MLLMs are Deeply Affected by Modality Bias
On the map: Multimodal LLMs / vision-language models → MLLM training and visual instruction tuning
Abstract (arXiv)
Recent advances in Multimodal Large Language Models (MLLMs) have shown promising results in integrating diverse modalities such as texts and images. MLLMs are heavily influenced by modality bias, often relying on language while under-utilizing other modalities like visual inputs. This position paper argues that MLLMs are deeply affected by modality bias. Firstly, we diagnose the current state of modality bias, highlighting its manifestations across various tasks. Secondly, we propose a systematic research road-map related to modality bias in MLLMs. Thirdly, we identify key factors of modality bias in MLLMs and offer actionable suggestions for future research to mitigate it. To substantiate these findings, we conduct experiments that demonstrate the influence of each factor: 1. Data Characteristics: Language data is compact and abstract, while visual data is redundant and complex, creating an inherent imbalance in learning dynamics. 2. Imbalanced Backbone Capabilities: The dominance of pretrained language models in MLLMs leads to overreliance on language and neglect of visual information. 3. Training Objectives: Current objectives often fail to promote balanced cross-modal alignment, resulting in shortcut learning biased toward language. These findings highlight the need for balanced training strategies and model architectures to better integrate multiple modalities in MLLMs. We call for interdisciplinary efforts to tackle these challenges and drive innovation in MLLM research. Our work provides a fresh perspective on modality bias in MLLMs and offers insights for developing more robust and generalizable multimodal systems-advancing progress toward Artificial General Intelligence.
Key facts. Cited by 40 papers in the map (data through 14 Oct 2026). Placed in MLLM training and visual instruction tuning, one of 16 directions in Multimodal LLMs / vision-language models. Builds on 74 of its 89 references that are in the map. First posted on arXiv 24 May 2025.
Builds on
74 of 89 references.
Closest ancestors · 16
Same direction, newest first
- 2025Debiasing Multimodal Large Language Models via Noise-Aware Preference Optimization23 Mar
- 2024Expanding Performance Boundaries of Open-Source Multimodal Models with Model, Data, and Test-Time Scaling6 Dec
- 2024Looking Beyond Text: Reducing Language bias in Large Vision-Language Models via Multimodal Dual-Attention and Soft-Image Guidance21 Nov
- 2024Insight Over Sight: Exploring the Vision-Knowledge Conflicts in Multimodal LLMs10 Oct
- 2024A Survey on Multimodal Benchmarks: In the Era of Large AI Models21 Sep
- 2024Qwen2-VL: Enhancing Vision-Language Model's Perception of the World at Any Resolution18 Sep
- 2024Visual Prompting in Multimodal Large Language Models: A Survey5 Sep
- 2024Assessing Modality Bias in Video Question Answering Benchmarks with Multimodal Large Language Models22 Aug
- 2024A Survey on Benchmarks of Multimodal Large Language Models16 Aug
- 2024Quantifying and Mitigating Unimodal Biases in Multimodal Large Language Models: A Causal Perspective27 Mar
- 2024Strengthening Multimodal Large Language Model with Bootstrapped Preference Optimization13 Mar
- 2024Debiasing Multimodal Large Language Models via Penalization of Language Priors8 Mar
4 more
- 2024Eyes Wide Shut? Exploring the Visual Shortcomings of Multimodal LLMs11 Jan
- 2023MMICL: Empowering Vision-language Model with Multi-Modal In-Context Learning14 Sep
- 2023MMBench: Is Your Multi-modal Model an All-around Player?12 Jul
- 2023LVLM-eHub: A Comprehensive Evaluation Benchmark for Large Vision-Language Models15 Jun
Other references · 58
- 2021Learning Transferable Visual Models From Natural Language Supervisioncited by 33,744
- 2021Swin Transformer: Hierarchical Vision Transformer using Shifted Windowscited by 12,139
- 2025Qwen2.5-VL Technical Reportcited by 5,587
- 2024GPT-4o System Cardcited by 4,588
- 2024Gemini 1.5: Unlocking multimodal understanding across millions of tokens of contextcited by 3,224
- 2023MMMU: A Massive Multi-discipline Multimodal Understanding and Reasoning Benchmark for Expert AGIcited by 2,080
