Wall Street trims gains as survey adds to Brexit worries

A survey by polling firm Opinium showed the campaign for Britain to leave the EU holds a one-point lead over the "In" camp ahead of Thursday's referendum. Investors are also keeping an eye on U.S. Federal Reserve Chair Janet Yellen's second day of testimony to Congress. On Tuesday, Yellen played down the risk of a recession, but warned that the British referendum and a U.S. hiring slowdown posed risks to the economic outlook.