Nathan Bailey, 34, a research fellow at St Andrews University School Of Biology, was driving south in his Renault Clio on the M9 near Stirling when he lost control and hit a car being driven by Ronald Highcock.
Stirling Sheriff Court heard Mr Highcock and wife Elizabeth, 79, were returning from a weekend away when Bailey hit the side of their Citroen Saxo, causing both vehicles to leave the road and plunge down an embankment.
The Highcocks' car then smashed head-on into a tree. The couple were trapped in the crumpled vehicle and had to be cut free by the fire service.
Both were taken to Forth Valley Royal Hospital, Larbert, but Mr Highcock died weeks later from the severe injuries he sustained.
Bailey, an American citizen now living in Dundee, was not injured.
Mr Highcock was interviewed in hospital by police before his health deteriorated and he told them about the crash.
He said the couple were heading back from Dunblane Hydro to Prestwick when the accident happened.
He told police: "We were travelling slowly, about 40-45 miles per hour, then I felt I was being forced by something in the overtaking lane. I felt I was going sideways and went down the embankment and hit a tree and a fence."
The accident happened on April 28 last year and Mr Highcock died on May 20 from chest injuries sustained in the crash and a secondary cause of heart disease.
Mrs Highcock was also severely injured, suffering fractures, cuts and bruises.
Bailey pleaded guilty yesterday to causing death by careless driving.
His lawyer, advocate Gavin Anderson, said the accident appeared to have been caused by a "momentary lapse of concentration" on Bailey's part, which caused him to lose control.
Mr Anderson said speed had played no part and it was likely it may have involved a late unplanned overtaking manoeuvre.
He handed four letters of reference to the court from academics at St Andrews University. One letter from Dr Louise Richardson, the university principal, said Bailey was an "extremely accomplished scientist" who appeared to be devastated by the accident and "filled with remorse".
She said she hoped his "promising career" would not be cut short.
Sheriff William Gilchrist deferred sentence until September 16.