Soon after midday on Wednesday, panic selling seemed to be seizing the United States stock market.

The Dow Jones industrial average was down more than 500 points, and all the fears that have driven this year’s declines — including concerns that the world economy is slowing — seemed as potent as ever.

Then investors’ mood changed.

Buyers rushed in, sending the Dow up over 400 points. What could have been a rout ended up being just the sort of down day that investors have had to get used to this year.

The Dow closed off 249 points, or 1.56 percent, to 15,766.74. The Standard & Poor’s 500-stock index, a broader benchmark, fell 22 points, or 1.17 percent, to 1,859.33. The Nasdaq composite index ended barely changed, 5.26 points lower, or 0.12 percent.

And after spiking, the Vix index, which measures investors’ expectations of volatility and is known as Wall Street’s fear gauge, tumbled.