with MMDBS, including access control by the manufacturer, the potential for over-use, and aggravation of societal inequalities. These are very difficult problems whose solution will require the concerted effort of democratic societies.
Second, MMDBS could be enormously beneficial if developed responsibly. It could help people exercise, thus attenuating the healthcare crisis in general and the obesity epidemic in particular. It could be combined with learning tutorials to aid people with learning difficulties. It could be used to motivate people to complete scientific research protocols, thus accelerating research in critical fields such as renewable energy and biomedicine. Moreover, it might offer a more dynamic alternative to pharmacological modulators of monoamines such as stimulants and antidepressants.
www.iPlant.eu is a discussion group promoting the development of beneficial electronic modulation of monoamines for human use. For more information, please visit our website or join our forum.
References
Bean AJ & Roth RH (1991) Extracellular dopamine and neurotensin in rat prefrontal cortex in vivo: effects of median forebrain bundle stimulation frequency, stimulation pattern, and dopamine autoreceptors. Journal of Neuroscience, 11(9), p2694-2702
Burgess ML, Davis MJ, Borg TK & Buggy J (1991) Intracranial self-stimulation motivates treadmill running in rats. Journal of Applied Physiology 71(4), p1593-1597
Fiorino DF, Coury A, Fibiger HC, Phillips AG. (1993) Electrical stimulation of reward sites in the ventral tegmental area increases dopamine transmission in the nucleus accumbens of the rat. Behavioral brain research, 55(2), p131-141
Garner RP, Terracio L, Borg TK & Buggy J (1991) Intracranial self-stimulation motivates weight-lifting exercise in rats. Journal of Applied Physiology 71(4), p1627-1631.
Garris PA, Christensen JR, Rebec GV, Wightman RM. (1997) Real-time measurement of electrically evoked extracellular dopamine in the striatum of freely moving rats. Journal of Neurochemistry, 68(1), p152-61
Hermer-Vazquez L, Hermer-Vazquez R, Rybinnik I, Greebel G, Keller R, Xu S, Chapin JK (2005) Rapid learning and flexible memory in "habit" tasks in rats trained with brain stimulation reward. Physiology & Behavior 84(5), p753-9
Hernandez G, Hamdani S, Rajabi H, Conover K, Stewart J, Arvanitogiannis A, Shizgal P (2006) Prolonged rewarding stimulation of the rat medial forebrain bundle: neurochemical and behavioral consequences. Behavoral Neuroscience. 120(4), p888-904
Schmidt EM, Bak MJ, Hambrecht FT, Kufta CV, O'Rourke DK, Vallabhanath P. (1996) Feasibility of a visual prosthesis for the blind based on intracortical microstimulation of the visual cortex. Brain 119(pt2), p507-522
Schwalb JM, Hamani C (2008) The history and future of deep brain stimulation. Neurotherapeutics. 5(1), p3-13
Christopher Harris is interested in realistic modifications of brain function and what they tell us about the human condition. He aims to develop iPlants - monoamine regulating brain implants that could allow people to program their own behaviour and mood. He works in the electrophysiology lab at the University of Sussex, manages the iPlant research website and writes a blog.