NEW YORK, July 21 (Reuters) - The Nasdaq briefly turned positive on Tuesday to trade near break-even, along with the S&P 500 index, as optimism about the latest corporate earning results tempered a cautious economic assessment from U.S. Federal Reserve Chairman Ben Bernanke.
* The Dow was underpinned by U.S. machinery maker Caterpillar Inc, which posted a stronger-than-expected quarterly profit.
* After the benchmark S&P 500 hit a fresh 2009 intraday high of 956.53 shortly after the open, some investors opted to book profits, fueling some choppiness.
* The Dow Jones industrial average gained 50.49 points, or 0.57 percent, to 8,898.64. The Standard & Poor's 500 Index rose 0.38 points, or 0.04 percent, to 951.51. The Nasdaq Composite Index dipped 5.75 points, or 0.30 percent, to 1,903.54.