Ever find yourself doing something without knowing exactly why? You might swat at a fly before you consciously realize it is there, or you might catch yourself scratching a bug bite you were trying to forget about. A new study published in Psychological Science could help explain why: half of the brain can be subliminally motivated while the other half is left in the dark.

Researchers at INSERM in Paris first measured how hard 33 subjects could squeeze a grip with each hand. Then they presented the subjects with images on a computer screen of either a one-euro coin or a one-cent coin. The coins were visible to only one eye at a time, and they appeared for only 17 milliseconds­—long enough for sub­liminal, but not conscious, processing. After each coin image flashed, the subjects squeezed the grip with whatever hand they were holding it in—they were told they would win a fraction of the coin’s value depending on the amount of effort they exerted. Each subject got to try all four possible combinations of eyes and hands: right eye with right or left hand and left eye with right or left hand.

Although the subjects could not correctly guess which coin they had seen—confirming that they were not conscious of what they saw—they squeezed harder when presented with the larger coin if the hand grip was on the same side of the body as the eye that had seen it. Their squeezes did not change depending on what the opposite eye saw, indicating that only half the brain was being motivated at a time. Motivation, therefore, is sometimes not only subconscious, explains co-author and INSERM cognitive neuroscientist Mathias Pessiglione, but it can also be “subpersonal,” in that “one part of a person can be motivated while the other is not.” So next time you are surprised to find yourself midaction, consider blaming it on the independent halves of your brain.