Lia Kvavilashvili sat in her office at the University of Hertfordshire, mentally reviewing a study she had recently published. She knew that there was a particular statistical measure that might have been useful in the study, but she could not remember its name. Frustrated, she got up to make a cup of tea.

Suddenly the word "hurdle" popped into her mind, unannounced, uninvited. Kvavilashvili—who grew up in Georgia speaking Georgian, Russian and Estonian, and only started to learn English at age 13—had no idea what "hurdle" meant. She looked it up in her dictionary:

Hurdle (noun) 1. A portable barrier over which athletes jump in a race. 2. A difficult problem to be overcome; obstacle.

The second definition was underlined. Although she had no conscious recollection of it, Kvavilashvili had evidently looked up the meaning of "hurdle" before. Somehow, she concluded, her subconscious knew that the word was relevant to her difficulty remembering the name of the useful statistical measure. She had just experienced what she and a few other psychologists call "mind-pops"—fragments of knowledge, such as words, images or melodies that drop suddenly and unexpectedly into consciousness.

In most cases, mind-pops seem completely irrelevant to the moments in time and thought into which they intrude. But Kvavilashvili is discovering that mind-pops are not truly random—they are linked to our experiences and knowledge of the world, albeit with hidden threads. Research on mind-pops is preliminary, but so far studies suggest that the phenomenon is genuine and common. Some people notice their mind-pops far more often than others and frequent mind-popping could quicken problem solving and boost creativity. However, in some people's minds—such as those with schizophrenia—mind-pops might evolve from benign phenomena into unsettling hallucinations.

Beyond Proust's legacy
In everyday life, people often search their memory for specific information: Where did I leave the car keys? Did I really turn the oven off? Other times, they actively reminisce about the past: Remember that crazy night out last week? Man, that was crazy. But not all recall is a choice; some forms of memory are involuntary. Perhaps the most famous example of involuntary memory is a scene from French novelist Marcel Proust's In Search of Lost Time (also called Remembrance of Things Past). As the narrator drinks some tea and eats a small lemony cake known as a madeleine, the taste resurfaces a memory of eating the same treat at his aunt's house when he was young. More recently, Pixar adapted Proust's madeleine episode for the animated movie Ratatouille, in which the eponymous stewed vegetable dish immediately transports a cynical food critic to the dinner table in his boyhood home.

Although they qualify as a type of involuntary memory, mind-pops differ from the classic Proustian example. Mind-pops are more often words or phrases than images or sounds and they usually happen when someone is in the middle of a habitual activity that does not demand much concentration—perhaps when they are brushing their teeth or tying their shoes. Most notably, identifying a trigger for a mind-pop in the surrounding environment or even in previous thoughts is extremely difficult—they seem to come out of nowhere.

Kvavilashvili became interested in mind-pops because she experienced them so frequently. In the summer of 1995 she started to keep a diary of the mental hiccups, eventually recording more than 100 incidents. When she searched databases of research papers for mind-pops, she found almost no mention of such unannounced, seemingly arbitrary memories—except in the studies of George Mandler, currently an emeritus of professor of psychology at the University of California, San Diego, who coined the term "mind-pop."

Recognizing a mutual interest and a gap in the research literature, Kvavilashvili and Mandler began to collect data, keeping careful diaries of their thought patterns and asking volunteers to do the same. Kvavilashvili personally recorded more than 400 mind-pops over nine months. More than 90 percent of them occurred when she was alone and more than 80 percent happened during routines and chores that did not require great mental effort, such as brushing her teeth, getting dressed and ironing. Mind-pops seem to be most common when the mind is free to wander and not neccesarily fixated on the task at hand.

Most of the mind-pops surprised Kvavilashvili with their irrelevance to her current actions and thoughts. In each case, she carefully searched her mind and her surroundings for a cue—something that might have triggered the mind-pop. She could only identify cues 20 to 30 percent of the time, many of which were subliminal. One time, for example, the phrase "millennium dome" fluttered across her mind like a banner lost to the wind just after she had been looking in the direction of a shelf on which sat a pack of sesame wheat wafers. When Kvavilashvili examined the package she discovered the words "Miller's Damsel" printed in a semicircle.

