The Senate banking committee has scheduled a hearing today about high-frequency trading. Executives from NYSE Euronext, Nasdaq OMX Group and other firms are set to testify. Eric Noll, executive vice president at Nasdaq, is expected to defend the trading practice and exchanges' role. According to his prepared testimony, "academic evidence" on HFT practices "supports the fact that they generally add value to the market." Meanwhile, market participants and observers continue to express concerns about how computerized trading affects markets.