Researchers can now turn memories on and off, and this is good news for PTSD and Alzheimer's sufferers.
File under "things likely to make someone insane": first having a memory switched off, just long enough to make one wonder if it was even there in the first place, and then turning it back on. The memory was there, helping in the formation of core awareness and perception, and then it wasn't. The brain adapts to its absence, and then is forced to readapt to its reappearance. That might make for a very unique variety of torture, an absolute crisis of identity.
This sort of rewiring is now possible, according to a study out this week in the journal Nature. And it doesn't really take rewiring at all, just a beam of light. Using the relatively new techniques of optogenetics, in which animals are genetically engineered such that their brain tissue becomes sensitive to photo stimuli (lasers), it's possible to quickly alter the neurological functioning of an animal by boosting the electrical potential between certain synapses. It's a mechanism learned from algae, which alters its behavior in accordance with sunlight.
The good news is that memory-based torture isn't the goal of this research (and this sort of research generally). What we're after is the very nature of memory in the first place, just what it even is within the brain's physiology. The effectiveness of these photogenic techniques is further evidence that memories are the result of strengthened connections between certain neurons, subtle depressions or channels that form memory-storing circuits. Silence these channels/circuits and the memory goes away; reinstate them and the memory returns.
Roberto Malinow, a professor at the University of California, San Diego and study co-author, summarizes as such: "Our results add to mounting evidence that the brain represents a memory by forming assemblies of neurons with strengthened connections, or synapses. Further, the findings suggest that weakening synapses likely disassembles neuronal assemblies to inactivate a memory."
Malinow et al's experiment worked like this. A mouse is conditioned to fear a certain tone by delivering a shock to the animal's foot at the same time as the tone is played. Once conditioned, the mouse freezes up and ceases its reward-seeking behaviors whenever it hears the tone. This circuit is already well-known as the site of auditory fear memory, and the researchers were able to effectively replace the tone stimuli with direct brain stimulation via laser. Using different patterns of light, it was possible to strengthen and weaken neural connections along this particular pathway, turning the fear memory up and then back down.
After the fear memory tunings, the mice are then put out of their misery. Postmortem examinations revealed alterations in their brains' chemical messenger systems, consistent with what would be expected from changes made to this particular memory circuit.
Applications for this knowledge are not hard to find. Alzheimer's, for example, involves the weakening of synapse connections as the result of accumulating harmful compounds. It's not hard to imagine a reversal of the process by refortifying those connections. On the other hand, it might be desirable to quiet certain memory circuits in patients with some mental illnesses.
In a statement, Thomas R. Insel, the director of the National Institute of Mental Health, noted, "Beyond potential applications in disorders of memory deficiency, such as dementia, this improved understanding of how memory works may hold clues to taking control of runaway emotional memories in mental illnesses, such as post-traumatic stress disorder."