- 2025InternVL3: Exploring Advanced Training and Test-Time Recipes for Open-Source Multimodal Modelscited by 1,834
- 2019Generalized Intersection over Union: A Metric and A Loss for Bounding Box Regressioncited by 1,368
- 2020DocVQA: A Dataset for VQA on Document Imagescited by 1,317
- 2019Event-based Vision: A Surveycited by 1,016
- 2024Are We on the Right Way for Evaluating Large Vision-Language Models?cited by 914
- 2020A Survey on Visual Transformercited by 654
38 more
- 2023A Comprehensive Overview of Large Language Modelscited by 648
- 2024DeepSeek LLM: Scaling Open-Source Language Models with Longtermismcited by 642
- 2024MMMU-Pro: A More Robust Multi-discipline Multimodal Understanding Benchmarkcited by 512
- 2022Multimodal Learning with Transformers: A Surveycited by 400
- 2023LanguageBind: Extending Video-Language Pretraining to N-modality by Language-based Semantic Alignmentcited by 370
- 2019High Speed and High Dynamic Range Video with an Event Cameracited by 345
- 2020Counterfactual VQA: A Cause-Effect Look at Language Biascited by 266
- 2022Balanced Multimodal Learning via On-the-fly Gradient Modulationcited by 264
- 2024MMBench-Video: A Long-Form Multi-Shot Benchmark for Holistic Video Understandingcited by 182
- 2024Paying More Attention to Image: A Training-Free Method for Alleviating Hallucination in LVLMscited by 179
- 2023VALOR: Vision-Audio-Language Omni-Perception Pretraining Model and Datasetcited by 140
- 2018Overcoming Language Priors in Visual Question Answering with Adversarial Regularizationcited by 105
- 2023Deep Learning for Event-based Vision: A Comprehensive Survey and Benchmarkscited by 90
- 2023TinyCLIP: CLIP Distillation via Affinity Mimicking and Weight Inheritancecited by 90
- 2023CLIP-KD: An Empirical Study of CLIP Model Distillationcited by 87
- 2023An Inverse Scaling Law for CLIP Trainingcited by 77
- 2020Removing Bias in Multi-modal Classifiers: Regularization by Maximizing Functional Entropiescited by 57
- 2022CLIPPO: Image-and-Language Understanding from Pixels Onlycited by 55
- 2024The Curse of Multi-Modalities: Evaluating Hallucinations of Large Multimodal Models across Language, Visual, and Audiocited by 53
- 2024UniBind: LLM-Augmented Unified and Balanced Representation Space to Bind Them Allcited by 49
- 2024VLind-Bench: Measuring Language Priors in Large Vision-Language Modelscited by 45
- 2023Enhancing multimodal cooperation via sample-level modality valuationcited by 44
- 2024CLIP the Bias: How Useful is Balancing Data in Multimodal Learning?cited by 35
- 2024Semantics, Distortion, and Style Matter: Towards Source-free UDA for Panoramic Segmentationcited by 34
- 2022Masked Jigsaw Puzzle: A Versatile Position Embedding for Vision Transformerscited by 33
- 2024CAFuser: Condition-Aware Multimodal Fusion for Robust Semantic Perception of Driving Scenescited by 33
- 2024Multimodal Visual-Tactile Representation Learning through Self-Supervised Contrastive Pre-Trainingcited by 31
- 2024Learning Modality-agnostic Representation for Semantic Segmentation from Any Modalitiescited by 31
- 2022A Review on Methods and Applications in Multimodal Deep Learningcited by 29
- 2024360SFUDA++: Towards Source-free UDA for Panoramic Segmentation by Learning Reliable Category Prototypescited by 28
- 2025OmniSAM: Omnidirectional Segment Anything Model for UDA in Panoramic Semantic Segmentationcited by 28
- 2023Multimodality Representation Learning: A Survey on Evolution, Pretraining and Its Applicationscited by 27
- 2023Learning Disentangled Identifiers for Action-Customized Text-to-Image Generationcited by 24
- 2024Centering the Value of Every Modality: Towards Efficient and Resilient Modality-agnostic Semantic Segmentationcited by 22
- 2024Bi-VLA: Vision-Language-Action Model-Based System for Bimanual Robotic Dexterous Manipulationscited by 21
- 2022On Modality Bias Recognition and Reductioncited by 20
- 2024OmniBind: Teach to Build Unequal-Scale Modality Interaction for Omni-Bind of Allcited by 20
- 2025Benchmarking Multi-modal Semantic Segmentation under Sensor Failures: Missing and Noisy Modality Robustnesscited by 18
50 most cited of 58 All 58 (JSON)
Built on by
Citing papers per year: 2025: 12; 2026: 28.
