 Some people can also show symptoms belonging to both subtypes.ADHD is linked to abnormalities in dopamine levels in the brain as well as to noticeable impairments in time perception.Numerous imaging studies have demonstrated that the caudate nucleus and globus pallidus (striatum) which contain a high density of DA receptors are smaller in ADHD than in control groups, ADHD groups have smaller posterior brain regions (e.g.,occipital lobes) and areas involved in coordinating activities of multiple brain regions (e.g., rostrum and splenium of corpus collosum and cerebellar vermis).These volumetric differences between the two groups correlate with severity of ADHD and its symptoms.