A lesion is damage to a part of the brain that results in destruction to the neurons. Lesions can come from natural sources (like brain injury or disease) but can also be deliberately caused.
Lesion methods are when lesions to a specific part of the brain are deliberately created. This can be done by surgically cutting and/or removing part of the brain, by electrical currents, or by chemicals that kill neurons. Reversible lesions can be caused by using extreme temperatures.
Lesion methods are used in research as a means of seeing which parts of the brain are responsible for specific functions. For example, if the amygdala is lesioned it can cause a reduction in fear and emotional responses. Lesion methods can also be used as a treatment for severely damaging and dangerous disorders that are a threat to an individual such as severe epileptic seizures.