The carotid arteries in the neck supply blood to the brain.
A build-up of material on their walls can restrict blood flow to the brain or release harmful pieces of debris.
Scientists studied 67 people with a condition called asymptomatic carotid stenosis (ACS) which results in a 50% reduction in carotid artery diameter.
They were compared with 60 individuals with risk factors such as diabetes, high cholesterol and heart disease but no ACS.
Members of the ACS group performed significantly worse in tests of thinking and reaction speed, learning and memory.
Language scores did not differ between the two groups.
Study leader Dr Brajesh Lal, from the University of Maryland School of Medicine in Baltimore, US, said: "To date, the focus of diagnosis and management of carotid artery blockages has been prevention of stroke since that was the only harm that these blockages were thought to cause to patients.
"These results underscore the importance of assessing the status of memory and thinking in people with carotid artery narrowing."
Dr Lal presented the research at the annual meeting of the American Academy of Neurology in Philadelphia.