A perception in this context is any sensory experience, and the three broad types of perceptual disturbance are hallucinations, pseudohallucinations and illusions. A hallucination is defined as a sensory perception in the absence of any external stimulus, and is experienced in external or objective space (i.e. experienced by the subject as real). An illusion is defined as a false sensory perception in the presence of an external stimulus, in other words a distortion of a sensory experience, and may be recognized as such by the subject. A pseudohallucination is experienced in internal or subjective space (for example as "voices in my head") and is regarded as akin to fantasy.