Light stimulus triggers memory recollection

Have you ever had a single scent, melody or picture transport you to a distant memory? MIT scientists have done something similar but with neurons as the trigger. In other words, they have been investigating whether a few neurons can bring to mind an entire memory.

This feat, achieved for the first time recently, is called optogenetics, and it is powerful enough to use brief pulses of light to control genetically modified neurons and artificially turn on memories.

First, the researchers identified the neurons in mice that become activated when a new memory is being formed. Their focus was on the hippocampus, which has a significant role in learning and memory, and is known for its ability to recognize places. They then inserted a gene that codes for a protein sensitive to light into neurons in the hippocampus. This enabled them to use light to control the neurons.

They then set out to give the mouse a new memory. The mice received a mild electric shock to a foot in a specific environment. The shock resulted in behaviors characteristic of mice when they experience fear: freezing in place. The mice then developed an association between the environment where the shock was delivered and the shock.

At this point, the scientists were poised to figure out whether they could activate the memory of fear through controlling the neurons. By directing light to the hippocampus, they activated some of the neurons involved in forming the memory. As a result, the mice froze in place, expressing fear. The entire memory seemed to have been triggered through merely stimulating the neurons.

To verify their results - i.e. that the mice were indeed recalling the original memory - the researchers performed several tests. They gave some mice the same protein that was sensitive to light, but did not shock them. Other mice were not given the protein but were given the shock.

Still other were tested in a different environment that was not associated with fear.

These tests all supported the original finding, since none of these subsequent tests resulted in the mice freezing up. Thus, it seems that the pulse of light did indeed result in the activation of the memory of fear.

Optogenetics had been introduced by a research group at Stanford, led by Karl Deisseroth, who contributed to the present study, in 2004. Its advantage over traditional techniques like electrical or chemical stimulation is that it is fast and precise. In fact, traditional techniques can only alter unspecified brain activity and does not allow for precise timing. However, using optogenetics, it is possible to control a small number of neurons on a millisecond time scale.

Previous research over the past few years has used optogenetics in highly potent ways to study disorders such as anxiety, depression, Parkinson's disease and schizophrenia. This new study now shows that a few neurons can be artificially stimulated to trigger an old memory, in essence controlling animal behavior without necessitating sensory input.

However, the possibility of applying this technology to human brains is still distant. The first steps toward this possibility, though, have been taken. Researchers at Brown are developing a wireless optical electrode that is meant to deliver light to human neurons.