topo-omni

Topo-Omni

Topo-Omni is Qwen2.5-Omni-3B fine-tuned with a spatial smoothness loss that induces a brain-like topographic organization across its vision, audio, and language layers. Every unit in the network is assigned a fixed 2-D coordinate on a shared cortical sheet, and training encourages units that are close on the sheet to respond similarly — mirroring how functionally similar neurons cluster together in the cortex. The result is emergent, spatially clustered functional regions (face-, scene-, object-, and speech-selective areas) that can be localized, ablated, stimulated, and compared against human fMRI data.

Code, evaluation scripts, and further documentation: github.com/epflneuroailab/topo-omni

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Model Details

• Base model: Qwen/Qwen2.5-Omni-3B (Thinker module — vision encoder, audio encoder, and language model)

• Architecture: Qwen2_5OmniThinkerForConditionalGeneration with a CorticalAdaptor projection inserted at each layer that maps hidden activations onto a shared cortical sheet and back (via a learned, near-identity linear map and its pseudo-inverse), so the underlying computation is preserved while exposing a spatially organized representation

• Cortical sheet: a single 304 × 512 2-D grid spanning the entire model

  Rows	Component
  0 – 159, cols 0–255	Vision encoder layers
  0 – 159, cols 256–511	Audio encoder layers
  160 – 303	Thinker (language model) layers

• Spatial loss: computes pairwise response similarity within local neighborhoods (Chebyshev distance) on the sheet and penalizes dissimilarity between nearby units; weighted by alpha = 20 relative to the task loss

• Modalities: text, image, video, and audio in; text out

How It Works

Standard transformer networks have no notion of spatial layout: any unit can interact with any other regardless of "position." Topo-Omni injects a topographic inductive bias, inspired by TDANN and TopoLM, by:

1. Assigning every unit in the vision encoder, audio encoder, and language model a fixed coordinate on a shared 2-D cortical sheet.
2. Adding a spatial smoothness loss during fine-tuning that penalizes nearby units on the sheet for responding dissimilarly to the same input.
3. Letting functionally selective regions (e.g., for faces, scenes, objects, bodies, speech) emerge and spatially cluster, much like functional maps in the human brain.

The learned sheet can then be probed with the same tools used in visual/auditory neuroscience: localizer-based selectivity mapping, ablation/stimulation, cluster overlap (IoU), and alignment to human fMRI recordings (NSD, SpaceTop).

Usage

Note: This checkpoint requires the custom Qwen2_5OmniThinkerForConditionalGeneration implementation (with the CorticalAdaptor modules) from the topo-omni repository — loading it with the stock transformers class will not reconstruct the cortical sheet projections correctly.

git clone https://github.com/epflneuroailab/topo-omni.git
cd topo-omni
pip install -r requirements.txt

from transformers import Qwen2_5OmniThinkerConfig, Qwen2_5OmniProcessor
from src.models.qwen2_5_omni import Qwen2_5OmniThinkerForConditionalGeneration

repo_id = "epfl-neuroai/topo-omni"

config = Qwen2_5OmniThinkerConfig.from_pretrained(repo_id)
config.audio_config.is_training = False
config.vision_config.is_training = False
config.text_config.is_training = False

model = Qwen2_5OmniThinkerForConditionalGeneration.from_pretrained(
    repo_id, dtype="auto", device_map="auto", config=config,
).eval()
processor = Qwen2_5OmniProcessor.from_pretrained(repo_id)

conversation = [
    {"role": "user", "content": [
        {"type": "image", "image": "path/or/url/to/image.jpg"},
    ]},
]
text = processor.apply_chat_template(conversation, tokenize=False, add_generation_prompt=True)
inputs = processor(text=text, images=[...], return_tensors="pt").to(model.device)

output_ids = model.generate(**inputs, max_new_tokens=128)
print(processor.batch_decode(output_ids, skip_special_tokens=True)[0])

To inspect or intervene on the cortical sheet representations directly (selectivity mapping, ablation, stimulation), see the evaluation scripts under src/eval/ in the repository.

Training

Fine-tuned from Qwen/Qwen2.5-Omni-3B with TRL's SFTTrainer plus an additional spatial smoothness loss term.

Framework versions

• TRL: 0.25.1
• Transformers: 4.57.3
• PyTorch: 2.9.1
• Datasets: 4.4.1
• Tokenizers: 0.22.1

Evaluation

The cortical sheet learned by this model is evaluated for:

• Selectivity — ON-vs-OFF contrasts (faces, bodies, objects, scenes, speech, etc.) with FDR-corrected t-tests
• Spatial clustering — Island Moran's I spatial autocorrelation of selectivity maps
• Brain alignment — correlation of model topography with human fMRI t-maps (NSD, SpaceTop)
• Causal relevance — ablation and stimulation of localizer-defined regions
• Cluster overlap — IoU of thresholded activation masks across stimulus categories, against a random-sampling null

See src/eval/ and the project README for scripts and metric definitions.

Citation

If you use this model, please cite the Topo-Omni repository and the works it builds on:

TODO

This model is built on Qwen2.5-Omni and the topographic-DNN approaches introduced by TDANN and TopoLM.

License

Released under the same license as the base model, Qwen2.5-Omni-3B (Apache 2.0). Please review the base model's license terms before use.