# 07 - 2. Disorders of perception

# 2. Disorders of perception

© SPMM Course 
2. Disorders of perception 
Perception consists of two parts – receiving information from a sensory modality (bottom up) and 
interpretation or processing of the sensation instantaneously using cognitive faculties (top down). 
Normally, any perceived object corresponds to the stimulus that elicited it. 
Perception occurs in visual, auditory, tactile, gustatory, olfactory, kinaesthetic or proprioceptive 
modalities – any distortions in perception could also occur in any of these domains. 
Perceptual errors can occur at different levels – 
Perceptual disorder 
Stimulus 
present? 
Corresponding object 
perceived? 
Error 
Perceptual distortions 
Yes 
Yes 
Object’s quality altered 
Illusions 
Yes 
No 
A different object is perceived 
Hallucinations 
No 
Yes 
Perception without a stimulus 
Negative hallucinations 
Yes 
No 
No object is perceived 
 
 If a stimulus is perceived as corresponding object but not accurately – changes in physical properties 
e.g. size, shape, intensity and colour - this is a perceptual distortion. In depression and hypoactive 
delirium there is dulled perception; intense perceptions can occur in mania, hyperactive delirium and 
drug-induced states (hallucinogens). Hyperacusis especially is seen in migraine and alcohol hangover. 
 
Changes in the shape of objects especially with the loss of symmetry are called dysmegalopsia. 
 
The objects can shrink in size – micropsia or enlarge - macropsia. 
 
These are usually organic – could be ictal (parietal) or ocular (accommodation errors – 
paralysed accommodation can cause micropsia), rarely in acute schizophrenia. Hallucinogens 
(Mescalin) can also change the colour of perceived objects or make components of an object e.g. 
body parts – to be seen detached in space. 
 Stimulus is perceived as an object but not corresponding to the source – both stimulus and object are 
present, but different from each other – illusions. 
 There is no stimulus but perception occurs – hallucinations. 
 There is a stimulus but no perception occurs – negative hallucinations.