Using neuroimaging to expose the unconscious influences of priming

In 1996, a group of researchers at NYU conducted an interesting experiment. First, they had NYU students work on unscrambling letters to form words. Unbeknownst to the students, they had been split up into three groups, and each group unscrambled letters that formed slightly different words. One group unscrambled words with a "rude" connotation like aggressively, bold, and interrupt. Another group unscrambled "polite" words like considerate, patiently, and respect. And the third group unscrambled neutral words like watches and normally.

The students were told they should come find the experimenter, who would be waiting in a different room, after they finished the unscrambling task. This, however, was just another part of the experiment. When the students walked up to the experimenter, he was engaged in a conversation with someone else (who was actually in on the experiment). The experimenter stood in such a way that it was clear he knew the student was waiting for him, but he nevertheless continued his conversation and didn't acknowledge the student.

In fact, the experimenter continued talking for 10 minutes unless the student interrupted to draw attention to the fact that he or she was done with the unscrambling task (and being somewhat rudely ignored). What the experiment really had set out to determine was if the type of words the students unscrambled seemed to have an influence on whether or not they interrupted the experimenter. Interestingly, about 80% of students who unscrambled polite words waited a full 10 minutes without interrupting, while only 35% of the students who unscrambled rude words waited that long. On average the rude-word group only waited 5.4 minutes, compared to the polite-word group's 8.7 minutes. The students, of course, were not aware that the words they unscrambled had any effect on their patience, or lack thereof.

Now, think about the implications of this experiment in your daily life. If its findings are valid--and it's worth noting that this particular area of research has been criticized for the publication of studies that others have been unable to replicate--it suggests that information that we are not consciously aware of shapes our thoughts and behavior. Taken a step further, we could begin to question how much of our behavior is even under our own conscious control. For example, you might swear that fight you got into with your significant other was about doing the dishes and it never would have happened if he/she hadn't blatantly disregarded your strong opinions--yet again--about leaving dirty dishes in the sink. But maybe your inclination towards hostility had been influenced by that jerk who cut you off in traffic an hour prior, causing you to overreact negatively and call it quits on an a relationship that was pretty good despite a lack of harmony on the relatively minor issue of timely dish washing.

The influence a previous experience has on our likelihood of responding in a particular way later on is known as priming. It was first discovered in the 1970s through a series of simple experiments exploring response time in tasks like determining if groups of letters represented English words. For example, in one such experiment researchers presented participants with pairs of words. Sometimes the words used were actual English words (e.g. butter), other times they were nonsense words (e.g. nart), and they were presented in different combinations of each. The researchers found that participants were able to identify something as an English word more rapidly if the word presented previous to it had a related meaning (e.g. the first word was nurse and the second was doctor). Since then, a number of experiments have investigated this effect a previous experience can have on a subsequent response, showing that it can influence everything from reaction time to subtleties of behavior like the speed at which someone walks.

Priming and memory

Priming is considered an an example of implicit memory, a term that describes a type of memory that can influence behavior even though we aren't consciously aware of it. We use a form of implicit memory called procedural memory every day when we engage in tasks that we have performed countless other times, like tying our shoes. In these cases we don't consciously think of the process involved in doing the job (often we are thinking of something quite different), but clearly we retain a memory of how to perform the task, and that memory facilitates its execution.

The influence of priming extends much further than shoe-tying, however. Although it may be difficult for us to accept, our implicit memory seems to affect the beliefs we hold and the decisions we make. Because our brains are so good at forming connections between things we see around us and things we have seen or learned in the past, our implicit memory is being accessed on a continuous basis. For example, in another study researchers put participants in two groups: one group filled out a questionnaire in a room that smelled strongly of citrus all-purpose cleaner, while the other filled out a questionnaire in a room with no apparent odor. Then, the researchers had both groups eat a crumbly biscuit. The group that had been exposed to the citrus smell was significantly more likely to clean up the crumbs from their biscuit. Even though they weren't consciously thinking about it, the citrus scent (hypothetically) conjured up implicit associations with cleanliness, which prompted the participants to clean up after themselves.

