Brain damage doesn't just erase memories. In some cases, it may actually mislead the brain to falsely remember a new object as being familiar, according to new simulations and animal studies.
The surprise finding may help explain similar memory problems for human patients suffering from amnesia or Alzheimer's disease. Both of those conditions can damage a specific part of the brain called the perirhinal cortex – a region buried in the middle part of the brain that helps form memories by sifting through incoming information from the senses and building a complex picture of an object.
"The perirhinal cortex is one of the first regions that is affected in Alzheimer's disease, and it is very often damaged in cases of amnesia, so specific damage in this region is highly relevant to both of these conditions," said Lisa Saksida, a psychologist at the University of Cambridge in England.
Damage in that part of the brain disrupts the ability to form memories based on complex, detailed representations of objects seen in the real world. That means the brain must rely instead upon simpler features that commonly appear across many objects.
"The remaining representations of simpler features of objects are relatively easily confused, and as a result, false memories are generated," Saksida explained.
For instance, a certain line or curve on one object is very likely to show up in plenty of other objects.
Finding the hint
The first clue about false memories came from a computer simulation that allows researchers to test the effects of virtual brain damage. That simulation predicted that animals that suffered damage to the perirhinal cortex should see novel objects as familiar.
"The prediction was quite counterintuitive, and we were indeed surprised, so we thought we'd figure out a way to test it," Saksida told LiveScience.
Saksida's colleagues in the U.K., U.S. and Canada tested the prediction by creating a series of experiments involving both normal lab rats and rats that had been selectively brain-damaged through surgery as part of the experiment. Their study is detailed in the Dec. 3 issue of the journal Science.
The researchers first gave the rats three minutes to familiarize themselves with a specific "junk object" – a small, complex and colorful object similar to a small garden gnome or plastic toy train that you might find in a dollar store. Next, they held the rats in a separate cage environment for an hour before releasing the rats into an environment where they encountered either the old object or a new object.
Seeing the light
Normal rats spent more time exploring the new objects, and less time with the old familiar object. But the brain-damaged rats spent less time exploring the new object – a sign that they treated the new object as if it were familiar. If the rats had simply forgotten about having encountered the old object, they would have spent more time exploring both the old and new objects.
Saksida's group followed up by repeating the experiment and holding rats in a dark environment rather than an open cage during the hourlong delay between the two experiment phases.
The researchers predicted the brain-damaged rats' false sense of familiarity with the new objects came from seeing similar simple but unrelated features in the environment during the holding period, and so a dark environment should prevent the rats from seeing anything that could build up the false memory.
That suspicion proved correct when the brain-damaged rats performed normally after spending time in the dark environment rather than in the open cage.
From rats to humans
Such findings support a 10-year-old model of memory and thought that Saksida and her colleagues have developed. The representational hierarchy theory of cognition suggests that visual information is organized into more complex representations of real-world objects. This information organization supposedly takes place in certain parts of the brain, such as the perirhinal cortex. Without it, brain-damaged rats or humans would be unable to produce such representations, leaving open the chance for a memory mix-up
The researchers have tested other predictions that support their theory in human patients who have amnesia or Alzheimer's, but have not yet repeated the rat experiment with humans – something Saksida hopes to do in the future.
"There is already some evidence in the literature that people with various types of memory impairment (including Alzheimer's disease) perform better on memory tests if they spend time before the test in a dark, quiet environment, which is consistent with what we found here," Saksida said in an e-mail.
She cautioned that humans rarely experience the selective brain damage that was tested in the lab rats, but suspects that the main study findings do apply to humans – it just won't explain the broader range of symptoms resulting from wider brain damage.