Psychiatric disturbances, notably depression, occur frequently as co-morbid conditions with epilepsy. A complex, probably bidirectional relationship between epilepsy and depression has been postulated. Both epilepsy and depression also interact with stressful life events, but only some patients develop these disorders after a stressful event, indicating the possibility of a "vulnerable" population. Animal and human studies have looked at the role of brain derived neurotrophic factor (BDNF) in this context. Low serum and/or CSF levels of BDNF are associated with higher incidence of depression, and thus indicate the vulnerable population.