40 papers.
Sort:
By area:
- 2025A Survey of Context Engineering for Large Language Modelscited by 126
- 2025MemER: Scaling Up Memory for Robot Control via Experience Retrievalcited by 67
- 2025Multimodal Spatial Reasoning in the Large Model Era: A Survey and Benchmarkscited by 33
- 2026When Vision Overrides Language: Evaluating and Mitigating Counterfactual Failures in VLAscited by 30
- 2025When Language Overrules: Revealing Text Dominance in Multimodal Large Language Modelscited by 18
- 2025Multi-speaker Attention Alignment for Multimodal Social Interactioncited by 9
- 2026Omni-Modal Dissonance Benchmark: Systematically Breaking Modality Consensus to Probe Robustness and Calibrated Abstentioncited by 6
- 2025Omnidirectional Spatial Modeling from Correlated Panoramascited by 5
- 2026When Audio-LLMs Don't Listen: A Cross-Linguistic Study of Modality Arbitrationcited by 5
- 2026Vesta: A Generalist Embodied Reasoning Modelcited by 5
- 2026AMPS: Adaptive Modality Preference Steering via Functional Entropycited by 4
- 2026Are Audio-Language Models Listening? Audio-Specialist Heads for Adaptive Audio Steeringcited by 4
28 more
- 2026MM-JudgeBias: A Benchmark for Evaluating Compositional Biases in MLLM-as-a-Judgecited by 4
- 2025Can Large Multimodal Models Actively Recognize Faulty Inputs? A Systematic Evaluation Framework of Their Input Scrutiny Abilitycited by 3
- 2026When Vision-Language Models Judge Without Seeing: Exposing Informativeness Biascited by 3
- 2026Pareto LoRA: Mitigating Modality Imbalance in Unified Multimodal Models via Pareto-Optimal Gradient Integrationcited by 3
- 2025FinCap: Topic-Aligned Captions for Short-Form Financial YouTube Videoscited by 2
- 2025Unleashing the Intrinsic Visual Representation Capability of Multimodal Large Language Modelscited by 2
- 2025Generative Giants, Retrieval Weaklings: Why do Multimodal Large Language Models Fail at Multimodal Retrieval?cited by 2
- 2026MIBench: Evaluating LMMs on Multimodal Interactioncited by 2
- 2026Senses Wide Shut: A Representation-Action Gap in Omnimodal LLMscited by 2
- 2026CAAD: Contrastive Audio-Aware Distillation for Efficient Speech Language Modelscited by 2
- 2026C3PO: Evaluating Cross-Modal Composition and Counterfactual Performance in Omnimodal Modelscited by 2
- 2026Towards Mitigating Modality Bias in Vision-Language Models for Temporal Action Localizationcited by 1
- 2026Seeing Beyond 8bits: Subjective and Objective Quality Assessment of HDR-UGC Videoscited by 1
- 2026Chain of Modality: From Static Fusion to Dynamic Orchestration in Omni-MLLMscited by 1
- 2026Measuring Cross-Modal Synergy: A Benchmark for VLM Explainabilitycited by 1
- 2026Omni-Streaming Thinkingcited by 1
- 2025CogGuide: Human-Like Guidance for Zero-Shot Omni-Modal Reasoningcited by 0
- 2025Understanding Temporal Logic Consistency in Video-Language Models through Cross-Modal Attention Discriminabilitycited by 0
- 2026Token-Efficient Multimodal Reasoning via Image Prompt Packagingcited by 0
- 2026A Systematic Study of Cross-Modal Typographic Attacks on Audio-Visual Reasoningcited by 0
- 2026Simulating Validity: Modal Decoupling in MLLM Generated Feedback on Science Drawingscited by 0
- 2026Do Composed Image Retrieval Benchmarks Require Multimodal Composition?cited by 0
- 2026Semantic Generative Tuning for Unified Multimodal Modelscited by 0
- 2026EgoCITE: Context-Augmented Indexing and Time-Aware Retrieval for Long-Horizon Egocentric Memorycited by 0
