I distinctly remember the first time I was motion sick. I was nine, and in true nine-year-old fashion, had stuffed four buckeyes into my mouth just as my mom called me to board our flight. Maybe it was the turbulence or my hyperactivity or the fact that I had eaten just a little too much, but I had the urge to vomit. After what, to my adolescent mind, seemed like a near-death experience, I vowed to never eat another buckeye. And almost a decade later, I still can’t even think of consuming that dreadful confection without getting sick to my stomach.
Often, I have wondered what exactly causes the mind to associate unrelated events so strongly with one another after only one experience. Long known in psychology as Pavlovian, or classical, conditioning, this phenomenon of linking two independent experiences with one another has been the focus of many neurobiologists. And for good reason. If the brain’s mechanism of creating and storing memories is understood, more effective treatments for Alzheimer’s and Post Traumatic Stress Disorder (PTSD) patients can be developed. Currently, cases of PTSD are treated mostly through psychotherapy in an attempt to subdue memory of and weaken associations to particular events. But what if the memory of the event itself could be completely erased? What if a new memory, something that the individual had never actually experienced, could replace the disturbing recollections?
Neurologists over the last fifty years have been moving away from the theory that memories are stored in specific brain cells and are slowly accepting its storage in neural connections. Each neuron, when it is activated, sends an electrical signal down its length using sodium and potassium channels. Once it reaches the end, it secretes a unique neurotransmitter in order to communicate with and activate the next neuron in the chain. It is in these spaces between neurons, called synapses, that memories are stored by means of synaptic plasticity, the flux in synaptic strength. A factor in this change of strength is long-term potentiation (LTP), a mechanism that enhances the strength of a synapse for an extended length of time. Several experiments have shown LTP’s association with memory and learning through observations of the effects when LTP is blocked with inhibitory drugs. However, none, until now, had shown a direct causal link between the activation of LTP and the storage of certain memories.
Robert Manilow, a neurologist at the University of California, San Diego, used the latest technological tool, optogenetics, to view the neural circuits of rodents subjected to classical conditioning. In optogenetics, a gene that produces a light-sensitive protein, opsin, is injected into the brain via a virus, and the gene is translated into opsin. Then, this protein is activated by a blue light that is delivered by optical fibers implanted in the brain. The activation of these opsin proteins controls synaptic connections, and, based on the type of light signal received, can strengthen or weaken these connections. So, with the optical equipment in place, Manilow carried out a standard classical conditioning experiment. An audio sound was played right before a shock was administered to the rat’s foot, and soon, the rat showed symptoms of paralysis after hearing the tone, even if it wasn’t shocked. This paralysis demonstrated that the rat had formed a memory to associate the auditory input with fear, which was nothing ground-breaking. However, what Manilow did next both transformed our understanding of the human mind and finally confirmed the suspicions of neurologists. He delivered an optogenetic signal that weakened LTP in the neurons connecting the part of the brain that processed sound to the part that processed fear. By weakening the connections between the neurons, Manilow was testing to see whether the rodent’s association between sound and fear had also weakened. In fact, the association was not only diminished, but actually completely erased. Through this experiment, Manilow proved that synaptic strength is directly involved in the formation and storage of memories. When the strength is decreased, the same rat, which had cowered in fear when a certain pitch was played, now paid no heed to the sound, blissfully unaware of its former pain.
Manilow then took it a step further. He reasoned that if a memory could be taken away by weakening the intercellular connection, its inverse was also possible. In other words, through optogenetics, completely new memories could be formed, independent of experience. He tested this by delivering an optogenetic signal, increasing the strength of LTP in the same part of the brain as before. Sure enough, the rat, upon perceiving the audio input, showed signs of fear, demonstrating the reactivation of its memory. “We were playing with memory like a yo-yo,” Manilow said.
Although it is hypothesized that many molecular mechanisms, in addition to LTP, are involved in the storage of complex memories, the implications of this experiment are undeniably great. Soon, there will be a time when the recollections of grotesque war images and the mental recreations of rape will evaporate with the click of a button. Or, a time when a few tiny wires are all it takes to re-form an Alzheimer patient’s memory of her granddaughter. But with this advancement, as with most discoveries, there exists a great potential for abuse. I think it’s safe to say that we will not experience any Orwellian brainwashing by the government in the near future, but what about the decisions that will plague psychiatrists and physicians? Once this technology becomes commercially available as a potential treatment method, healthcare professionals must choose which memories to keep and which to discard, with no objective means of making that decision. As the expense of the treatment decreases, the possibility for overuse increases. I am, by no means, a Luddite, and the benefits—cure of certain mental disorders and improvement of learning capabilities—incontrovertibly outweigh the consequences. However, companies should still consider the bioethical concerns before placing their financial support behind such a profitable and potentially beneficial treatment.
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