What is agnosia? Development and explanations of disorder.

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Agnosia means 'without knowledge', it is a perceptual disorder where there is an inability to recognise objects. Agnosia can occur in any sensory modality despite an intact sensory system.

Bauer (1993) defined agnosia as:
'a failure of recognition that cannot be attributed to elementary sensory defects, mental deterioration, attentional disturbances, aphasic misnaming, or unfamiliarity with sensorially presented stimuli.'

The most commonly studied types of agnosia are:
1). Visual agnosia
2). Auditory agnosia
3). Tactile agnosia

Visual Agnosia
Patients with visual agnosia have a severe inability to recognise visual stimuli, despite intact sensory abilities.

Lissauer (1890) distinguished between two forms of visual agnosia, these were 'APPERCEPTIVE AGNOSIA' (the inability to recognise visual objects owing to a deficit in perceptual processing), and 'ASSOCIATIVE AGNOSIA' (here perception is relatively normal, but the process of associating an object's percepts with its meaning was impaired).
N.B. These were originally titled 'mindblindness'

For a patient to be diagnosed with agnosia they must not have a sensory impairment.

The term 'agnosia' was first introduced by Freud (1891), Freud argued that the deficits seen in 'mindblindness' could not only be considered a sensory disorder, but also an impairment of existing knowledge of objects.

Agnosia was therefore the 'inability to bring together visual elements into a complete, recognizable whole'.

These definitions have, however been criticised. De Renzi & Lucchelli (1993) suggested that apperceptive agnosia has 'fuzzy boundaries', and is less distinguishable that associative agnosia.

APPERCEPTIVE (VISUAL) AGNOSIA
This is also known as 'Visual object agnosia', it is the inability to recognise objects owing to a deficit in forming stable representations of visual stimuli. Common causes of visual object agnosia include;

1). Carbon monoxide/mercury poisoning
2). Cardiac arrest
3). Stroke/bilateral paterior cortical atrophy

Patients with this disorder are unable to copy drawings or match objects, although they are able to identify them. They may find pictures of letters difficult to identify. They also find it difficult to identify objects when shadows are cast upon them (Warrington, 1982).

These impairments are associated with right inferior parietal lobe lesions.

Humphreys & Riddoch (1987) suggest that the inability to recognise the 'shape' of an object and an inability to discriminate between different sharpes is termed 'shape agnosia'.

The inability to intergrate stimuli to create a whole is called ...