LME copper closed down $50 at $6,540/mt, after moving between $6,494.50 and $6,673.50. In after-hours trade, it was trading back up above $6,600.

"On one hand, you have the economic optimism crowd from the S&P 500 turning positive, but then you also have the currency followers watching the dollar move higher, which is bearish for the metals," said Adam Klopfenstein, senior market strategist with MF Global Ltd's Lind-Waldock division in Chicago.

"With both markets giving different signals right now, it's no surprise why copper is pretty much unchanged today. It's primarily a reflection of the broader market searching for a meaningful direction."

Chairman Ben Bernanke said, "policymakers may raise the discount rate before long."

"The global economic recovery is still on track, Chinese demand is still very robust, policy makers are in no rush to reduce liquidity, and many base metals are still facing structural supply shortages," Leon Westgate, an analyst at Standard Bank, said in a note.

Closing prices for Wednesday:

Copper    $6,540   -$50

Aluminium $2,030   -$26

Zinc      $2,114   +$10

Lead      $2,045   +$13

Tin      $15,700  +$200

Nickel   $17,710  +$160