The truth is that we all live by leaving behind …
—Jorge Luis Borges,
Funes the Memorious
The title character of Jorge Luis Borges’s short story Funes the Memorious lives a tortured life: a riding accident left him without the ability to forget. He spends his days alone in the dark, trying to avoid adding to the store of memories that constantly impinge on his consciousness.
It would be impossible to function if we were not able to block out all but the relevant memories. Indeed, the ability to not dwell on distracting or unpleasant experiences is a vital coping mechanism. Thus, the brain must have ways to suppress memories.
Sigmund Freud studied not only ways in which the brain might unconsciously block unwanted memories, but also the idea that we might actively, consciously help ourselves forget. ©Science Photo Library/ Photo Researchers, Inc
Influenced strongly by the ideas of Sigmund Freud, Western society has long entertained the idea that the mind has mechanisms to suppress especially traumatic unwanted memories. We take it for granted that Freud was focused only on ways in which the mind might unconsciously, and automatically, block access to unwanted memories, but Freud also proposed that such forgetting may be served by a more active, conscious process. The mechanisms that might underlie such an ability have remained unclear and difficult to investigate, but recently psychologists and neuroscientists have taken fresh aim at this subject.
In a study in 2001, researcher Michael Anderson of the University of Oregon demonstrated that the brain could be made to suppress unwanted memories. His method, termed “think/no think,” was quite simple: he asked subjects to study a pair of words. The next time they saw one of those words, he asked them to not think about the word with which it had been paired. With repetition, subjects indeed had a harder time recalling the response (or target) word than subjects who had not been instructed to suppress the word.
So is the memory for the suppressed words still there? Based on related research, Anderson is inclined to think so, though there is no direct evidence of whether the memory itself has been diminished, or simply the ability to access it.
Anderson wonders whether his think/no think paradigm replicates what might be at work with more complex memories, especially traumatic memories. “When we see something in the environment that’s upsetting, we put up a mental hand that signifies, ‘I don’t want to think about that right now,’ without necessarily having the intention to forget anything,” says Anderson.
Once More, with Emotion
Anderson’s findings have been thrust into some highly charged debates, including those about the recovery of “lost” memories in cases of childhood abuse. But they may also be relevant to some psychiatric disorders in which people suffer from the intrusion of unwanted thoughts: posttraumatic stress disorder, obsessive compulsive disorder, and some forms of anxiety disorders and depression.
Anderson acknowledges the critics who question whether something as simple—and emotionally neutral—as memory for words proves that more complex, emotionally charged personal experiences could be actively forgotten. Marie Banich and her colleagues Brendan Depue and Tim Curran at the University of Colorado have taken an initial step in expanding Anderson’s findings. In a recently completed study that has been submitted for publication, they have adapted Anderson’s think/no think method to investigate whether the same effects would hold for words with emotional content, as well as more complex information such as very unpleasant visual scenes.
In a first round of experiments, Banich and colleagues used photographs of faces as cues and paired these with word targets that were either emotionally neutral (such as “carriage”) or likely to evoke an emotional response (such as “murder”). The researchers found that even with the image cues and emotion-laden word targets, study subjects were able to suppress the memories when instructed not to think about them.
Still, because research has shown that emotional content helps solidify memories, Banich and colleagues expected to find it more difficult to suppress emotional words. Surprisingly, they found the opposite: subjects did a better job of suppressing the memories of emotion-laden words than of neutral words.
Researchers found that while study subjects tried to suppress memories, the prefrontal cor-tex became more active and the hippocampus became less active. This finding indicates inhibition of memory processes. © Michael C. Anderson, associate professor of cognitive neuroscience, University of Oregon
“Our interpretation of these effects is that the more salient an item is, the better your ability to exhibit cognitive control over it,” says Banich. A similar result was found in subsequent experiments when the word targets were replaced by image targets, either neutral scenes such as a wildlife photo or emotional ones such as a photo of a burn victim.
At the University of Washington, Susan Joslyn and Mark Oakes have focused on real-life memories of their subjects to assess whether truly complex memories are subject to the same voluntary forgetting as word lists. In a soon-to-be-published series of experiments, Joslyn and Oakes asked subjects to write short accounts of daily episodes in their lives. Then, on an invented pretext, some of the subjects were asked to forget those memories, and continue writing up accounts of new episodes (a method called “directed forgetting”).
The researchers even set up one of their experiments to run during the week of Valentine’s Day, the idea being to ensure that many of the memories thus recorded would have emotional value. Later, when tested on their recall of those events, subjects who had been directed to forget about the Valentine’s week events were less able to remember those events than subjects who had received no such instructions.
The next things to uncover, Joslyn says, are the psychological mechanisms and strategies that people employ to forget things. “With directed forgetting of word lists, it has been suggested that people remember items they are directed to forget less well because they rehearse them less often. Another possibility is that people suppress or inhibit the memory so that it’s less available at recall,” she says. For the think/no think form of forgetting that Anderson and Banich’s group studies, the latter—suppression of the memory—appears to be the case.
Forgetting in the Brain
Another direction for this research is to understand how the brain controls which memories are available for access. Last year Anderson, in collaboration with John Gabrieli and others at Stanford University, reported on a study using magnetic resonance imaging to track brain activity as study subjects obey the “don’t think” command. They found that two important brain areas—the prefrontal cortex and the hippocampus, a structure critical for memory formation—changed their activity levels while subjects were trying to suppress memories. The prefrontal cortex became more active, while the hippocampus became less active, suggesting an inhibitory effect on memory processes. Indeed, Anderson and colleagues were able to predict how much memory suppression a subject would show on a later test by how much activity they measured in the prefrontal cortex while the subject obeyed the “no think” instruction.
The involvement of the prefrontal cortex was particularly interesting to Anderson because it suggests that the suppression of memory is effected in the same way, by the same brain centers, as the suppression of movements. He likes to use an example from personal experience to illustrate: he once reached to grab a plant falling from a window sill, but halted his reflexive action when he realized it was a cactus.
Such overriding of reflexes (called “executive control”) is exercised by regions of the prefrontal cortex, which can directly influence the brain centers that control reflexive behaviors. Anderson proposes that in his think/no think paradigm, executive control is also being exerted by the prefrontal cortex on the hippocampus to suppress memory in much the same way.
Borges’s character Funes the Memorious was buffeted by memories to the point that it made it hard to sleep. At night Funes tried to sleep by focusing on some newly built houses that he had never seen. “Funes imagined them black, compact, made of a single obscurity; he would turn his face in this direction in order to sleep,” writes Borges. Perhaps the research being conducted into the suppression of memory will offer more permanent solutions to patients with disorders that force unwanted memories into their consciousness.