Have you ever met someone and recognized their face, but without any distinct memory from where? Or have you ever smelled something that invokes a strong emotion in you, but you had no idea why?
These questions involve the processing of unconscious memories. Until recently, medial temporal lobes, like the hippocampus, were thought to be uninvolved in unconscious memory formation. There has been little knowledge as to exactly what brain structures are involved in unconscious memories.
Wei-chun Wang, a psychology graduate student at UC Davis, and colleagues recently discovered that the perirhinal cortex, a small area in the brain, is critical for forming unconscious conceptual memories.
Prior to this research, the perirhinal cortex was thought to be involved, like the hippocampus, in conscious memories, called “declarative” memories. These memories are those you can describe when telling an anecdote beginning “I remember when …” They are memorable events from your past you can voluntarily retrieve at any time.
Researchers believed that if the perirhinal cortex were to be damaged, declarative memories would be affected, but not your conceptual unconscious memories. This is true of the hippocampus, but as seen by Wang’s research, this is not true of the perirhinal cortex, neighbor to the hippocampus.
Wang said that previous exposure to an idea gets that idea to stick in the unconscious memory. With a little prompting, that idea will be the first to pop up.
For example, test subjects who were shown the word “soccer” during the experiment were more likely to respond with “soccer” when asked for the sport that first jumped to mind.
According to Ranganath, there has been a lot of work showing that people with amnesia have brain damage that makes it impossible to form new memories. But, even these patients can retain unconscious memories.
“In general, the common assumption is that, if patients with amnesia can learn some things without awareness, maybe they can learn anything, as long as it doesn’t require awareness … that’s what led to this experiment,” Ranganath said.
Wang and colleagues carried out memory tests on people diagnosed with amnesia. These patients all had damage to the perirhinal cortex or other brain areas. To peer inside the brain, researchers also carried out functional magnetic resonance imaging (fMRI) scans of healthy volunteers while they performed the same memory tests.
Typically, researchers gave all subjects a long list of words, such as chair, table or spoon, and asked them to think about how pleasant they were. Then they asked the subjects to think of words in different categories, such as “furniture.”
Amnesiacs with damage to the perirhinal cortex performed poorly on the tests. In healthy control subjects, activity was shown in the peripheral cortex in fMRI scans. These results indicate that these types of conceptual unconscious memories involve the perirhinal cortex, which was previously thought to only be involved in conscious memory processing.
“I think what’s most exciting is that our study helps to advance past theories that assume that the brain is composed of “modules,” or separate components in the mind. Modules for language, memory, etc.,” Wang said. “What we show is that the brain is far too complex to be thought of this way, rather it is interactive and many types of memories and functions overlap in the brain.”
According to a 2004 study published in the National Center for Biotechnology Information, Alzheimer’s disease first affects hippocampus and the perirhinal cortex. Now that scientists know what the perirhinal cortex is involved in, they can use their knowledge to study some of the unknowns of diseases like Alzheimer’s.
“I think that this study really helps advance our understanding of the role of the medial temporal lobes in memory,” Wang said. “It will hopefully help researchers understand the cognitive functions that are affected in diseases that affect the perirhinal cortex … like Alzheimer’s disease and semantic dementia.”
CAMMIE ROLLE can be reached at science@theaggie.org.