- 2026Tri-PvP: Exposing Modality Bias in Omni-Modal Large Language Models through Perceptual-Propositional Evidence Conflictscited by 0
- 2026TryOnReward: Learning Foveated Consistency for Reinforcement Fine-Tuning of Virtual Try-Oncited by 0
- 2026GraphWrit3R: End-to-End 3D Scene Graph Writingcited by 0
- 2026Scaling Laws for Multimodal Data Mixturescited by 0
- 2026Scaling Laws for Multimodal Data Mixtures10 Oct 2026
- 2026GraphWrit3R: End-to-End 3D Scene Graph Writing25 Sep 2026
- 2026Omni-Streaming Thinking14 Sep 2026
- 2026TryOnReward: Learning Foveated Consistency for Reinforcement Fine-Tuning of Virtual Try-On6 Sep 2026
- 2026Tri-PvP: Exposing Modality Bias in Omni-Modal Large Language Models through Perceptual-Propositional Evidence Conflicts5 Sep 2026
- 2026EgoCITE: Context-Augmented Indexing and Time-Aware Retrieval for Long-Horizon Egocentric Memory12 Aug 2026
- 2026C3PO: Evaluating Cross-Modal Composition and Counterfactual Performance in Omnimodal Models5 Aug 2026
- 2026CAAD: Contrastive Audio-Aware Distillation for Efficient Speech Language Models22 Jun 2026
- 2026Vesta: A Generalist Embodied Reasoning Model18 Jun 2026
- 2026Pareto LoRA: Mitigating Modality Imbalance in Unified Multimodal Models via Pareto-Optimal Gradient Integration15 Jun 2026
- 2026Measuring Cross-Modal Synergy: A Benchmark for VLM Explainability21 May 2026
- 2026Semantic Generative Tuning for Unified Multimodal Models18 May 2026
28 more
- 2026Do Composed Image Retrieval Benchmarks Require Multimodal Composition?14 May 2026
- 2026Senses Wide Shut: A Representation-Action Gap in Omnimodal LLMs13 May 2026
- 2026MM-JudgeBias: A Benchmark for Evaluating Compositional Biases in MLLM-as-a-Judge20 Apr 2026
- 2026When Vision-Language Models Judge Without Seeing: Exposing Informativeness Bias20 Apr 2026
- 2026Chain of Modality: From Static Fusion to Dynamic Orchestration in Omni-MLLMs16 Apr 2026
- 2026Simulating Validity: Modal Decoupling in MLLM Generated Feedback on Science Drawings5 Apr 2026
- 2026A Systematic Study of Cross-Modal Typographic Attacks on Audio-Visual Reasoning5 Apr 2026
- 2026Token-Efficient Multimodal Reasoning via Image Prompt Packaging2 Apr 2026
- 2026Omni-Modal Dissonance Benchmark: Systematically Breaking Modality Consensus to Probe Robustness and Calibrated Abstention28 Mar 2026
- 2026MIBench: Evaluating LMMs on Multimodal Interaction13 Mar 2026
- 2026Are Audio-Language Models Listening? Audio-Specialist Heads for Adaptive Audio Steering6 Mar 2026
- 2026Seeing Beyond 8bits: Subjective and Objective Quality Assessment of HDR-UGC Videos1 Mar 2026
- 2026When Vision Overrides Language: Evaluating and Mitigating Counterfactual Failures in VLAs19 Feb 2026
- 2026AMPS: Adaptive Modality Preference Steering via Functional Entropy13 Feb 2026
- 2026When Audio-LLMs Don't Listen: A Cross-Linguistic Study of Modality Arbitration12 Feb 2026
- 2026Towards Mitigating Modality Bias in Vision-Language Models for Temporal Action Localization28 Jan 2026
- 2025Generative Giants, Retrieval Weaklings: Why do Multimodal Large Language Models Fail at Multimodal Retrieval?22 Dec 2025
- 2025Unleashing the Intrinsic Visual Representation Capability of Multimodal Large Language Models6 Dec 2025
- 2025Multi-speaker Attention Alignment for Multimodal Social Interaction22 Nov 2025
- 2025Multimodal Spatial Reasoning in the Large Model Era: A Survey and Benchmarks29 Oct 2025
- 2025MemER: Scaling Up Memory for Robot Control via Experience Retrieval23 Oct 2025
- 2025Understanding Temporal Logic Consistency in Video-Language Models through Cross-Modal Attention Discriminability9 Oct 2025
