Examination and Disturbance of Perception (MSE)

1. Overview & Definitions

Perception = the uptake of stimuli through the sensory organs and their integration in the mind (Gajdos). Perception is the bridge between the external world and the inner mental representation; when it breaks down, the patient may act on a world that is not there.

The Debrecen (Gajdos) lecture classifies perceptual disorders into three groups — memorise this skeleton, the examiner builds the whole topic on it:

1. Perceptual distortions — a real stimulus is perceived but altered in size, shape, colour, intensity or familiarity.
2. Productive disorders — the illusion and the hallucination (the high-yield core).
3. Loss symptoms — agnosias after temporal/parietal lesions (the brain receives the stimulus but cannot process it).

The two definitions every examiner wants, word-perfect:

• Illusion (PSY-3.11, 3.24): febrile child sees the moving curtain and reports "an animal is coming through the window." The stimulus (the curtain, its movement) is real and present — only the interpretation is wrong. Illusions have normal variants (we all misread a coat-rack as a figure in the dark), are not a necessary sign of psychosis, and are commoner in organic states (fever, delirium, intoxication, sensory-organ disease). They appear more readily when attention is unfocused or when there is a strong affective state (affect-driven illusions — fear makes the shadow a burglar).
  • Pareidolia = illusions with fantastic content read into formless objects (faces in clouds, animals in wallpaper) — a normal-variant illusion.
• Hallucination (PSY-3.3, 3.6): a perception with no external stimulus, indistinguishable in quality from real perception. Both illusion and hallucination are symptoms of perception — not of thinking, mood or attention (PSY-3.6). Distinguish sharply from a delusion, which is a disorder of thought content (a false belief, not a false perception).

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