Thursday marked another wild day for biotechnology firms as a highly-anticipated presentation from Biogen Inc. took the spotlight and results from neurology-focused peers triggered huge stock swings amid a flurry of reports.

When the dust settled in afternoon trading, the presentation from Biogen, combined with key study data from Sage Therapeutics Inc. and Acadia Pharmaceuticals Inc., resulted in a combined $7.6 billion shift in value for the three companies. The huge moves were yet another reminder of how biotechnology investing -- and, more specifically, betting on drug development for diseases affecting the brain -- can often resemble a roulette wheel.

Biogen’s standing-room-only presentation of detailed results for its Alzheimer’s disease drug at a California medical meeting initially sank its shares by 5%. The stock later rallied to add about $2 billion to the company’s market value. Despite Biogen’s advance, some analysts on Wall Street were skeptical, including Mizuho’s Salim Syed, who called the data “not that impressive” and added that “old questions remain, new ones raised.”

For Sage, Thursday’s trading action was a day to forget. The Cambridge, Massachusetts-based company saw its shares plummet about 60%. That was worth a whopping $4.6 billion as some investors quickly pointed out that sales of the company’s lone drug on the market are projected to top out at less than $500 million annually, Bloomberg data show.

Acadia, a company focused on developing drugs to address unmet medical needs in central nervous system disorders, was among the day’s big winners after data drew praise across Wall Street. The company added a cool $1 billion to its value after presenting better than anticipated data at a conference Wednesday night for patients with dementia.

Small-cap drug developer Aurinia Pharmaceuticals Inc., while not focused on neurological diseases, briefly doubled Thursday after promising data.