Priming and the brain

Understanding the neuroscientific correlates of priming has not been simple, in part because it seems to involve a diverse selection of brain areas. One general finding has been that there is a reduction in brain activity during exposure to a primed stimulus (i.e. a stimulus that has been preceded by priming) vs. an unprimed stimulus. For example, if you prime someone by exposing them to words related to transportation, then ask them to unscramble letters that could readily be formed into words like traffic or drive, you will see less activity in their brains than if you hadn't primed them. This should make intuitive sense, as the brain that has been primed is not having to work as hard. It can rely on cues from implicit memory to bring to mind potential words the letters might form.

One reason we tend to see many brain regions involved in priming is that different systems are used to process different types of stimuli--as well as different aspects of the same stimulus. For example, if the primed stimulus involved the meaning of a word, then we would see a decreased response to the primed stimulus in a number of areas of the brain associated with processing different aspects of a word, like meaning, spelling, phonology, and so on. If the primed stimulus involved an odor, we would see a reduction in brain activity in very different regions.  

There are also, however, some commonalities in the neural activity underlying priming across different types of stimuli. For example, regions of the inferior temporal cortex and inferior frontal gyrus have been found to respond to abstract qualities of stimuli, and thus they are activated even when the prime and the primed stimulus are presented in different ways. One study, for instance, saw activity in these areas when the prime involved normally-oriented words and the primed stimulus involved mirror-reversed words. The inferior temporal cortex and inferior frontal gyrus are also activated in response to primed stimuli of different perceptual modalities (e.g. auditory and visual), and they are still activated when the prime and the primed stimulus are each presented in a different modality. Thus, it may be that areas like these mediate the priming of concepts, regardless of how the stimulus is introduced and initially processed.

Neuroimaging evidence also suggests that the prefrontal cortex may play an especially important role in priming, as it is another area where activity is reduced in response to a number of different types of primed stimuli. The prefrontal cortex is frequently associated with executive functions, and as such it is involved in managing the activity of a network of brain areas in retrieving memories and handling other cognitive duties. Having an implicit memory to draw upon, however, may make its job a little easier, allowing the prefrontal cortex to work more efficiently to complete the task at hand. Thus, reduced activity in the prefrontal cortex during exposure to a primed stimulus may generally represent a decreased reliance on the conscious processing of a stimulus due to the contributions of implicit memory.

These are some of the patterns of brain activity that we can associate with priming, but what's really going on still remains somewhat unclear. Regardless, priming seems to be a process that is occurring in our minds all the time. And this feature of the way our brains work is also opportunistically manipulated by others every day, especially those who are trying to sell you something. Take Cinnabon, for example, the baked goods chain known for their American-sized cinnamon rolls. They intentionally place their ovens near the front of their stores so the smell of fresh-baked rolls drifts toward the entrance. Because they strategically put their stores primarily in enclosed buildings like malls and airports, these drifting aromas are more likely to be smelled by those who are passing by. Some of these passersby will then make an impulsive decision to stop in and buy a cinnamon roll. Is this decision free of any priming effects induced by the enticing odors emanating from the store? It's hard to say, but considering that Cinnabon's sales were lower when they tried putting the ovens at the back of stores, it would seem priming is playing a role in many decisions to indulge.

Even your desire to read this article might be able to be traced back to some priming influence that occurred earlier today, last week, or last year. The same could be said for my intention to write it. All of this leads to the inevitable question: with these multitudinous influences on our behavior from unconscious associations with words, sounds, smells, colors, etc., how much of our behavior do we really control? Don't think too hard about trying to answer that question, because you've likely already been primed to respond in a particular way.

Schacter, D., Wig, G., & Stevens, W. (2007). Reductions in cortical activity during priming Current Opinion in Neurobiology, 17 (2), 171-176 DOI: 10.1016/j.conb.2007.02.001