Blanks for the Memory

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There are many types of memory, but fundamentally, humans remember in two ways.

Declarative memory consists of ordinary recollections consciously summoned from the brain. It is dependent on the medial temporal lobe (MTL), which in turn contains key cognitive structures, including the hippocampus and entorhinal cortex. Declarative memory allows us to explicitly recall or recognize past events or facts.

Nondeclarative memory is more mysterious. It is accessed without conscious thought. It can be triggered implicitly and is expressed through performance. The skill of riding a bike, which changes and improves with practice, is a product of nondeclarative memory. Another form is the ability to improve one’s perception of the surrounding world through practice, which is called perceptual learning.

But not all forms of perceptual learning are equally understood.

In a new paper, published online in the May 11, 2021 issue of the journal PNAS, researchers at the University of California San Diego School of Medicine and VA San Diego Healthcare, describe testing “one-trial perceptual learning” with memory-impaired patients, reporting that while the patients performed well on the actual task, they struggled to retain declarative memories of what they had done.

Most people cannot identify what is depicted in this degraded image. However, when an intact version of the image is presented just once immediately after presentation of the degraded version, the ability to later identify the degraded image is greatly improved, even after many months. Image depicts a Dalmatian, with the dog’s head in upper left corner, facing left.

In the study, senior author Larry Squire, PhD, Distinguished Professor of Psychiatry, Neurosciences and Psychology at UC San Diego, and colleagues presented a degraded, black-and-white image of an object, which appeared unrecognizable and meaningless upon first presentation. Yet when subsequently exposed for a single time to the original, intact image, the degraded image became easily identifiable to the memory-impaired patients, even months later.

“This effect was described decades ago in volunteers, but it has not been known whether this kind of learning depends on conscious memory of the images that were presented, which would involve the hippocampal region of the brain, or whether it involves an unconscious change in perception, independent of the hippocampus and independent of the ability to remember the images,” said Squire.

Five memory-impaired patients participated, each with lesions disrupting function in the MTL region of their brains. Yet compared to unimpaired volunteers, they performed equally well, with perceptual learning improvement persisting more than five months, even though they subsequently struggled to remember the testing format or the images themselves.

The researchers also directly compared perceptual learning and remembering, asking study participants to take a naming test or a recognition memory test of the images they had viewed seven days earlier. “The patients did as well as the volunteers at identifying the degraded images,” wrote the authors, “but were severely impaired at remembering them.”

The distinction between conscious and unconscious forms of memory has been fundamental to brain science. One-trial perceptual learning had long eluded classification, said Squire, but can now be understood to be a form of nondeclarative or unconscious memory.

“Here arise the dispositions, habits and preferences that are inaccessible to conscious recollection, yet are shaped by past events, influence our behavior and mental life, and are a fundamental part of who we are,” Squire said. “The diagnosis and treatment of these behaviors when they go awry, as in phobias or self-destructive habits like addiction, comprise much of the world of psychiatry.”

Co-authors include: Jennifer C. Frascino and Charlotte S. Rivera, UC San Diego and VA San Diego Healthcare; Nadine C. Heyworth, UC San Diego; and Biyu J. He, New York University.

Funding for this research came, in part, from the Medical Research Service of the Department of Veterans Affairs (program 5IK6CX001644) and the National Institute of Mental Health (grant 24600).