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= Tactile Agnosia = Tactile agnosia refers to the inability to identify objects or their purpose using solely the sensation of touch without the presence of other somatosensory deficits or cognitive deficits to explain the impairment. The organization of input of tactile sensory information has allowed the deconstruction for the types of tactile agnosia. Tactile agnosia can be present both unilaterally or bilaterally, but its rarity and variance has prevented an exact structural location that links to its presentation in individuals. Nonetheless, there are several supported research hypotheses about what brain areas could be involved. It is important to note that tactile agnosia involves only an object identification impairment of tactile processing, and therefore individuals who show impairments regarding connection between tactile-to-verbal processing or deficits in object discrimination would not be cases of agnosia.

History
Tactile agnosia historically has been a controversial topic, which is not dissimilar to what visual agnosia has encountered. However, tactile agnosia stands as much more rare than visual agnosia and thus, has presented with more complication and speculation in the separation between other tactile deficits. One of the first cases of tactile recognition impairment without any other somatosensory deficits for explanation was reported by Carl Wernicke in 1895. It was then that Wernicke proposed the separation of primary (recognition of qualities) versus secondary (recognition of use and name) identification of an object. Since then, continuous speculation and research has been done into how exactly tactile information moves through the brain and what results when certain connections are broken. The point of confusion was with individuals who showed they had the ability to tactually distinguish certain qualities like texture, weight, and temperature of an object, but could not sufficiently discriminate between certain sizes and shapes (and vice versa).

It was not until 1935 that Delay set forth the separation of primary tactile processing into hylognosia (the recognition of structure of an object) and morphagnosia (the recognition of size and shape of an object). This created separate routes in the organization of tactile processing that could be disrupted and lead to tactile agnosia. Since then, research of tactile agnosia has further referenced the schematic layout of tactile processing initially foretold by Wernicke and Delay.

Nonetheless, there are many researchers who have hypothesized against tactile agnosia and its existence. Arguments against tactile agnosia mainly take two stances, (1) tactile agnosia is secondary to impairments in spacial perception and (2) motor deficits in patients can explain the lack of recognition of an object because there is a lack of ability to explore an object features or effectively manipulate the object. Each stance has been directly rebutted. Reed, Caselli, and Farah reintroduced E.C., a subject diagnosed with tactile agnosia, and conducted several spacial tasks that rule out the suspected supra-model influence of impaired spacial abilities. E.C., along with other cases of tactile agnosia since, performed splendidly on the spacial tasks once used against the credibility of the diagnosis' existence. Regarding the argument of motor deficits, hemiparesis or initial numbness can be found in some individuals with tactile agnosia due to how the vasculature of the brain is structured and the lack of locality of lesions. However, there are exploration and motor task assessments that have been performed on cases to ensure that exploration and motor deficits are not present, or that they do not explain the severity of the impaired tactile object recognition. Instead it has been argued that the motor deficits seen specifically dealing with the agnostic hands are actually a result, not the cause, of the tactile agnosia.

While tactile agnosia can still be an open topic of debate by some research, there continues to be more recognition in the development of tactile agnosia and how to dissociate the diagnosis.

Organization and Types
The different divisions of tactile agnosia was compared similarly to that of visual agnosia for quite some time, and the types are named after those of visual agnosia. Although, unlike the understood schematic model of visual agnosia involving one route of visual processing divided into simply apperceptive and associative, a single route of tactile information could not fully explain the cases of those with impaired tactile processing. So when Delay determined the separation of structural and spacial qualities of tactile processing, the categorization of tactile agnosia could then be grounded. Tactile agnosia can be described as either apperceptive or associative, but apperceptive can present in three different ways.

Apperceptive Tactile Agnosia
Apperceptive tactile agnosia is defined by a disruption in the lower levels of tactile processing between the input of elementary sensory data and the formation of a structural description of an object. This could present in three different ways :


 * 1) Individuals could have the inability to process intermediate qualities of an object (such as weight, temperature, or texture)
 * 2) Individuals could have the inability to process spacial qualities of an object (size and shape)
 * 3) Individuals could have the inability to process all of the qualities of an object.

The key is that all three of the routes must be followed by severe deficits in the tactile object recognition of objects to be defined as tactile agnosia. Detecting apperceptive agnosia can be done in many ways, but one is that as seen in apperceptive visual agnosia where drawings of objects after tactile exploration would appear distorted or unidentifiable based on severity.

Associative Tactile Agnosia
Associative tactile agnosia can be defined by a disconnect between higher levels in tactile processing—disturbance between having a preserved description of an object and accessing the semantic memory. It is purely based on recollecting prior knowledge about objects. Individuals with associative tactile agnosia are able to tactually formulate an objects description. However, despite describing the object accurately in detail, they cannot recognize the object or recall its purpose. Parallel to associative visual agnosia, individuals are able to produce accurate drawings of the objects after tactually exploring them, yet are still unable to identify the object or its use.

Regional/Topographical Links
Discovering the exact brain regions and structures that play a part in tactile agnosia is difficult, as most individuals with tactile agnosia have lesions in several areas of the brain and/or across a certain span of regions. Due to this, discerning which areas are part of the concern is not clean-cut. However, studies have shown some overlap of areas. Additionally, cases of tactile agnosia like E.C. where there are very focal lesion sites have supported hypotheses for what regions play a role.

Lesions that have been linked to tactile agnosia have involved the inferior parietal lobule, and more specifically, the supramarginal gyrus and parts of the angular gyrus. There have also been several cases with additional lesions upon the middle third of the postcentral gyrus, which is also the primary somatosensory cortex. It is important to note that this is in either hemisphere of the brain. Depending on the case, tactile agnosia would normatively presents in the hand contralateral to the lesion site. Although less common, tactile agnosia can occur bilaterally (in both hands).

How these areas are involved in tactile processing is still not clear. And with a relatively recent case of tactile agnosia reporting only a lesion on the posterior portion of the body of the corpus callosum, researchers are still determining the topographical mapping of tactile agnosia.