Incremental Prompting: Episodic Memory Prompt for Lifelong Event Detection

AuthorsMinqian Liu, Shiyu Chang, Lifu Huang

2022

TL;DR

Incremental Prompting with Episodic Memory Prompts entangles span features with type-specific prompts to reach 50.12% micro F1 on MAVEN Task 5, +10.78 over KT.

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THE PROBLEM

Lifelong event detectors catastrophically forget old types in class incremental learning

Class-incremental event detection suffers from catastrophic forgetting, where performance on old types drops sharply after training on new data.

When BERT-ED is trained to Task 5 on MAVEN, micro F1 falls to 22.69%, severely harming downstream event detection reliability over the full ontology.

HOW IT WORKS

Episodic Memory Prompts for incremental prompting

Incremental Prompting introduces Episodic Memory Prompting, combining Span-based Event Detection, Episodic Memory Prompts, Experience Replay, and Knowledge Distillation to store and reuse type-specific knowledge.

You can think of Episodic Memory Prompts as a growing card catalog, where each new event type gets its own learnable card that stays accessible across future tasks.

By entangling span representations with all accumulated prompts, Incremental Prompting lets the classifier consult a soft episodic memory rather than relying only on a limited context window.

DIAGRAM

Training flow with prompts, replay, and distillation

This diagram shows how Incremental Prompting trains on each task using Episodic Memory Prompts, experience replay, and prediction plus feature-level knowledge distillation.

DIAGRAM

Evaluation pipeline across incremental tasks

This diagram shows how Incremental Prompting is evaluated on MAVEN and ACE05-EN across five class-incremental tasks with shared test sets.

PROCESS

How Incremental Prompting Handles a Class Incremental Event Detection Stage

  1. 01

    Span-based Event Detection

    Incremental Prompting freezes BERT and uses Span-based Event Detection to encode sentences and build span representations for candidate event triggers.

  2. 02

    Episodic Memory Prompting

    Incremental Prompting initializes new type-specific prompts, concatenates them with accumulated prompts, encodes them, and entangles span features with prompt representations.

  3. 03

    Lifelong Learning with Experience Replay and Knowledge Distillation

    Incremental Prompting replays exemplars from the memory buffer and applies prediction-level and feature-level Knowledge Distillation to align with the previous model.

  4. 04

    Optimization

    Incremental Prompting optimizes the combined loss L = L̃C + αLER + β(LPD + LFD) and updates the exemplar memory using the herding algorithm.

KEY CONTRIBUTIONS

Key Contributions

  • 01

    Episodic Memory Prompts for class incremental event detection

    Incremental Prompting introduces Episodic Memory Prompting with type-specific prompts that are carried across tasks, improving MAVEN Task 5 micro F1 from 39.34% for KT to 50.12%.

  • 02

    Prompt based lifelong event detection framework

    Incremental Prompting is the first to apply prompt learning to class-incremental event detection, combining Span-based Event Detection, Experience Replay, and Knowledge Distillation.

  • 03

    Comprehensive analysis of new and old event types

    Incremental Prompting analyzes per-type F1 on old and new types across tasks, showing gains on both categories compared with KT on MAVEN.

RESULTS

By the Numbers

Micro F-1 MAVEN Task 5

50.12%

+10.78 over KT

Micro F-1 MAVEN Task 3

58.61%

+11.37 over KT

Micro F-1 ACE05 Task 5

43.28%

+0.66 over KT

Micro F-1 MAVEN Task 1

67.86%

+4.35 over BERT-ED

On MAVEN and ACE05-EN class-incremental benchmarks, Incremental Prompting is evaluated over five tasks with a shared test ontology. The gains in micro F-1, especially +10.78 on MAVEN Task 5 versus KT, show that Episodic Memory Prompts substantially reduce catastrophic forgetting while improving base event detection.

BENCHMARK

By the Numbers

On MAVEN and ACE05-EN class-incremental benchmarks, Incremental Prompting is evaluated over five tasks with a shared test ontology. The gains in micro F-1, especially +10.78 on MAVEN Task 5 versus KT, show that Episodic Memory Prompts substantially reduce catastrophic forgetting while improving base event detection.

BENCHMARK

Comparison on MAVEN Task 5 micro F-1

Micro F-1 on MAVEN Task 5 for Incremental Prompting and strong lifelong learning baselines.

BENCHMARK

Ablation on MAVEN Task 5 micro F-1

Effect of prompt initialization, entangled optimization, knowledge distillation, and discrete prompts on MAVEN Task 5 micro F-1.

KEY INSIGHT

The Counterintuitive Finding

Incremental Prompting with zero exemplars (buffer size 0) still benefits from Episodic Memory Prompts, yet performance drops sharply compared with using 20 exemplars.

This is surprising because prompts are designed as a memory, but the results show that even strong prompt-based episodic memory still heavily depends on stored examples.

WHY IT MATTERS

What this unlocks for the field

Incremental Prompting unlocks a way to grow a soft episodic memory of event types via prompts, without expanding backbone parameters or requiring task identifiers.

Builders can now design lifelong event detectors that retain knowledge across many tasks using compact prompt vectors, making continual updates more practical under tight memory and retraining budgets.

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