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The circulatory system is responsible for transporting oxygen and nutrients to the brain. Part of the complex cerebral blood supply includes the left and right posterior cerebral arteries. These two arteries supply oxygenated blood to the lower back of the brain, in the occipital portion of the skull.
Blood is supplied to the head and brain by four large arteries, the two carotid arteries and the two vertebral arteries. The right and left vertebral arteries join together to form the basilar artery. This artery in turn separates into the two posterior cerebral arteries near the brain stem, at the upper border of the pons.
The carotid artery separates in the neck into the external and internal carotid arteries. The internal carotids join to the posterior cerebral arteries through the posterior communicating artery. This communication forms part of the arterial structure known as the Circle of Willis. The Circle of Willis allows the circulation between different parts of the brain to be equalized.
The posterior cerebral artery supplies blood to the center of the occipital lobes, the inferior sections of the temporal lobes, the brain stem and the cerebellum. This area contains the calcarine cortex, also known as the primary visual cortex. The short branches of the posterior cerebral artery supply blood to the hippocampus, the thalamus, part of the optic pathways and the midbrain.
Strokes affecting the posterior cerebral artery can affect the occipital cortex and cause alexia, which is an inability to read. Alternately, these strokes might affect visual learning, visual recognition or visual spatial orientation. Strokes involving the posterior cerebral artery might also affect the cerebellum or brain stem, causing slow or slurred speech. This is called dysarthria, and is the result of damage to nerves affecting the muscles that control the tongue and jaw. Most strokes tend to be unilateral, affecting only half of the brain.
Anton's syndrome, however, results from a stroke involving the vertebral artery that affects both hemispheres. The Anton’s syndrome patient experiences cortical blindness, of which they are honestly unaware and might vigorously deny. Patients cannot identify simple objects and might even be unable to tell if room lights are on or off. The pupils of the eye, however, still respond appropriately to light.
The brain of an Anton's syndrome patient will "lie" about what the patient is seeing. The patient is unaware of this falsehood and blames visual mistakes on other problems, such as not having his or her glasses. This unusual reaction could be because the damaged visual cortex is separated from other parts of the brain, including the speech and language sections. This reaction does not persist, and over time, the patient does become aware of his or her cortical blindness.