A 78-year-old female patient presents with sudden onset
violent, flinging movements involving the right side of her body. On
examination she is conscious and alert, there is no speech defect and her
vision is intact. The limbs are difficult to examine because of the abnormal
movements, which are present at rest, but there does not appear to be an
impairment of tone, power or reflexes.
The site of the neurological lesion is likely to be: