In after-hours trading, medical device maker Dexcom plunged ...

In after-hours trading, medical device maker Dexcom plunged about 38% after releasing disappointing fiscal full-year guidance. Footwear company Deckers reported fiscal first-quarter earnings and revenue that exceeded analysts’ expectations, boosting shares by roughly 10%.

Stocks are poised to end the week with declines, as investors on Thursday added to the previous session’s steep losses by dumping some megacap tech and artificial intelligence-tied stocks. The activity seemed to be part of a broader rotation into small caps and more cyclical areas of the market.

The S&P 500 and tech-heavy Nasdaq Composite slid by about 0.5% and 0.9%, respectively, on Thursday. The 30-stock Dow bucked the trend and added roughly 81 points, or just 0.2%.

“Volatility came back with a vengeance this week as selling pressure in the megacap space dragged down the broader market,” LPL Financial chief technical strategist Adam Turnquist said in a note to clients, adding that overbought conditions have also contributed to the recent weakness. ″Counterbalancing weakness in these heavyweight names poses a challenge for the rest of the market.”

The broad-market index is down 1.9% this week, while the Nasdaq has lost nearly 3.1%. The Dow is down roughly 0.9% week to date.