Disorders of Perception

Overview of Perceptual Abnormalities

Perceptual abnormalities or aberrations are common features of psychiatric and neurologic illness. Non-ill persons also experience occasional perceptual disturbances, but the presence of frequent or intense aberrations indicates serious system disease.

Disorders of perception occur in all sensory modalities. They include misinterpretations and distortions of environmental stimuli, as well as self-generated hallucinations.

Patients vary in their ability to explain their subjective perceptual experiences. The brain constantly receives large amounts of perceptual information via the five special senses of vision, hearing, touch, taste, and smell; the muscle, joint, and internal organ proprioceptors; and the vestibular apparatus.

The majority of this information is processed unconsciously and only a minority reaches conscious awareness at any one time. An external object is represented internally by a sensory percept that combines with memory and experience to produce a meaningful internal percept in the conscious mind.

In health, we can clearly distinguish between percepts which represent real objects and those which are the result of internal imagery or fantasy, which may be vividly experience in the mind but are recognized as not real.

Abnormal perceptual experiences may be divided into two subgroups:

  1. Altered perceptions—including sensory distortions and illusions Opens in new window—in which there is a distorted internal perception of a real external object.
  2. False perceptions—including hallucinations Opens in new window and pseudo-hallucinations—in which there is an internal perception without an external object.

Sensory distortion

Sensory distortions are changes in the perceived intensity or quality of a real external stimulus. They are associated with organic conditions and with drug ingestion or withdrawal.

Hyperacusis (experiencing sounds as abnormally) and micropsai (perceiving objects as smaller and further away, as if looking through the wrong end of a telescope) are examples of sensory distortions.


Illusion is defined as “misinterpretations of a sensory stimulus.”

Illusions Opens in new window occur in conditions of inattention, low sensory stimuli, and lower level of consciousness or anxious state. Illusions lack diagnostic significance and are different from hallucination by the presence of a stimulus.


Hallucination is defined as “a false perception which is not a sensory distortion or a misinterpretation, but which occurs at the same time as real perceptions.”

Individual believes and reacts to hallucinations as a real perception. Hallucinations are described according to the involved sensory modality.

  1. Auditory hallucinations

These are the most common type of hallucinations seen in psychiatric patients.

  • The patient may hear clear voices communicating to the patient (second person hallucination),
  • talking among themselves about the patient (third person hallucination),
  • commenting about the patient’s activities (running commentary type hallucination),
  • or instructing the patient (commanding hallucination).
  1. Visual hallucinations

These are less common than auditory hallucinations. The patient may see people, objects, or animals and sometimes as a complete scene like a movie (scenic hallucination).

Their presence in the absence of auditory hallucination should raise suspicion of some organic brain disorder.

  1. Olfactory and gustatory hallucinations

Olfactory and gustatory hallucinations involve smell and taste, respectively.

Their presence in isolation should raise suspicion of temporal lobe epilepsy or other organic brain disease. They may be present in schizophrenia.

  1. Tactile hallucination

These can be a superficial sensation over or under the skin (tactile or haptic hallucination) or a felling of sensations like pressure, tearing or twisting in deep visceral organs (visceral hallucination).

Special types of hallucinations are given in Box X-1.

Box X-1 | Special types of hallucinations
Reflex hallucination: Stimulation in one sensory modality induces hallucination in other sensory modality
Autoscopic hallucination: Seeing own body in external space
Extracampine hallucination: Hallucinatory experiences beyond the normal sensory range
Lilliputin hallucination: A type of visual hallucination seen in delirium where the patient sees dwarf figures

Other than the above types of perceptual abnormalies, depersonalization and derealization are also considered to belong to the group.

Depersonalization is characterized by a feeling of unreality or unfamiliarity with oneself whereas derealization is unfamiliarity with surroundings.

    Adapted from:
  1. Essentials of Psychiatry By Dr Sandeep K Goyal