Projections from the caudate nucleus to the superior colliculus also modulate saccadic eye movement. Altered patterns of pars reticulata firing such as single-spike or burst firing are found in Parkinson's disease and epilepsy. Pars compacta The most prominent function of the pars compacta is motor control, though the substantia nigra's role in motor control is indirect; electrical stimulation of the substantia nigra does not result in movement, due to mediation of the striatum in the nigral influence of movement. The pars compacta sends excitatory input to the striatum via D1 pathway that excites and activates the striatum, resulting in the release of GABA onto the globus pallidus to inhibit its inhibitory effects on the thalamic nucleus. This causes the thalamocortical pathways to become excited and transmits motor neuron signals to the cerebral cortex to allow the initiation of movement, which is absent in Parkinson's disease. However, lack of pars compacta neurons has a large influence on movement, as evidenced by the symptoms of Parkinson's. The motor role of the pars compacta may involve fine motor control, as has been confirmed in animal models with lesions in that region. The pars compacta is heavily involved in learned responses to stimuli. In primates, dopaminergic neuron activity increases in the nigrostriatal pathway when a new stimulus is presented. Dopaminergic activity decreases with repeated stimulus presentation. However, behaviorally significant stimulus presentation (i.e. rewards) continues to activate dopaminergic neurons in the substantia nigra pars compacta. Dopaminergic projections from the ventral tegmental area (bottom part of the "midbrain" or mesencephalon) to the prefrontal cortex (mesocortical pathway) and to the nucleus accumbens (mesolimbic pathway – "meso" referring to "from the mesencephalon"... specifically the ventral tegmental area) are implicated in reward, pleasure, and addictive behavior. The pars compacta is also important in spatial learning, the observations about one's environment and location in space. Lesions in the pars compacta lead to learning deficits in repeating identical movements, and some studies point to its involvement in a dorsal striatal-dependent, response-based memory system that functions relatively independent of the hippocampus, which is traditionally believed to subserve spatial or episodic-like memory functions. The pars compacta also plays a role in temporal processing and is activated during time reproduction. Lesions in the pars compacta leads to temporal deficits. As of late, the pars compacta has been suspected of regulating the sleep-wake cycle, which is consistent with symptoms such as insomnia and REM sleep