University of Cape Town professor Wayne Derman, who has worked with the athlete for six years, said Pistorius would look around in the dining hall, in a team setting, and during one-on-one consultations in Derman's room.
He described hypervigilance as a "restless looking around and a constant scanning of potential threats".
Pistorius was an anxious individual who suffered from hand tremors and sleep disorder, which Derman had to medicate, the court heard.
He said during the opening and closing of sporting events, such as the Olympics, Pistorius would have exaggerated startled responses to the fireworks.
He said Pistorius would cover his head and ears and cower until the noise ended.
Derman is a medical practitioner who teaches sports and exercise medicine at the university and had worked with Pistorius through assessments, consultations, and as part of the South African team at both the Beijing and London Olympics in his capacity as a physician.
He described Pistorius as being cautious not to consume prohibited substances when he was sick.
Derman said the athlete had never tested positive for doping.
On one occasion, Pistorius called Derman to ask for a prescription for medication for sinusitis. During the call Derman asked how the athlete was doing after taking part in the 2012 London Olympics.
Pistorius told Derman he was "lying next to the most wonderful girl he had ever met" and could not wait to introduce her to him.
Derman read out an e-mail he had received from a massage therapist he had met in London, who used her legs to massage people after being born with a disability of the hands.
In the e-mail she explained to Derman how people without disabilities did not understand how heightened a disabled individual's response to an unsafe situation could be.
Nel objected before Derman could conclude reading the e-mail, citing that it was hearsay because the woman was not a witness and her opinion played no part in the trial.
Pistorius is charged with murdering his girlfriend Reeva Steenkamp. He shot her through the locked door of the toilet in his Pretoria home, apparently thinking she was an intruder about to open the door and attack him. She was struck in the hip, arm, and head.
After firing the shots, Pistorius used a cricket bat to break open the door to get to a dying Steenkamp.
He has pleaded not guilty to the charge of murder and to three firearm-related charges.
The State argues he killed her during an argument.
Court was adjourned for lunch.