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People with associative agnosia fail in assigning meaning to an object, animal or building that they can see clearly. Most cases have injury to the occipital and temporal lobes and the critical site of injury appears to be in the left occipito-temporal region, often with involvement of the splenium of the corpus callosum. The clinical "definition" of the disorder is when an affected person is able to copy/draw things that they cannot recognise. The disorder appears to be very uncommon in a "pure" or uncomplicated form and is usually accompanied by other complex neuropsychological problems such as impaired language or memory. The affected individual may not realise that they have a visual problem and may complain of becoming "clumsy" or "muddled" when performing familiar tasks such as setting the table or simple DIY.
There has been debate about whether the fundamental problem in associative agnosia implicates
- Higher-order visual perception - perhaps in integrating the parts of an object into a structured whole
- A disconnection syndrome - a failure of linking vision and perception with language
- Damage to a modality-specific meaning process (semantic system).
Any or all of these interpretations may be appropriate for an individual patient but the same explanation will not necessarily work for all.
Evidence in favour of the semantic hypothesis can be inferred from studies showing that affected people can often show partial knowledge of the things they cannot recognise - such as knowing that an object is a container but not being able to determine whether it is a jug or a mug. They may also have difficulties in deciding whether two different visual forms (visual synonyms) are "the same" - for example, they may be unable to determine that a wine glass and a tumbler are more alike than a tumbler and a vase.