Agnosia is the inability to process sensory information. Often there is a loss of ability to recognize objects, persons, sounds, shapes, or smells while the specific sense is not defective nor is there any significant memory loss. It is usually associated with brain injury or neurological illness, particularly after damage to the occipitotemporal border, which is part of the ventral stream. Agnosia only affects a single modality, such as vision or hearing. More recently, a top-down interruption is considered to cause the disturbance of handling perceptual informationAgnosia can result from strokes, dementia, or other neurological disorders. It may also be trauma-induced by a head injury, brain infection, or hereditary. Additionally, some forms of agnosia may be the result of developmental disordersDamage causing agnosia usually occurs in either the occipital or parietal lobes of the brain. Although one modality may be affected, cognitive abilities in other areas are preserved.

Patients who experience dramatic recovery from blindness experience significant to total agnosia.

The effect of damage to the superior temporal sulcus is consistent with several types of neurolinguistic deficiencies, and some contend that agnosia is one of them. The superior temporal sulcus is vital for speech comprehension because the region is highly involved with the lexical interface. According to the 1985 TRACE II Model, the lexical interface associates sound waves (phonemes) with morphological features to produce meaningful words. This association process is accomplished by lateral inhibition/excitement of certain words within an individual's lexicon (vocabulary). For instance, if an experimenter were to say DOG aloud, the utterance would activate and inhibit various words within the subjects lexical interface:

DOG activates 3, and inhibits 0 letters in DOG. – +3

DOG activates 2, and inhibits 1 letters in FOG. – +2

DOG activates 1, and inhibits 2 letters in DAN. – +1

The consistency of this model to agnosia is shown by evidence that bilateral lesions to the superior temporal sulcus produces 'pure word deafness' (Kussmaul, 1877), or as it's understood today—speech agnosia. Patients with pure word deafness demonstrate the inability to recognize and process speech sounds with normal auditory processing for non-speech sounds below the level of the cortex.

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Journal of Phonetics and  Audiology
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