A practical map of perception #
Perception is the experienced interpretation of sensory information. An account of an unusual experience should establish whether there was a relevant stimulus, how it was experienced, and what meaning the person gave it. A perceptual change, a belief about that change and behaviour in response are different findings. Record them separately.
| Phenomenon | Central distinction | Synthetic example |
|---|---|---|
| Sensory distortion | An existing stimulus is experienced with altered intensity, quality or spatial properties. | A familiar sound seems unusually intense. |
| Illusion | A real stimulus is misinterpreted. | A coat in poor light is momentarily mistaken for a person. |
| Hallucinatory experience | A perception-like experience occurs without a corresponding external stimulus. | A voice is heard while no one is speaking. |
| Imagery | A mental representation may be recognized as imagined; vividness and control vary. | Recalling the sound of a familiar song. |
These categories help describe form. None identifies a cause by itself. Experiences around sleep, during sensory impairment, with neurological illness, under substance effects or in psychiatric conditions require different contextual questions.
Sensory distortions #
| Term | Description | Clarification |
|---|---|---|
| Hyperaesthesia | Increased sensitivity or intensity of sensation. | Specify the sensory modality and the person’s description. |
| Hyperacusis | Reduced tolerance or heightened sensitivity to ordinary sounds. | Distinguish the sound experience from fear of a particular sound. |
| Micropsia | Objects appear smaller than their usual size. | Ask about onset, duration, vision and associated symptoms. |
| Macropsia | Objects appear larger than their usual size. | Separate altered perceived size from a belief about the object. |
| Altered colour or spatial experience | Changes in colour, shape, depth, distance or movement. | Describe the actual experience rather than relying on a rare label. |
Terminology in older descriptions is not always consistent. An accurate ordinary-language description can communicate more than an uncertain technical word. New visual or other sensory changes may require ophthalmic, neurological or medical assessment.
Explore each relevant modality #
- Auditory: voice or other sound, words, number, identity, location, loudness, frequency and relation to surrounding sounds.
- Visual: formed image or elementary light, colour or shape; movement, field and relationship to eye opening or visual conditions.
- Olfactory and gustatory: smell or taste, persistence, source attribution and associated neurological or physical symptoms.
- Tactile and bodily: surface sensation, internal bodily experience, movement or position, and the explanation attached to it.
- Across modalities: whether experiences occur together, in sequence or independently.
A voice experienced “inside the head” is not automatically ordinary thought, and an experience described as external does not establish psychosis. Clarify perception-like quality, agency, control, conviction and context instead of classifying on location alone.
Context, insight and consequences #
Establish wakefulness, sleep transition, sensory impairment, fever, seizures, medications, substances and changes in attention. Ask whether the experience occurs only in a particular situation and whether the person can compare it with other people’s perceptions. Insight can vary within the same person and across time.
Clinical importance includes distress, sleep, attention, daily activities and behaviour. Some experiences are neutral or comforting. Others lead to avoidance, self-neglect or actions in response. Assessment must describe these consequences rather than assume them from the perceptual form.
Observe and document carefully #
Looking around, pausing or talking when alone can prompt questions, but does not prove hallucinations. Conversely, lack of visible responding during an interview does not invalidate a reported experience at another time. Specify the observation window and separate reported symptoms from inference.