Rabu, 17 Desember 2014

Alexia (from Greek ἀ- (a-), meaning "absence of, without", and λέξις (lexis), meaning "word") is a brain disorder in which a person is unable to understand written words. It refers specifically to the loss, usually in adulthood, of a previous ability to read.

Alexia is most often associated with damage to the cortex in the temporal lobe, parietal lobe or occipital lobe, which may arise from brain injury, stroke, or a progressive illness. "Developmental dyslexia", in contrast, has genetic origins. However, both alexia and developmental dyslexia share similar symptoms.

Alexia may be accompanied by expressive aphasia and/or receptive aphasia, which is the acquired inability to produce (expressive aphasia) and / or comprehend (receptive aphasia) spoken language. Alexia can also co-occur with agraphia, the specific loss of the ability to produce written language even when the necessary motor skills seem to be intact.

There are two subgroups of alexia: central alexias and peripheral alexias. Basically, central alexia is alexia as a part of aphasia, or alexia with agraphia, whereas peripheral alexia is an isolated impairment, without agraphia.

Central alexias



Central alexias include surface alexia, semantic alexia, phonological alexia, and deep alexia.

Surface alexia

In surface alexia, words whose pronunciations are 'regular' (highly consistent with their spelling e.g. mint) are read more accurately than words with irregular pronunciation, such as colonel. Difficulty distinguishing homophones is diagnostic of some forms of surface alexia. This disorder is usually accompanied by (surface) agraphia and fluent aphasia.

Surface alexia is associated with lesions in the temporo-parietal region of the left hemisphere, especially the superior temporal gyrus. Some research shows that the most dramatic cases of surface alexia are caused by lesions in the inferolateral left temporal regions, which are considered to involve semantic processing.

Phonological alexia

In phonological alexia, patients can read familiar words but have difficulty reading unfamiliar words (such as invented pseudo-words). It is thought that they can recognize words by accessing lexical memory orthographically but cannot 'sound out' novel words.

Phonological alexia is associated with lesions in varied locations within the territory of the middle cerebral artery. The superior temporal lobe is often also involved. Research has pointed towards the theory that phonological alexia is a development of deep alexia.

A treatment for phonological dyslexia is the Lindamood Phoneme Sequencing Program (LiPS). This program is based on a three way sensory feedback process. The subject uses their auditory, visual, and oral skills to learn to recognize words and word patterns. This is considered letter-by-letter reading using a bottom-up processing technique. Case studies with a total of three patients found a significant improvement in spelling and reading ability after using LiPS.

Deep alexia

Patients with deep alexia experience semantic paralexia, which happens when the patient reads a word, and says a related meaning instead of the denoted meaning. Deep alexia is more recently seen as a severe version of phonological alexia.

Deep alexia is caused by lesions that are often widespread and include much of the left frontal lobe. Research suggests that damage to the left perisylvian region of the frontal lobe causes deep alexia, as both the phonological and lexical routes of language are impaired.

Peripheral alexias



Peripheral alexias have been described by Bub as "impairment to processes that convert letters on the page into an abstract representation of visual word forms". These include hemianopic alexia, neglect alexia, attentional alexia, and pure alexia (also known as alexia without agraphia).

Pure alexia

Pure alexia, also known as agnosic alexia, alexia without agraphia, and pure word blindness; is alexia due to difficulty recognizing written sequences of letters (such as words), or sometimes even letters. It is 'pure' because it is not accompanied by other (significant) language-related impairments. Pure alexia does not include speech, hand writing style, language, or comprehension impairments.

Pure alexia is caused by lesions on the visual word form area (VWFA). The VWFA is composed of the left lateral occipital sulcus and is activated during reading. A lesion in the VWFA stops transmission between the visual cortex and the left angular gyrus. It can also be caused by a lesion involving the left occipital lobe and the splenium of the corpus callosum. It is usually accompanied by a homonymous hemianopsia in the right side of the visual field.

Multiple oral re-reading (MOR) is a treatment for pure alexia. It is considered a top-down processing technique in which patients read and re-read texts a predetermined number of times or until reading speed and/or accuracy improves a predetermined amount. The idea behind MOR is to learn how to use context, syntax, and semantics of the text to process written information rather than using bottom-up processing techniques in which letter by letter (LBL) reading is necessary. The theory that the MOR technique only uses top-down processing has been questioned and some studies have shown that in fact, bottom-up processing is in part responsible for reading improvement. This has been proven by reading tests that are engineered to use as few of the same words as possible that are used in training texts during MOR treatment. In these studies, patients did not significantly improve in reading speed or accuracy when reading untrained passages. Untrained passages are defined by having differing vocabulary from the texts used in reading practice. This supports the findings that MOR also has bottom-up processing components. In addition to tactile and kinesthetic reading techniques, other therapies that are aimed to improve letter-level reading include: timed semantic and lexical association tasks and limited-time single word identification.

Hemianopic alexia

Commonly considered to derive from visual field loss due to damage to the primary visual cortex. Sufferers may complain of slow reading but are able to read individual words normally. This is the most common form of peripheral alexia, and the form with the best evidence of the (possibility of) effective treatment.

Neglect alexia

In neglect alexia, some letters are neglected (skipped or misread) during reading - most commonly the letters at the beginning or left side of words. This alexia is associated with right parietal lesions.

Use of prism glasses in treatment has been demonstrated to produce substantial benefit.

Attentional Alexia

Patients with attentional alexia complain of letter crowding or migration, sometimes blending elements of two words into one. The lesion usually affects the left parietal lobe (Warrington et al., 1993). Patients perform better when word stimuli are presented in isolation rather than flanked by other words and letters. Using a large magnifying glass may help as this should reduce the effects of flanking interference from nearby words; however, no trials of this or indeed any other therapy for left parietal syndromes have been published.

Dual-route cascaded model of reading



The dual-route theory of reading aloud was first described in the early 1970s. This theory suggests that two separate mental mechanisms, or cognitive routes, are involved in reading aloud, with output of both mechanisms contributing to the pronunciation of a written stimulus. One mechanism is the lexical route, which is the process whereby skilled readers can recognize known words by sight alone, through a “dictionary” lookup procedure. The other mechanism is the nonlexical or sublexical route, which is the process whereby the reader can “sound out” a written word. This is done by identifying the word's constituent parts (letters, phonemes, graphemes) and, applying knowledge of how these parts are associated with each other, for example how a string of neighboring letters sound together.

History



In 1892 Joseph Jules Dejerine discovered alexia after studying the case of Oscar C., an educated French merchant who lost the ability to understand written words after suffering a stroke. Oscar C. suddenly couldn’t interpret letters or words and saw them as obscure symbols which were devoid of meaning. According to Dejerine, the white matter of Oscar C.’s lesion had severed the connection between his interpretation of visual words and his visual cortices. The patient could therefore no longer access the stored orthography of words from vision, while his spelling capacities (from non-visual memory) had not been damaged. Dejerine also observed that Oscar C.’s ability to understand numbers had remained intact, although the ophthalmologist who examined the patient before turning him over to Dejerine had found that Oscar C.’s numeral reading was very slow and fraught with error.

Notable cases



Canadian novelist Howard Engel suffered from alexia (sine agraphia) following a stroke in 2000. His case was subsequently publicised by neurologist Oliver Sacks.



 
Sponsored Links