- 2025FinCap: Topic-Aligned Captions for Short-Form Financial YouTube Videos30 Sep 2025
- 2025CogGuide: Human-Like Guidance for Zero-Shot Omni-Modal Reasoning8 Sep 2025
- 2025Omnidirectional Spatial Modeling from Correlated Panoramas2 Sep 2025
- 2025When Language Overrules: Revealing Text Dominance in Multimodal Large Language Models14 Aug 2025
- 2025Can Large Multimodal Models Actively Recognize Faulty Inputs? A Systematic Evaluation Framework of Their Input Scrutiny Ability6 Aug 2025
- 2025A Survey of Context Engineering for Large Language Models17 Jul 2025
- 2025When Language Overrules: Revealing Text Dominance in Multimodal Large Language Modelscited by 18
- 2026AMPS: Adaptive Modality Preference Steering via Functional Entropycited by 4
- 2026Are Audio-Language Models Listening? Audio-Specialist Heads for Adaptive Audio Steeringcited by 4
- 2026MM-JudgeBias: A Benchmark for Evaluating Compositional Biases in MLLM-as-a-Judgecited by 4
- 2025Can Large Multimodal Models Actively Recognize Faulty Inputs? A Systematic Evaluation Framework of Their Input Scrutiny Abilitycited by 3
- 2026When Vision-Language Models Judge Without Seeing: Exposing Informativeness Biascited by 3
- 2025FinCap: Topic-Aligned Captions for Short-Form Financial YouTube Videoscited by 2
- 2025Unleashing the Intrinsic Visual Representation Capability of Multimodal Large Language Modelscited by 2
- 2025Generative Giants, Retrieval Weaklings: Why do Multimodal Large Language Models Fail at Multimodal Retrieval?cited by 2
- 2026C3PO: Evaluating Cross-Modal Composition and Counterfactual Performance in Omnimodal Modelscited by 2
- 2026Towards Mitigating Modality Bias in Vision-Language Models for Temporal Action Localizationcited by 1
- 2026Measuring Cross-Modal Synergy: A Benchmark for VLM Explainabilitycited by 1
- 2026C3PO: Evaluating Cross-Modal Composition and Counterfactual Performance in Omnimodal Models5 Aug 2026
- 2026Measuring Cross-Modal Synergy: A Benchmark for VLM Explainability21 May 2026
- 2026MM-JudgeBias: A Benchmark for Evaluating Compositional Biases in MLLM-as-a-Judge20 Apr 2026
- 2026When Vision-Language Models Judge Without Seeing: Exposing Informativeness Bias20 Apr 2026
- 2026Are Audio-Language Models Listening? Audio-Specialist Heads for Adaptive Audio Steering6 Mar 2026
- 2026AMPS: Adaptive Modality Preference Steering via Functional Entropy13 Feb 2026
- 2026Towards Mitigating Modality Bias in Vision-Language Models for Temporal Action Localization28 Jan 2026
- 2025Generative Giants, Retrieval Weaklings: Why do Multimodal Large Language Models Fail at Multimodal Retrieval?22 Dec 2025
- 2025Unleashing the Intrinsic Visual Representation Capability of Multimodal Large Language Models6 Dec 2025
- 2025FinCap: Topic-Aligned Captions for Short-Form Financial YouTube Videos30 Sep 2025
- 2025When Language Overrules: Revealing Text Dominance in Multimodal Large Language Models14 Aug 2025
- 2025Can Large Multimodal Models Actively Recognize Faulty Inputs? A Systematic Evaluation Framework of Their Input Scrutiny Ability6 Aug 2025
- 2025Multimodal Spatial Reasoning in the Large Model Era: A Survey and Benchmarkscited by 33
- 2026Vesta: A Generalist Embodied Reasoning Modelcited by 5
- 2025CogGuide: Human-Like Guidance for Zero-Shot Omni-Modal Reasoningcited by 0
- 2026Token-Efficient Multimodal Reasoning via Image Prompt Packagingcited by 0
- 2026A Systematic Study of Cross-Modal Typographic Attacks on Audio-Visual Reasoningcited by 0
