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- Somatosensory agnosia or astereognosia or ahylognosia is a form of tactile disorder and is the inability to identify an object by active touch of the hands without other sensory input. An individual with astereognosis is unable to identify objects by handling them, despite intact sensation. With the absence of vision (i.e. eyes closed), an individual with astereognosis is unable to identify what is placed in their hand. As opposed to agnosia, when the object is observed visually, one should be able to successfully identify the object. People finds it difficult to recognize objects by touch based on its texture, size and weight. However, they may be able to describe it verbally or recognize the same kind of objects from pictures or drawings.
Ahylognosia is one of the four forms of tactile recognition disorder proposed by Daley in 1935. He distinguished it from impairment of the knowledge about the size and shape of an object this he called amorphognosis, or the identity of objects he which he called tactile asymboly
If only one hand is affected individuals are said to have with tactile agnosia and may be able to identify the name, purpose, or origin of an object with their left hand but not their right, or vice-versa, or both hands. Astereognosis refers specifically to those who lack tactile recognition in both hands. In the affected hand(s) they may be able to identify basic shapes such as pyramids and spheres (with abnormally high difficulty) but still not tactilely recognize common objects by easily recognizable and unique features such as a fork by its prongs (though the individual may report feeling a long, metal rod with multiple, pointy rods stemming off in uniform direction). These symptoms suggest that a very specific part of the brain is responsible for making the connections between tactile stimuli and functions/relationships of that stimuli, which, along with the relatively low impact this disorder has on a person's quality of life, helps explain the rarity of reports and research of individuals with tactile agnosia.
Astereognosis is associated with lesions of the parietal lobe or dorsal column or parieto-temporo-occipital lobe (posterior association areas) of either the right or left hemisphere of the cerebral cortex. 
Thought to be connected to lesions or damage in somatosensory cortex.
- ↑ 1.0 1.1 1.2 O'Sullivan, S.B.; Schmitz, T.J. (2007). Physical Rehabilitation, 5th Edition, 1180–1181, Philadelphia: F.A. Davis Company.
- ↑ Beaumant J.G., Kenealy, P.M. & Rogers, M.J.C. (1999). The Blackwell Dictionary of Neuropsychology. Oxford:Blackwell
- ↑ 3.0 3.1 3.2 Gerstmann, J. (2001). Pure Tactile Agnosia Cognitive Neuropsychology, 267–274.