Bullish investors lost their nerve on Thursday and a brief rally in stocks went into reverse as concerns about overvalued A.I. companies crept back into the market.

The midday fade came after strong gains in early morning trading in what investors and analysts initially attributed to relief after solid earnings reports from Nvidia, the flag-bearer of the move toward artificial intelligence that has propelled technology stock prices higher, and Walmart, a bellwether of consumer health.

But investors’ early enthusiasm quickly gave way to afternoon selling pressure.

The S&P 500 tumbled after climbing as much as 1.9 percent in morning trading. The benchmark ended the day 1.6 percent lower, a reversal of more than 3 percent from its highest point.

Nvidia opened over 5 percent higher before erasing its gain by the afternoon, and ending with a loss of more than 3 percent. Advanced Micro Devices, a rival chipmaker, and other tech companies also fell Thursday.

Investors had come into this week after a bumpy ride in November. The prolific rally in stocks linked to the growth of A.I. had shown signs of flagging, with investors questioning whether rampant investment in the space had gone too far. Investors also worried about the health of lower-income consumers, even while Americans with higher incomes continued to spend. And a divided Federal Reserve had led to doubts over whether the central bank would cut interest rates next month.

After reaching a new high on Oct. 28, the S&P 500 has fallen over 5 percent, led lower by technology stocks.

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