Your first cup of coffee each morning increases your alertness, but a new study suggests caffeine--potentially because of how it interacts with neurons in the brain--might actually hinder your short-term recall of certain words. That is, it may temporarily suppress access to information locked in your memory and unrelated to your current train of thought. For example, you might struggle to remember an acquaintance's name after meeting many new people at a morning meeting.

The findings come from a study in the latest issue of APA's Behavioral Neuroscience (Vol. 118, No. 3) in which researchers Steve Womble, PhD, and Valerie Lesk, both of the International School for Advanced Studies in Trieste, Italy, examined the tip-of-the-tongue (TOT) phenomenon--a form of memory retrieval failure in which someone knows an answer for certain, yet is at a particular moment unable to recall it. They sought to provide a potential neurological explanation of how caffeine affects short-term memory.

Previous research has found that caffeine binds with neural receptors that control excitability--thereby preventing the calming substance adenosine from binding with the neurons and instead enhancing excitement and arousal. When caffeinated people recall short-term information, these caffeine-attracting neurons fire rapidly within the neocortex, which has previously been found to enhance recall and reduce TOT instances.

In the current study, the researchers theorized that when people must recall words that sound like others they've recently heard, caffeine helps further reduce TOT because their caffeinated brains can more effectively focus on similar information.

But, in a departure from previous TOT theories, the researchers hypothesized that TOTs would decrease not so much because of caffeine's well-known alertness effects, but because caffeine-excited neurons would not tire as easily as noncaffeinated ones when people process competing information. Caffeinated neurons, they theorized, would exhibit more plasticity, bolstering short-term memory by not tiring from seeing--and in fact incorporating--extraneous information.

To test the theory, researchers randomly sorted 32 college students into two conditions: a control group and a group ingesting 200 milligrams of caffeine, the equivalent of two cups of coffee. The participants answered 100 general knowledge questions with simple, one-word "target" answers. (For example, one question was, "Name the ancient Egyptian writing," with a target answer of "hieroglyphics.") For each question, they read 10 "priming" words displayed briefly on a monitor. The participants viewed the 10-word lists, where between two and eight of the 10 priming words were phonologically related to the target word (such as "hierarchy" or "glycine"); the rest of the lists were composed of unrelated words like "paint" or "compute." After seeing the 10th word, participants attempted to answer the general knowledge question by providing the target word. If they couldn't, researchers prompted them by saying the word's first phonemes. If participants recalled the target, that trial was scored as a TOT.

The results displayed a pure interaction effect, which surprised the researchers: As expected, caffeinated participants had fewer TOT experiences than the control groups in the related condition--suggesting better memory recall. But surprisingly, the caffeine group did worse than the control in the unrelated condition, where disparate information interfered with memory retrieval. The researchers posit that the frequency of TOTs appears dependent on both caffeine and the relevancy of any extra information.

"The results mean we could rule out a general advantageous effect of caffeine on attention," Womble says. Interestingly, adds co-author Lesk, "The questions that caused the most TOTs were those that centered on information that people had learned well at school but had not used since."