The basal ganglia (BG) are a group of interconnected nuclei which play a pivotal partin limbic, associative and motor functions. This role is mirrored by the wide range ofmotor and behavioral abnormalities directly resulting from dysfunctions of the BG.Studies of normal behavior have found that BG neurons tend to phasically modulatetheir activity in relation to different behavioral events. In the normal BG, thismodulation is highly specific, with each neuron related only to a small subset ofbehavioral events depending on specific combinations of movement parameters andcontext.. In many pathological conditions involving BG dysfunction and motorabnormalities, this neuronal specificity is lost. Loss of specificity (LOS) manifests inneuronal encoding of a larger spectrum of events and consequently a large overlap ofmovement-related activation patterns between different neurons. We review theexisting evidence for LOS in BG-related movement disorders, the possible neuralmechanisms underlying LOS, its effects on frequently used measures of neuronalactivity and its relation to theoretical models of the BG. The prevalence of LOS in amany BG-related disorders suggests that neuronal specificity may represent a keyfeature of normal information processing in the BG system. Thus, the concept ofneuronal specificity may underlie a unifying conceptual framework for the BG role innormal and abnormal motor control.