- 2026Vesta: A Generalist Embodied Reasoning Model18 Jun 2026
- 2026A Systematic Study of Cross-Modal Typographic Attacks on Audio-Visual Reasoning5 Apr 2026
- 2026Token-Efficient Multimodal Reasoning via Image Prompt Packaging2 Apr 2026
- 2025Multimodal Spatial Reasoning in the Large Model Era: A Survey and Benchmarks29 Oct 2025
- 2025CogGuide: Human-Like Guidance for Zero-Shot Omni-Modal Reasoning8 Sep 2025
- 2026Scaling Laws for Multimodal Data Mixturescited by 0
- 2026Scaling Laws for Multimodal Data Mixtures10 Oct 2026
- 2025A Survey of Context Engineering for Large Language Modelscited by 126
- 2025MemER: Scaling Up Memory for Robot Control via Experience Retrievalcited by 67
- 2025Multi-speaker Attention Alignment for Multimodal Social Interactioncited by 9
- 2026Omni-Modal Dissonance Benchmark: Systematically Breaking Modality Consensus to Probe Robustness and Calibrated Abstentioncited by 6
- 2025Omnidirectional Spatial Modeling from Correlated Panoramascited by 5
- 2026When Audio-LLMs Don't Listen: A Cross-Linguistic Study of Modality Arbitrationcited by 5
- 2026Pareto LoRA: Mitigating Modality Imbalance in Unified Multimodal Models via Pareto-Optimal Gradient Integrationcited by 3
- 2026MIBench: Evaluating LMMs on Multimodal Interactioncited by 2
- 2026Senses Wide Shut: A Representation-Action Gap in Omnimodal LLMscited by 2
- 2026Seeing Beyond 8bits: Subjective and Objective Quality Assessment of HDR-UGC Videoscited by 1
- 2026Chain of Modality: From Static Fusion to Dynamic Orchestration in Omni-MLLMscited by 1
- 2026Omni-Streaming Thinkingcited by 1
12 most cited of 18 All 18 (JSON)
- 2026GraphWrit3R: End-to-End 3D Scene Graph Writing25 Sep 2026
- 2026Omni-Streaming Thinking14 Sep 2026
- 2026Tri-PvP: Exposing Modality Bias in Omni-Modal Large Language Models through Perceptual-Propositional Evidence Conflicts5 Sep 2026
- 2026Pareto LoRA: Mitigating Modality Imbalance in Unified Multimodal Models via Pareto-Optimal Gradient Integration15 Jun 2026
- 2026Semantic Generative Tuning for Unified Multimodal Models18 May 2026
- 2026Do Composed Image Retrieval Benchmarks Require Multimodal Composition?14 May 2026
- 2026Senses Wide Shut: A Representation-Action Gap in Omnimodal LLMs13 May 2026
- 2026Chain of Modality: From Static Fusion to Dynamic Orchestration in Omni-MLLMs16 Apr 2026
- 2026Simulating Validity: Modal Decoupling in MLLM Generated Feedback on Science Drawings5 Apr 2026
- 2026Omni-Modal Dissonance Benchmark: Systematically Breaking Modality Consensus to Probe Robustness and Calibrated Abstention28 Mar 2026
- 2026MIBench: Evaluating LMMs on Multimodal Interaction13 Mar 2026
- 2026Seeing Beyond 8bits: Subjective and Objective Quality Assessment of HDR-UGC Videos1 Mar 2026
12 newest of 18 All 18 (JSON)
Similar, not cited
Close in wording, no citation either way
- 2025Rethinking the Text-Vision Reasoning Imbalance in MLLMs through the Lens of Training Recipes
- 2025Same Content, Different Answers: Cross-Modal Inconsistency in MLLMs
- 2026Language Bias in LVLMs: From In-Depth Analysis to Simple and Effective Mitigation
- 2025Some Modalities are More Equal Than Others: Decoding and Architecting Multimodal Integration in MLLMs
- 2024Unveiling the Ignorance of MLLMs: Seeing Clearly, Answering Incorrectly
- 2023How to Bridge the Gap between Modalities: Survey on Multimodal Large Language Model
- 2026When Words Speak Louder than Images: Towards Understanding Language Bias in Vision-Language Models
- 2024The Instinctive Bias: Spurious Images lead to Illusion in MLLMs