Lesion is used both as a noun and as a verb to describe damage done to the central nervous system (a lesion) and the process of making it (to lesion). Lesions may be present in patient groups or may be inflicted deliberately on experimental animals. Lesioning is one of the main techniques used experimentally to investigate brain-behaviour relationships, the rationale being that if a selected piece of brain tissue is removed, subsequent behavioural changes (compared to non-lesioned subjects) will reveal something about the normal function of the lost tissue. Lesions have revealed much about brain function, but caution has to be used in assessing their effects. For instance, the absence of a particular function does not necessarily imply that, normally, lesioned tissue exclusively mediates that function.
Experimental lesions are made in several ways. Fibre pathways can be interrupted by a KNIFE CUT LESION. This involves insertion of a microknife which can be moved to sever fibre pathways. Tissue at the surface of the brain can be destroyed by an ASPIRATION LESION (which sucks tissue away) or by SPRITZING (dusting NEUROTOXINS onto the brain). Within the brain, lesions can be made by passing electric current via an electrode, insulated except at the tip (ELECTROLYTIC LESIONS or RADIOFREQUENCY LESIONS).
All these techniques are relatively non-selective in terms of tissue destruction. Greater selectivity can be achieved using chemical neurotoxins. EXCITOTOXINS destroy neurons but spare fibre systems, while GLIOTOXINS destroy GLIAL CELLS rather than neurons. However, often the goal of experimental lesioning is to produce as selective a lesion as possible, which means targeting chemically identified neurons. 6-HYDROXYDOPAMINE (6OHDA) acts only on CATCHOLAMINE neurons and 5,7-DIHYDROXYTRYPTAMINE (5,7DHT) acts only on SEROTONIN neurons. Other than these though there are no neurotoxins selective for particular neurochemical systems. One way round this difficulty is to raise an ANTIBODY selective for a molecule which uniquely identifies a population of neurons and which can be internalised (taken inside a neuron). If the antibody is tagged with a neurotoxin (such as SAPORIN) normally excluded from the inside of the neuron, it can provide entry for the toxin into the neuron. This technique has been used successfully but is still in development.
In order to relate behavioural change to damage done, it is critical to have as accurate an understanding as possible of the damage present in the central nervous system. Lesions must be verified therefore by HISTOLOGY, measurement of brain neurochemistry, or some form of FUNCTIONAL NEUROIMAGING.
