While DBS has been traditionally viewed as a tool for delivering a static set of stimulation parameters, recent advances are beginning to allow it to be used in a more versatile manner. By nature, cognitive behavior such as learning and recall is highly dynamic, and therefore dynamic neuromodulatory devices are likely to be required to alter its function. For example, in primates delivery of striatal stimulation only after execution of correct responses has been shown to enhance formation of learned associations whereas delivery prior to motor execution or after execution of incorrect responses has little effect.40
More recent studies in humans have also attempted to use continuous neural stimulation for stroke rehabilitation, neuropathic pain, and seizure control.8,29
Whereas delivery of continuous stimulation in these cases may be effective, developing the ability to dynamically deliver stimulation in relation to an individual’s behavioral state may provide more substantial results. Such approaches would provide a more rational basis by which to modulate neuronal activity in a disease and behaviorally specific manner and is likely to be at least 1 component in the evolution of DBS.
In addition to developing “smart” dynamic tools for delivering stimulation, it is also important that our understanding of the effects of DBS continues to grow. Although microstimulation has been largely assumed to affect behavior via direct excitation or suppression of neuronal activity, recent studies also suggest that DBS may affect memory by alternative mechanisms. In a recent study by Toda et al.,35
for example, rodents received DBS in the anterior thalamus, a central relay area in the limbic memory system. The authors found that in stimulated animals, cell division in the subgranular layer of the hippocampus significantly increased. This was not observed in nonstimulated control groups and suggests that DBS may not only alter neuronal activity but may also enhance neurogenesis in certain areas involved in memory formation.
Our understanding of the effect of DBS on neuronal functioning and its dynamic implementation has proven to be a powerful tool for treating many of the negative aspects of disorders such as Parkinson disease, essential tremor, depression, and obsessive-compulsive disorder. However, it may also play a potential role in positively affecting cognitive behavior such as memory formation and recall. While such an approach could hold significant promise, it is also important to keep in mind its potential ethical limitations. Memories, in particular, are highly personal and are critical to our personality and conception of ourselves. The manipulation of memory, whether to obliterate unwelcome memories or reinforce fading ones, has implications for who we are as individuals. Therefore, as our ability to deliver real-time dynamic stimulation in the clinical setting grows, a rigorous ethical framework will need to be developed to ensure that such technology can be used in an appropriate and responsible manner.