Therapeutic neuromodulation, the direct delivery of electrical or chemical stimulation to the sites of malfunctioning neural activity in the brain, has been used for chronic pain management for over 50 years. Although not specifically related to pain, deep brain stimulation (DBS) is a promising neuromodulation technique for patients who fail to respond to conventional therapies. In a neurosurgical procedure, electrodes are implanted in predetermined brain regions. These electrodes are connected to a battery-powered pulse generator placed under the skin below the collarbone. This pulse generator sends electric pulses to the electrodes through a wire extending along the neck. The electrodes deliver continuous electrical stimulation to the malfunctioning neural circuits, the frequency of which can be regulated externally.
Evidence from brain imaging studies suggests that the cognitive, motor, and mood symptoms of common psychiatric illnesses, such as Parkinson’s disease or Major Depressive Disorder, result from the dysregulation or malfunction of specific neural circuits. Application of therapeutic electrical stimulation using DBS electrodes has shown promising results in the alleviation of some of these symptoms. For instance, empirical evidence suggests that electrical stimulation of the subthalamic nucleus of the basal ganglia improves the motor functions of patients with Parkinson’s disease. Moreover, these electrodes also provide data on the functional dynamics of brain circuits and their respective behavioural manifestations.
It seems that the benefits of this technique can also be extended to the treatment of Post-Traumatic Stress Disorder (PTSD). PTSD is a common consequence of exposure to a life-threatening event; it is a debilitating condition in which traumatic memories, often triggered without a cause, can lead to hypervigilance, avoidance, and disruption of daily functioning. Theoretically, the electrical stimulation produced as a part of DBS can be used to inhibit the activity of certain neural circuits (in this case, those associated with emotionally charged or anxiety inducing memory traces). According to the reconsolidation-blockade hypothesis, the maintenance of traumatic memories follows a process of consolidation and reconsolidation. Herein, for a brief window of time between the reactivation of a memory trace and its reconsolidation, the memory is liable to alteration. If the reconsolidation process is disrupted, the memory trace can be effectively erased. Earlier studies based on pharmacological interventions to disrupt the reconsolidation process using the non-adrenergic beta-blocker, Propanol, had shown promising results. However, such interventions may have severe side-effects on non-targeted memories. DBS, on the other hand, is argued to produce more focused electrical stimulation, thereby preventing impairment. Even so, critical questions regarding the safety and ethical connotations of this procedure have been raised; can it be done? More importantly, should it be done?
Let us try answering the first question. Emotionally charged memories get embedded in the basolateral amygdala, the fear network of the brain, upon exposure to a traumatic experience. Erasure of maladaptive memory traces using DBS would involve selectively inactivating neurons and synapses associated with the pathological memory trace. In order to achieve this, the aforementioned reconsolidation process would have to be disrupted. The frequency of electrical stimulation can be modulated to erase the emotional content of the particular memory trace, while sparing the declarative content of the traumatic memory. The question is, can the neurons constituting the maladaptive memory traces be localized precisely enough to successfully erase them without accidentally erasing other memories? One way to do this would be by using functional neuroimaging to target the specific neural groups that show higher activation when a patient is asked to recall the traumatic experience. However, the functional mechanisms of DBS are not that well understood. In the past, studies have demonstrated successful disruption of episodic memories by electrical stimulation of the hippocampus and the temporal lobes. For instance, Merkow, Burke, Ramayya et al. (2017) demonstrated that the application of direct electrical stimulation to the medial temporal lobe between the encoding and retrieval processes in a memory task lead to impaired memory performance. Whether the same process will result in the selective erasure of fear-based memories in the amygdala remains to be seen.
Moving on to the ethical considerations, it is pertinent that we consider that memories are an integral part of one’s narrative identity. Both positive and negative memories are essential for the continuity and connectedness of human experiences, and for constructing meaning from them. The availability of a procedure such as DBS may encourage people to erase memories (and thus, associated emotions), that are simply inconvenient, rather than pathological. Although erasing the emotional component of maladaptive memories preserves the adaptive value of negative memories, it has been argued to have profound effects on moral sensitivity, agency, and emotional learning. Deliberate forgetting of emotions such as remorse or guilt may make it harder for individuals to learn from their mistakes, as the declarative knowledge is stripped of associated emotions.
S. Matthew Liao, bioethics professor at NYU, sheds light on another potential ethical concern. The United States’ federal Defense Advanced Research Project Agency (DARPA) has been funding research in DBS, with the aim of providing relief to soldiers suffering from PTSD and major depression. Moreover, DARPA hopes to develop advanced closed-circuit deep brain stimulators capable of pre-empting PTSD in real time. This could be achieved by dampening the intensity of emotional reactions to traumatic events before they are consolidated into long term memory. It could also be used to fortify soldiers’ minds by modifying other emotions. For instance, it may be used to reduce fear and increase courage in the face of combat. Although it started out as a relief measure, DBS may, one day, be capable of ‘stimulating away’ soldiers’ conscience and thereby, giving rise to enhanced soldiers capable of indiscriminate violence.
It is unlikely that these grave ethical concerns will hamper the advancement of DBS as a viable technology in the coming years, considering the immense potential it carries as a clinical tool. However, it is important to not get carried away by the exciting possibilities, and to establish safeguards that minimize inherent risks, and prevent its abuse.
Isha Puntambekar
