Brain laterality as a determinant of susceptibility to depression in an animal model.

Rats exposed to stressors that cannot be controlled may develop a deficit in their ability to subsequently learn to control a new stressor. This phenomenon is known as 'learned helplessness' and is a well-accepted animal model of depression. Evidence is presented showing that rats having different directional biases of brain laterality, as indicated in tests of rotational behavior, differ greatly in their response to stressors and to the lack of stressor control. Differences in brain laterality appear to be an important source of variability within the animal model of depression. As with humans, only some rats are vulnerable to depression-like symptoms. These findings are relevant to biological theories of depression that are based upon lateralized specialization of the human brain for affect.