Question

Compare and contrast visual apraxia and visual agnosia in terms of the behaviours observed, the type of brain damage associated with each and the prognosis for recovery. Please answer this question inn long paragraph in more detail

Answer 1

Visual agnosia is an impairment in the visual recognition of objects unexplained by decreased visual acuity, reduced visual fields, impaired eye movements, aphasia, or dementia. Visual agnosia is defined as a disorder of recognition confined to the visual realm, in which a patient cannot arrive at the meaning of some or all categories of previously known nonverbal visual stimuli, despite normal or near-normal visual perception and intact alertness, attention, intelligence, and language. Typically, patients have impairments both for stimuli that they learned prior to the onset of brain injury (known as ‘retrograde’ memory), and for stimuli that they would normally have learned after their brain damage (known as ‘anterograde’ memory).

Visual agnosia occurs when there’s brain damage along the pathways that connect the occipital lobe of the brain with the parietal or temporal lobe. The occipital lobe assembles incoming visual information. The parietal and temporal lobes allow you to understand the meaning of this information.

Apperceptive visual agnosia

Apperceptive visual agnosia causes difficulty in perceiving shapes or forms of an object that you see. This condition may cause you to have difficulty in perceiving the difference from one object to another upon visual inspection. You may not be able to copy or draw a picture of an object. Instead, you may try to copy a picture of a circle and end up drawing a series of concentric scribbles.You can still use vision to navigate your environment and pick up objects without trouble, and knowledge of what the object is used for remains intact.Apperceptive visual agnosia is usually caused by lesions to the occipito-parietal cortex.

Associative visual agnosia

Associative visual agnosia is the inability to recall information associated with an object. This includes an object’s name and knowledge of its use.This form of agnosia doesn’t prevent you from being able to draw a picture of an object.Although you’re unable to name the object on visual inspection, you may be able to recognize and use an object shown to you when it’s accompanied by verbal or tactile cues.Associative visual agnosia is usually due to lesions of the bilateral occipito-temporal cortex.One of the most extreme instances of saccadic dysfunction is the complete inability to volitionally initiate saccades, which is termed ocular motor apraxia. (An apraxia is an inability to voluntarily initiate a movement that can be initiated by some other means, usually via a reflex, which reveals that a paralysis is not present.) Patients with congenital ocular motor apraxia characteristically use horizontal head thrusts past the point of interest, employing the VOR to move the eyes into extreme contraversion until foveation on the target is possible; this is followed by slower head rotation in the opposite direction to primary position while the eyes maintain fixation on the target. Nonvolitional saccades that occur as a reflex to a moving object or sound and vertical eye movements are normal. The location of the lesion that causes congenital ocular motor apraxia is not known but probably is above the brainstem centers that drive volitional saccades. Patients may have other neurologic abnormalities, including delayed development. Also, ocular motor apraxia is associated with several diseases, including ataxia telangectasia, Pelizaeus-Merzbacher disease, Niemann-Pick type C, Gaucher disease, Tay-Sachs disease, Joubert syndrome, abetalipoproteinemia (vitamin E deficiency), and Wilson disease.

Acquired ocular motor apraxia results from bilateral lesions of the supranuclear gaze pathways of the frontal and parietal lobes, usually from bilateral strokes, often as part of an anoxic encephalopathy following cardiac arrest, or post–coronary artery bypass grafting. Patients often blink to break the fixation and then turn their head toward a new point of interest. Bilateral lesions at the parieto-occipital junction may impair the guidance of volitional saccades. Such inaccurate saccades, together with inaccurate arm pointing (ie, a patient may misdirect his or her hand when attempting to shake yours, despite being able to see your hand) and simultanagnosia (disordered visual attention that makes it difficult for a patient to perceive all the major features of a visual scene at once), are known as the Balint syndrome; this syndrome is often associated with cognitive dysfunction