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The S&P 500 and Nasdaq Composite eked out new closing records Thursday following a rocky trading session.

Major stock indexes opened modestly higher but gave up those gains by midday and traded lower for most of the afternoon. A flurry of buying activity helped the S&P 500 and Nasdaq bounce back from their lows in the final minutes of trading. Technology stocks led the rebound, with an assist from the healthcare and communications sectors.

The broad index added 6.50 points, or 0.2%, to 3916.38, its highest close ever, while the Nasdaq rose 53.24 points, or 0.4%, to 14025.77, also a record. The late-session turnaround wasn’t enough to keep the Dow Jones Industrial Average out of the red; the blue-chip index closed down 7.10 points, or less than 0.1%, at 31430.70.

No significant catalysts appeared to support the moves. But some investors pointed to a continuing conversation around whether valuations are stretched that appears to be playing out across the market.

On one hand, traditional metrics like price/earnings ratios suggest stocks are trading near their priciest levels ever, a prospect investors generally fear since that suggests future returns will be muted. On the other, several investors say that picture fails to take into account the impact of low interest rates, which tend to increase the future earnings of companies.