**S&P 500 .SPX drops 1.9% in wild trading as jitters over lofty tech valuations and ambitious AI-spending plans resurface .N

** Dow .DJI loses 1.9%, while tech-heavy Nasdaq Composite .IXIC tumbles 2.7%

** SPX ends longest stretch of closes above its 50-day moving average since 2007 on Mon; then breaks its 100-DMA on Thurs, as traders eye bitcoin BTC=, more technical damage

** Indeed, when animal spirits slump, stocks struggle

** CBOE Volatility Index .VIX, Wall Street's "fear gauge", spikes to close at highest since late Apr before receding to roughly 23.40

** U.S. 10-Year Treasury yield US10YT=RR on course for weekly decline to around 4.07% as rate-cut bets rise US/

** Most sectors fried: Technology biggest burn out, while Healthcare and Communication Services most resilient

** Tech .SPLRCT dives 4.7%. Nvidia NVDA.O initially jumps on Thurs after stellar results, forecast, but then sputters and drags down chip stocks

Semiconductor index .SOX slides ~6%

Cloud firm Oracle ORCL.N extends its selloff on worries about AI

** Consumer Discretionary .SPLRCD retreats 3.3%. Home Depot HD.N sags after annual profit forecast cut, hurt by slowdown in big-ticket renovations and do-it-yourself projects

** Consumer Staples .SPLRCS up 0.8%. Walmart WMT.N rises after bumping up annual forecasts heading into the holiday season

Though Target TGT.N falls after bigger-than-expected drop in Q3 comparable sales as U.S. consumers pull back on discretionary spending on apparel, home decor

** Healthcare .SPXHC climbs 1.8%. Eli Lilly LLY.N hits record, becomes 1st drugmaker to hit $1 trln market value on weight-loss demand

** Communication Services .SPLRCL surges 3%. Google parent Alphabet GOOGL.O leaps after Berkshire Hathaway BRKa.N reveals new stake worth about $5 bln, adds to gains following positive views on its latest Gemini 3 AI model

** Meanwhile, individual investors fret over market concentration in Big Tech

** SPX performance YTD:

(Lance Tupper and Terence Gabriel are Reuters market analysts. The views expressed are their own)

((lance.tupper@tr.com ; terence.gabriel@tr.com))