A 54-year-old man has been sentenced to 18 months in jail after knocking another man unconscious in a road rage attack.

Robert Wilson Shepherd repeatedly punched and kicked 63-year-old doctor Ali Hussain on the side of the Kwinana Freeway in November 2011, after an altercation on the road.

Doctor Hussain was knocked unconscious and suffered a fractured nose and cheekbone.

Judge Simon Stone described the attack as cowardly and unprovoked, saying Shepherd had shown a callous disregard for the victim by driving off and leaving him unconscious on the side of the road.

But the judge said he accepted Shepherd was suffering from severe stress and psychological issues.

The minimum time Shepherd will serve is nine months.