Lead author Mary-Frances O'Connor of the University of California, Los Angeles, said that for some it's impossible to let go, and even years later, any reminder of their loss -- a picture, a memory -- brings on a fresh wave of grief and yearning.

The study analyzed whether those with "complicated," or long-term, grief had greater activity occurring in either the brain's reward network or pain network than those with non-complicated grief.

The researchers looked at 23 women who had lost a mother or a sister to breast cancer. The researchers found that, of that number, 11 had complicated grief, and 12 had the more normal, non-complicated grief.

The study participants looked at a photograph of a deceased loved one while undergoing brain scanning by functional magnetic resonance imaging and then a photograph of a female stranger.

The study, published in the the journal NeuroImage, found that while both groups had activation in the pain network of the brain after viewing a picture of their loved one, but only individuals with complicated grief showed significant activity in the region of the brain most commonly associated with reward and social attachment.