Which innovative linguistic feature was FIRST introduced in digital assistant products that revolutionized their ability to understand natural speech?

Voice-activated digital assistants have transformed how we interact with technology, but they're also fascinating products of applied linguistics. Behind every Siri and Alexa response is a complex system of natural language processing, semantic analysis, and machine learning. This poll tests your knowledge about the linguistic innovations that power these ubiquitous consumer products and how they've evolved to understand human speech patterns.

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Natural Language Understanding (NLU) with contextual memory to maintain conversation history
Hidden Markov Models for speech recognition patterns
Real-time neural network translation systems
Sentiment analysis to detect user emotional states