 Overactivity of the midbrain has been found at the level of the substantia nigra extended to the red nucleus and subthalamic nucleus, which all contribute to the production of dopamine.However, increased dopamine does not imply increased excitatory function since dopamine's effect can be both excitatory or inhibitory depending upon which dopamine receptors (labelled D1 – D5) have been stimulated.Some characteristics of stuttered speech are not as easy for listeners to detect.