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Because the definition of a lesion is so broad, the varieties of lesions are virtually endless. Lesions can occur anywhere in the body that consists of soft tissue or osseous matter, though most frequently found in the mouth, skin, and the brain, or anywhere where a tumour may occur. They are subsequently classified by their features. If a lesion is caused by a tumor it will be classified as malignant or benign. Lesions may be classified by the shape they form, as is the case with many ulcers, which can have a bullseye or 'target' appearance. Their size may be specified as gross or histologic depending on whether they are visible to the unaided eye or require a microscope to see.
An additional classification that is sometimes used is based on whether or not a lesion occupies space. A space-occupying lesion, as the name suggests, has a recognizable volume and may impinge on nearby structures, whereas a non space-occupying lesion is simply a hole in the tissue, e.g. a small area of the brain that has turned to fluid following a stroke.
Some lesions have specialized names, such as Ghon lesions in the lungs of tuberculosis victims. The characteristic skin lesions of a varicella zoster virus (VZV) infection are called chickenpox. Lesions of the teeth are usually called dental caries.
Sham lesion is the name given to a control procedure during a lesion experiment. In a sham lesion, an animal may be placed in a stereotaxic apparatus and electrodes inserted as in the experimental condition, but no current is passed, and therefore damage to the tissue should be minimal.
Lesions are caused by any process that damages tissues. Lesions can also be caused by metabolic processes, like an ulcer or autoimmune activity, as in the case with many forms of arthritis.
Note that lesions are not limited to animals or humans; damaged plants are said to have lesions.
Lesions to the brain can result from many factors, including vascular disorders, traumatic brain injuries, and tumors.
Vascular disorders of the brain, often called strokes, disrupt the flow of blood to the brain, resulting in a lesion called an infarct. Vascular disorders of the brain include thrombosis, embolisms, angiomas, aneurysms, and cerebral arteriosclerosis.
Traumatic brain injuries (TBI) damage the brain by causing swelling and bleeding inside the brain, leading to inter-cranial pressure. TBIs are divided into open-head injuries, in which the brain is penetrated, and closed head injuries, typically caused by blunt force to the head. Closed head injuries typically cause damage both at the site of the blow (referred to as the coup) and at the opposite side of the skull (referred to as the contrecoup).
Brain tumors increased inter-cranial pressure, causing brain damage.
Lesions are used as a treatment for epilepsy and in neuropsychological research using animals. These lesions can be induced with electric shocks (electrolytic lesions) to the exposed brain or commonly by infusion of exictotoxins to specific areas.
Studies show there is a correlation between brain lesion and language, speech, and category-specific disorders. However, lesions in Broca's and Wernicke's areas are not found to alter language comprehension.
Lesions to the visual cortex have different effects depending on the sub-area effected. Lesions to V1, for example, can cause blindness in different areas of the brain depending on the size of the lesion and location relative to the calcarine fissure. Lesions to V4 can cause color-blindness, and bilateral lesions to V5 can cause the loss of the ability to perceive motion.
Lesion in amygdala would eliminate the enhanced activation seen in occipital and fusiform visual areas in response to fear with the area intact. Amygdala lesions change the functional pattern of activation to emotional stimuli in regions that are distant from the amygdala.
Lesion size is correlated with severity, recovery, and comprehension.
In the Wisconsin Card Sorting Test with unilateral frontal or nonfrontal lesions, patients with left frontal lesions did more poorly but had high perseverative error scores. In right frontal and nonfrontal lesions are impaired but due to differences in patients. As a result, medial frontal lesions are associated with poor performance.
An impairment following damage to a region of the brain does not necessarily imply that the damaged area is wholly responsible for the cognitive process which is impaired, however. For example, in pure alexia, the ability to read is destroyed by a lesion damaging both the left visual field and the connection between the right visual field and the language areas (Broca’s Area and Wernicke’s area). However, this does not mean one suffering from pure alexia is incapable of comprehending speech -- merely that there is no connection between their working visual cortex and language areas -- as is demonstrated by the fact that pure alexics can still write, speak, and even transcribe letters without understanding their meeting. 
Lesions are useful to researchers in understanding how the components of the brain produce cognition. Research involving lesions is predicated on the formal logic that if impaired performance implies a model of damaged cognition and that the model of the damaged cognition is equal to the normal system plus the effect of the lesion, then the impaired performance implies the normal cognitive system plus the effect of the lesion.
Humans with brain lesions are often the subjects of research with the goal of establishing the function of the area where their lesion occurred.
A drawback to the use of human subjects is the difficulty in finding subjects who have a lesion to the area which the researcher wishes to study.
Using animal subjects gives researchers the ability to lesion specific areas in the subjects, allowing them to quickly acquire a large group of subjects. An example of such a study is the lesioning of rat hippocampi to establish the role of the hippocampus in object recognition and object recency.
Misc. Disease-Associated Lesions