Although Kvavilashvili had difficulty identifying cues in her current thoughts or immediate surroundings, in nearly half the cases she determined that she had encountered something directly related to a particular mind-pop a few hours or days earlier. Once, while throwing a used bag into the trash, the word "Acapulco" sprang into her consciousness. She did not know what the word meant until family member reminded her that a television news program they had watched 45 minutes earlier had mentioned the Mexican resort city. Another time, the phrase "corporal punishment" flung open her mind's door uninvited. The next day, Kvavilashvili discovered the phrase in some work documents that she had been reading five days earlier.

"One might think mind-pops are simply errors in cognitive functioning—accidental firings that bring up completely random content in your mind," Kvavilashvili says. "But once I started recording them, quite often I would notice that what popped into my mind wasn't entirely accidental. The contents of the mind-pop had been experienced in the recent past."

Kvavilashvili and Mandler also asked 58 psychology students to keep similar diaries of mind-pops for one week. Like Kvavilashvili, the students could only identify specific triggers for their mind-pops in a minority (37 percent) of the cases; they were more successful at recognizing that they had encountered something related to the mind-pops in the recent past, which they were able to do for 42 percent of their examples. The collected diary studies were published in 2004 in Cognitive Psychology.

Based on these diary studies, Kvavilashvili and Mandler propose that mind-pops are often explained by a kind of "long-term priming." Priming describes one way that memory behaves: every new piece of information that enters memory changes how the mind later responds to related information. If a psychologist gives a volunteer a list of words including the word "apple" and later asks the volunteer to write a complete word starting with "app," the subject is more likely to write apple than "appetite" or "application." Kvavilashvili and Mandler think that something similar, but subtler, more enduring and more capricious is happening with mind-pops.

"Most of information we encounter on a daily basis activates certain representations in the mind," Kvavilashvili explains. "If you go past a fish and chips shop, not only the concept of fish may get activated but lots of things related to fish, and they may stay activated for a certain amount of time—for hours or even days. Later on, other things in environment may trigger these already active concepts, which have the feeling of coming out of nowhere."

From intrusions to delusions
Kvavilashvili speculates that people who experience mind-pops most frequently might be super primers, which could in turn encourage creativity. "It could help us to process information more efficiently," she says. "If many different concepts remain activated in your mind, you can make connections more efficiently than if activation disappears right away."

Mind-pops and other kinds of involuntary memory are relatively new subjects for psychologists and so far no one has published any studies examining whether people who experience involuntary memories more frequently are better at priming tasks or more creative. Recently, however, Kvavilashvili and her colleagues published a study looking at a possible dark side of mind-pops. The researchers wondered just how similar everyday involuntary recall is to intrusive thoughts and hallucinations observed in mental disorders.

Repetitive, intrusive thoughts feature in many mental disorders, namely depression, post-traumatic stress disorder (PTSD) and obsessive-compulsive disorder. Imaginary sights and sounds—known as auditory and visual hallucinations—often haunt the minds of people with schizophrenia. Like mind-pops, hallucinations come in the form of words or phrases more often than they do as images or music, and obvious triggers are usually absent. In previous research, Ia Elua—one of Kvavilashvili's graduate students—suggested that hallucinations are constructed from mind-pops.

To begin testing this idea, Elua, Kvavilashvili and her U.H. colleague, psychology professor Keith Laws, surveyed 31 healthy adults, 31 depressed adults and 37 schizophrenic adults about their mind-pops. All the schizophrenic adults had experienced them, but six of the depressed adults and five of the healthy adults said they had never experienced one in their entire lives. Schizophrenic adults also reported experiencing mind-pops on average three to four times a week, whereas the average was once to twice a month for depressed adults and only once or twice every six months for healthy subjects. The results appeared online in March 2012 in Psychiatry Research.

This preliminary evidence suggests that mind-pops are more common among the mentally ill than among the healthy, but it is far too soon to definitely link the sudden memories to hallucinations. Kvavilashvili says she has been working on more studies about the phenomenon, in particular one on musical mind-pops and their relationship to various kinds of "ear worms"—songs that continually replay in people's heads. "The study of mind-popping is still in its infancy," Kvavilashvili says. "So far, recording these cases has made me aware of how wonderfully our perceptual system works. I got curious about them because they seemed so random and out of the blue, but these mind-pops are genuine fragments of knowledge about the world. What it shows us is that our subconscious often knows the meaning of an experience, even if consciously we don't."