WASHINGTON -- It seems like a simple question. With only 15 percent of Americans trusting the stock market and a declining percentage who invest in it, what can market bosses do to restore the people's faith?

That's what Sen. Elizabeth Warren (D-Mass.) wanted to know in a Senate Banking Committee hearing on Tuesday. Her query was sparked in part by revelations in Michael Lewis' recent book, Flash Boys, about how high-frequency stock trading rigs the markets.

Two witnesses answered Warren's question fairly easily, saying that increased transparency and a complete, open review of modern trading practices by the Securities and Exchange Commission would go a long way.

But for Thomas Wittman, executive vice president and global head of equities at the company that runs the NASDAQ and several international exchanges, it wasn't so simple. He wouldn't even agree that the data Warren cited on declining public trust and decreased investments were related.

High-frequency trading added $23 million to $29 million to NASDAQ's coffers in the first quarter of this year alone.