Jonathan Mathew, one of five former Barclays Plc employees accused of manipulating Libor, told a London jury that he was routinely humiliated by his boss when he made mistakes working on the money markets desk.

Mathew, who is partially deaf, said on his first day of testimony that he was hit on the back of the head with a 12-inch baseball bat, shouted at and forced to stand on a chair in the trading room to answer a quiz on world capitals by his boss, Peter Johnson.

"It wasn’t particularly hard, it was more designed to humiliate me," the 35-year-old trader said of Johnson. Jurors were told Wednesday that Johnson pleaded guilty to charges of Libor manipulation in October 2014.

"He was old school, a hard taskmaster and in the same respect he was a good teacher as well," said Mathew, who was diagnosed as being partially deaf in the left ear and dyslexic as a child.

Prosecutors say that Mathew and four of his colleagues conspired to rig the London interbank offered rate, a benchmark tied to trillions of dollars in securities and loans. Mathew, Stylianos Contogoulas, 44, Jay Merchant, 45, Alex Pabon, 37, and Ryan Reich, 34, all deny conspiracy to defraud charges dating from June 1, 2005 to Aug. 31, 2007.

Mathew, who joined Barclays as a 19-year-old, said he received no formal training when he moved to the bank’s money markets desk and worked as a Libor submitter with Johnson.

"How did you learn what to do?" asked his lawyer, Bill Clegg.

"From my boss, Peter Johnson," said Mathew, who failed the U.K. regulator’s so-called approved person exam three times. "Him explaining things to me, copying what he was doing, taking notes. On-the-job training."