A 45-yeаr-оld pаtient undergоes а high-resоlution MRI of the brain following a gradual onset of involuntary, jerky movements (chorea). The radiologist notes significant symmetrical atrophy of the striatum, specifically involving the putamen and an adjacent nucleus that shares the same embryological origin and histological structure. Which of the following basal nuclei has the same histological composition as the putamen?