The Journal of Neuroscience has recently published a study describing new findings on global similarity models, mathematical theories according to which a great quantity of memories can come flooding into the brain, including apparently unrelated ones, when it comes to familiarity.
Having conducted an fMRI (functional magnetic resonance imaging, a technique that measures brain activity by detecting changes in blood oxygenation and flow in response to neural activity) study on memory and categorization, the researchers, led by Tyler Davis, assistant director of Texas Tech’s Neuroimaging Institute and an assistant professor of psychology, specializing in neurobiological approaches to learning and memory, have come up with fresh evidence that these mathematical models seem to convey an accurate explanation for processing in the medial temporal lobes, the long-term memory-associated region of the brain.
“Since at least the 1980s, scientists researching memory have believed that when a person finds someone’s face or a new experience familiar, that person is not simply retrieving a memory of only this previous experience, but memories of many other related and unrelated experiences as well,” Davis said. “Formal mathematical theories of memory called global similarity models suggest that when we judge familiarity, we match an experience, such as a face or a trip to a restaurant, to all of the memories that we have stored in our brains. Our recent work using fMRI suggests these models are correct.”
Although it is often believed that only the most similar or recent memories are activated for comparison when one sees someone’s familiar face or takes a trip to a familiar restaurant, Davis states that this is not so. He gives the example of the taste of a familiar dish at a particular restaurant. It seems familiar because it draws on a spectrum of memories stored in the person’s brain, according to global similarity models. These may include a previous trip to the same restaurant, the décor, eating the same food at a similar restaurant or at the person’s home, and even seemingly loosely related memories such as a recent trip to another city.
“In terms of global similarity theories and our new findings, the important thing is when you are judging familiarity, your brain doesn’t just retrieve the most relevant memories but many other memories as well,” Davis explained. “This seems counter-intuitive to how memory feels. We often feel like we are just retrieving that previous trip to that one particular restaurant when we are asked whether we’d been there before, but there is a lot of behavioral evidence that we activate many other memories as well when we judge familiarity.”
However, for the researchers, stored memories do not contribute evenly to familiarity, though. The more similar a previous memory and the current experience are, the more the former will contribute to judging the latter familiar.
In terms of the restaurant meal example, Davis explained that despite dissimilar memories, such as a recent trip out of town, have less impact on familiarity than, say, previous trips to the restaurant, similarity from such less-related experiences can have a considerable effect in judgments of familiarity.
Following this logic, Davis and his colleagues used fMRI also to ascertain how memory similarity related to behavioral measures of familiarity, in terms of activation patterns in the medial temporal lobes.
“We found that peoples’ memory for the items in our experiments was related to their activation patterns in the medial temporal lobes in a manner that was anticipated by mathematical global similarity models,” Davis said. “The more similar the activation pattern for an item was to all of the other activation patterns, the more strongly people remembered it. This is consistent with global similarity models, which suggest that the items that are most similar to all other items stored in memory will be most familiar.”
The findings suggest that global similarity models may have a neurobiological Foundation, and raises the questions that similarity, as a neural process, may impact memory, since one finds things familiar both because they are identical to previously experienced things, and because they are similar to a number of things previously experienced.
Other researchers involved in this study include Gui Xue of the National Key Laboratory of Cognitive Neuroscience and Learning and IDG/McGovern Institute for Brain Research at Beijing Normal University; Bradley C. Love at University College London; and Alison R. Preston and Russell A. Poldrack in the Department of Neuroscience at The University of Texas at Austin.