An hallucination can be defined as a sensory experience that occurs in the absence of corresponding external stimulation of the relevant sensory organ(s). In addition, the experience has a sufficient sense of reality to resemble a veridical (genuine) perception and occurs when one is awake; furthermore, one does not feel that one has direct and voluntary control over it. Thus, an hallucinating person sees things that are not there or hears voices while nobody in the vicinity is speaking, for example. Although visual and auditory hallucinations are most common, hallucinations of taste, smell, and touch can also occur. The exact mechanisms underlying the experience of hallucination remain elusive, but significant strides have been made in recent years to uncover processes in the mind and brain that are associated with hallucinations.