A method of determining functional brain connectivity and its application as a biomarker is presented. Brain activity signals are measured to detect a functional connection between a first region of a brain and a second region of said brain. The direction of the functional connection is determined by determining whether said first region excites or inhibits the second region more strongly than the second region excites or inhibits the first region. The functional connectivity is then used as a biomarker to predict the existence of a condition, such as epilepsy or autism. The functional connection may be measured while said brain is in a resting state.
