Brain lesions in humans


 * For the experimental study of lesions in animals see: Brain lesions

Brain lesions in humans are lesions that occur in the brain due to a number of causes.

Causes
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.

Lesions are sometimes intentionally inflicted during neurosurgery, such as the carefully placed brain lesion used to treat epilepsy and other brain disorders. (See Ablative brain surgery.)

Note that lesions are not limited to animals or humans; damaged plants are said to have lesions.

Causes of Brain Lesions
Lesions to the brain can result from many factors, including vascular disorders, traumatic brain injuries, and tumors.

Vascular Disorders
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
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).

Tumors
Brain tumors increased inter-cranial pressure, causing brain damage.

Surgery
Lesions are used in neurosurgery as a treatment for a number of conditions including epilepsy. These lesions can be induced with electric shocks (electrolytic lesions) to the exposed brain or commonly by infusion of exictotoxins to specific areas.

Procedures that result in lesions include:
 * Endoscopic thoracic sympathectomy is the burning, severing, removing or clamping parts of the sympathetic nerve trunk.
 * Ganglionectomy is the excision of a ganglion.
 * Hemispherectomy is the surgical removal of one cerebral hemisphere.
 * Hypophysectomy is the surgical removal of the pituitary gland or hypophysis such as when presented with a tumor.
 * Lobectomy is the removal of a lobe.
 * Neurectomy is the removal of a nerve.
 * Sympathectomy is the cutting of the nerves.

Effects of Brain Lesions
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 fusiform gyrus often result in prosopagnosia, the inability to distinguish faces and other complex objects from each other.

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.

Research using Lesions
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.

Research with Humans
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.