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.