A person who laughs one minute and cries the next without any clear stimulus is said to have:

This patient is showing features of labile affect. Affect refers to a feeling or emotion, which is experienced by a person in response to an external event or a thought. A person with labile affect experience a sudden change of emotion from one type to other. Lability is associated with emotional intemperateness, an abrupt unreflective expression of heightened emotion. Labile mood is seen in mood disorders such as bipolar illness and in borderline personality disorder. It may also occur in frontotemporal dementia and pseudobulbar palsy. Ref: Nurcombe B., Ebe M.H. (2008). Chapter 4. The Psychiatric Interview. In M.H. Ebe, P.T. Loosen, B. Nurcombe, J.F. Leckman (Eds), CURRENT Diagnosis & Treatment: Psychiatry, 2e.