CAMBRIDGE, Mass. Neuroscientists at MIT Picower Institute of Learning and Memory have uncovered why relatively minor details of an episode are sometimes inexplicably linked to long-term memories. The work is slated to appear in the Jan. 13 issue of Neuron.

“Our finding explains, at least partially, why seemingly irrelevant information like the color of the shirt of an important person is remembered as vividly as more significant information such as the person's impressive remark when you recall an episode of meeting this person,” said co-author Susumu Tonegawa, Picower Professor of Biology and Neuroscience and director of the RIKEN-MIT Center for Neural Circuit Genetics.

The data also showed that irrelevant information that follows the relevant event rather than precedes it is more likely to be integrated into long-term memory.

Shaping a memory
One theory holds that memory traces or fragments are distributed throughout the brain as biophysical or biochemical changes called engrams. The exact mechanism underlying engrams is not well understood.

MIT neuroscientists Arvind Govindarajan, assistant director of the RIKEN/MIT Center for Neural Circuit Genetics; Picower Institute postdoctoral associate Inbal Israely; and technical associate Shu-Ying Huang; and Tonegawa looked at single neurons to explore how memories are created and stored in the brain.

Previous research has focused on the role of synapses—the connections through which neurons communicate. An individual synapse is thought to be the minimum unit necessary to establish a memory engram.

Instead of looking at individual synapses, the MIT study explored neurons’ branch-like networks of dendrites and the multiple synapses within them.

Boosting the signal
Neurons sprout dendrites that transmit incoming electrochemical stimulation to the trunk-like cell body. Synapses located at various points act as signal amplifiers for the dendrites, which play a critical role in integrating synaptic inputs and determining the extent to which the neuron acts on incoming signals.

In response to external stimuli, dendritic spines in the cerebral cortex undergo structural remodeling, getting larger in response to repeated activity within the brain. This remodeling is thought to underlie learning and memory.

The MIT researchers found that a memory of a seemingly irrelevant detail—the kind of detail that would normally be relegated to a short-term memory--may accompany a long-term memory if two synapses on a single dendritic arbor are stimulated within an hour and a half of each other.

“A synapse that received a weak stimulation, the kind that would normally accompany a short-term memory, will express a correlate of a long-term memory if two synapses on a single dendritic branch were involved in a similar time frame,” Govindarajan said.

This occurs because the weakly stimulated synapse can steal or hitchhike on a set of proteins synthesized at or near the strongly stimulated synapse. These proteins are necessary for the enlargement of a dendritic spine that allows the establishment of a long-term memory.

“Not all irrelevant information is recalled, because some of it did not stimulate the synapses of the dendritic branch that happens to contain the strongly stimulated synapse,” Israely said.

Source: “The dendritic branch is the preferred integrative unit for protein synthesis-dependent LTP,” by Arvind Govindarajan, Inbal Israely, Shu-Ying Huang and Susumu Tonegawa. Neuron, 13 January, 2011.

Funding: RIKEN, the Howard Hughes Medical Institute and the National Institutes of Health.
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Professor Bill Lionheart and his team, working with security firm Rapiscan, won the Defence and Security prize at The Engineer’s Technology & Innovation Awards.

Hosted by Robert Llewellyn, best known for his role as Kryten in Red Dwarf and from Scrapheap Challenge – the awards acknowledged engineering excellence emanating from collaborative engineering projects in ten categories.

The RTT80 3D scanner, which has been tested at Manchester Airport and by the US Government, combines the very high quality 3D data of a CT image with the high speed of an X-Ray system to detect potential explosives in the baggage of airline passengers.

At the moment, suitcases and large bags go through large X-ray systems and are sent to a second line of security if anything suspicious shows up – a CT scan, similar to those used in hospitals.

This double-layered security system is slow and requires extra operators, so the scanner was designed to replace the current layered system with a single CT scan.

Kenn Mann, Technical Director of Aviation Security at Rapiscan, said: “We expect the RTT to be a step change in baggage screening, bringing the highest detection performance at the fastest speeds.

“Our collaboration with The University of Manchester has been a great success and has produced some very impressive results.”

The RTT80 produces a 3D image of the contents of a case or bag in the same time as a conventional projection X-ray system’s 2D image

To achieve this, Rapiscan turned to Professor Lionheart, an applied mathematician at The University of Manchester.

Professor Lionheart realised that in this innovative new type of X-ray scanner the basic mathematics behind forming a 3D image was not yet known. Funded by a grant from the Engineering and Physical Sciences Research Council and Rapiscan, Professor Lionheart assembled a team to work on various aspects of the problem.

Dr Marta Betcke joined the team and together they devised an algorithm based on multiple surfaces to reconstruct the image in real time, so that it can be viewed by an operator. A patent has been filed and the method tested on data collected at the airport.  A production prototype of the RTT 80 is currently being tested by regulatory authorities.

Meanwhile the systems little brother, the prototype RTT20  is installed at the University where Lionheart and collaborators plan to use it for basic science.

Professor Lionheart says: “This project shows what can be done when mathematicians work closely with engineers in industry on problems that require new mathematics but are immediately applicable.

“The RTT80 is a great example of British innovation and we are proud to have been part of it. As well as making air travel safer real time X-ray scanners have enormous potential in other applications.”