Our thoughts are susceptible to external influence, even against our will

This nifty little cognitive trick is not only amusing -- the study also reveals that the stream of consciousness is more susceptible to external stimuli than had previously been proven. This research is the first demonstration of two thoughts in the stream of consciousness being controlled externally and against participants' will.

"Our conscious thoughts seem protected from our surroundings, but we found that they are much more tightly linked to the external environment than we might realize, and that we have less control of what we will think of next," said Ezequiel Morsella, associate professor of psychology and co-author of the study.

Morsella and his team showed the study's participants 52 black-and-white images corresponding to familiar words of varying lengths -- basic drawings including a fox, heart and bicycle. Participants were instructed not to subvocalize (speak in the mind) each word or how many letters the word had. On average, 73 percent subvocalized a word, and 33 percent counted its letters.

"We triggered with our experiment not one but two different kinds of unintentional thoughts, and each thought required a substantial amount of processing," Morsella said. "We think that this effect reflects the machinery of the brain that gives rise to conscious thoughts. When you activate the machinery -- and it can be activated even by being told not to do something -- the machinery cannot help but deliver a certain output into consciousness."

The study found that people were much more likely to experience counting subvocalizations of shorter words. For words with three letters, 50 percent of participants reported counting. At six or more letters, the rate dropped to just over 10 percent. "It shows you the limits of the unconscious machinery that generates conscious thoughts -- it seems that it can't count above four or five," Morsella said. He added that the limits to the automatic triggers are not clear, nor is it understood why they exist.

Morsella said that this research has important implications for the study of psychopathological disorders that afflict people with uncontrollable repetitive thoughts or, more commonly, the inability to stifle an obsession. "When people have a thought they can't control, this machinery may be at work," Morsella said. "We're learning not only that the brain does work this way, but that unfortunately, under most circumstances, the brain should work like this."

While it may seem counterintuitive, Morsella argues that the mind's inability to shut out unwanted thoughts is a good thing in most cases. "A lot of things that seem bad about the brain reflect part of its overall architecture, which was selected through evolution because, in most cases, it is adaptive," Morsella said.

Take guilt, for example. Just like most people can't stop themselves from subvocalizing the word "sun" in response to an image of one, it can also be difficult to repress negative feelings after doing something wrong. "If you could override these kinds of thoughts, it would not be adaptive," Morsella explained. "There is a reason why we feel guilt: to change future behavior. If you could snap your fingers and not feel guilty about something, guilt would cease to have a functional role."