Researchers from the Bielefeld University in Germany noted that the clue for bias comes from alpha brain waves -- a pattern of activity when the neurons in the front of the brain fire in rhythm together.
     
Alpha brain waves pop up when people make decisions, but it remains unclear what their role is, they said.
     
The study, published in the journal JNeurosci, used electroencephalography to monitor the brain activity of adults while they made a decision.
     
The participants saw a picture and heard a sound milliseconds apart, and then decided which one came first.
     
Prior to the experiment, the researchers determined if the participants possessed a bias for choosing the picture or sound.
     
Before the first stimulus appeared, the strength of the alpha waves revealed how the participants would decide.
     
Weaker alpha waves meant resisting the bias while as stronger alpha waves indicated succumbing to the bias, according to the researchers.