Posts in category: Google AI

Posted by Sandeep Tata, Software Engineer, Google Research, Athena Team

The last few years have seen rapid progress in systems that can automatically process complex business documents and turn them into structured objects. A system that can automatically extract data from documents, e.g., receipts, insurance quotes, and financial statements, has the potential to dramatically improve the efficiency of business workflows by avoiding error-prone, manual work. Recent models, based on the Transformer architecture, have shown impressive gains in accuracy. Larger models, such as PaLM 2, are also being leveraged to further streamline these business workflows. However, the datasets used in academic literature fail to capture the challenges seen in real-world use cases. Consequently, academic benchmarks report strong model accuracy, but these same models do poorly when used for complex real-world applications.

In “VRDU: A Benchmark for Visually-rich Document Understanding”, presented at KDD 2023, we announce the release of the new Visually Rich Document Understanding (VRDU) dataset that aims to bridge this gap and help researchers better track progress on document understanding tasks. We list five requirements for a good document understanding benchmark, based on the kinds of real-world documents for which document understanding models are frequently used. Then, we describe how most datasets currently used by the research community fail to meet one or more of these requirements, while VRDU meets all of them. We are excited to announce the public release of the VRDU dataset and evaluation code under a Creative Commons license.

Benchmark requirements

First, we compared state-of-the-art model accuracy (e.g., with FormNet and LayoutLMv2) on real-world use cases to academic benchmarks (e.g., FUNSD, CORD, SROIE). We observed that state-of-the-art models did not match academic benchmark results and delivered much lower accuracy in the real world. Next, we compared typical datasets for which document understanding models are frequently used with academic benchmarks and identified five dataset requirements that allow a dataset to better capture the complexity of real-world applications:

• Rich Schema: In practice, we see a wide variety of rich schemas for structured extraction. Entities have different data types (numeric, strings, dates, etc.) that may be required, optional, or repeated in a single document or may even be nested. Extraction tasks over simple flat schemas like (header, question, answer) do not reflect typical problems encountered in practice.
• Layout-Rich Documents: The documents should have complex layout elements. Challenges in practical settings come from the fact that documents may contain tables, key-value pairs, switch between single-column and double-column layout, have varying font-sizes for different sections, include pictures with captions and even footnotes. Contrast this with datasets where most documents are organized in sentences, paragraphs, and chapters with section headers — the kinds of documents that are typically the focus of classic natural language processing literature on long inputs.
• Diverse Templates: A benchmark should include different structural layouts or templates. It is trivial for a high-capacity model to extract from a particular template by memorizing the structure. However, in practice, one needs to be able to generalize to new templates/layouts, an ability that the train-test split in a benchmark should measure.
• High-Quality OCR: Documents should have high-quality Optical Character Recognition (OCR) results. Our aim with this benchmark is to focus on the VRDU task itself and to exclude the variability brought on by the choice of OCR engine.
• Token-Level Annotation: Documents should contain ground-truth annotations that can be mapped back to corresponding input text, so that each token can be annotated as part of the corresponding entity. This is in contrast with simply providing the text of the value to be extracted for the entity. This is key to generating clean training data where we do not have to worry about incidental matches to the given value. For instance, in some receipts, the ‘total-before-tax’ field may have the same value as the ‘total’ field if the tax amount is zero. Having token level annotations prevents us from generating training data where both instances of the matching value are marked as ground-truth for the ‘total’ field, thus producing noisy examples.

VRDU datasets and tasks

The VRDU dataset is a combination of two publicly available datasets, Registration Forms and Ad-Buy forms. These datasets provide examples that are representative of real-world use cases, and satisfy the five benchmark requirements described above.

The Ad-buy Forms dataset consists of 641 documents with political advertisement details. Each document is either an invoice or receipt signed by a TV station and a campaign group. The documents use tables, multi-columns, and key-value pairs to record the advertisement information, such as the product name, broadcast dates, total price, and release date and time.

The Registration Forms dataset consists of 1,915 documents with information about foreign agents registering with the US government. Each document records essential information about foreign agents involved in activities that require public disclosure. Contents include the name of the registrant, the address of related bureaus, the purpose of activities, and other details.

We gathered a random sample of documents from the public Federal Communications Commission (FCC) and Foreign Agents Registration Act (FARA) sites, and converted the images to text using Google Cloud’s OCR. We discarded a small number of documents that were several pages long and the processing did not complete in under two minutes. This also allowed us to avoid sending very long documents for manual annotation — a task that can take over an hour for a single document. Then, we defined the schema and corresponding labeling instructions for a team of annotators experienced with document-labeling tasks.

The annotators were also provided with a few sample labeled documents that we labeled ourselves. The task required annotators to examine each document, draw a bounding box around every occurrence of an entity from the schema for each document, and associate that bounding box with the target entity. After the first round of labeling, a pool of experts were assigned to review the results. The corrected results are included in the published VRDU dataset. Please see the paper for more details on the labeling protocol and the schema for each dataset.

Existing academic benchmarks (FUNSD, CORD, SROIE, Kleister-NDA, Kleister-Charity, DeepForm) fall-short on one or more of the five requirements we identified for a good document understanding benchmark. VRDU satisfies all of them. See our paper for background on each of these datasets and a discussion on how they fail to meet one or more of the requirements.

We built four different model training sets with 10, 50, 100, and 200 samples respectively. Then, we evaluated the VRDU datasets using three tasks (described below): (1) Single Template Learning, (2) Mixed Template Learning, and (3) Unseen Template Learning. For each of these tasks, we included 300 documents in the testing set. We evaluate models using the F1 score on the testing set.

• Single Template Learning (STL): This is the simplest scenario where the training, testing, and validation sets only contain a single template. This simple task is designed to evaluate a model’s ability to deal with a fixed template. Naturally, we expect very high F1 scores (0.90+) for this task.
• Mixed Template Learning (MTL): This task is similar to the task that most related papers use: the training, testing, and validation sets all contain documents belonging to the same set of templates. We randomly sample documents from the datasets and construct the splits to make sure the distribution of each template is not changed during sampling.
• Unseen Template Learning (UTL): This is the most challenging setting, where we evaluate if the model can generalize to unseen templates. For example, in the Registration Forms dataset, we train the model with two of the three templates and test the model with the remaining one. The documents in the training, testing, and validation sets are drawn from disjoint sets of templates. To our knowledge, previous benchmarks and datasets do not explicitly provide such a task designed to evaluate the model’s ability to generalize to templates not seen during training.

The objective is to be able to evaluate models on their data efficiency. In our paper, we compared two recent models using the STL, MTL, and UTL tasks and made three observations. First, unlike with other benchmarks, VRDU is challenging and shows that models have plenty of room for improvements. Second, we show that few-shot performance for even state-of-the-art models is surprisingly low with even the best models resulting in less than an F1 score of 0.60. Third, we show that models struggle to deal with structured repeated fields and perform particularly poorly on them.

Conclusion

We release the new Visually Rich Document Understanding (VRDU) dataset that helps researchers better track progress on document understanding tasks. We describe why VRDU better reflects practical challenges in this domain. We also present experiments showing that VRDU tasks are challenging, and recent models have substantial headroom for improvements compared to the datasets typically used in the literature with F1 scores of 0.90+ being typical. We hope the release of the VRDU dataset and evaluation code helps research teams advance the state of the art in document understanding.

Acknowledgements

Many thanks to Zilong Wang, Yichao Zhou, Wei Wei, and Chen-Yu Lee, who co-authored the paper along with Sandeep Tata. Thanks to Marc Najork, Riham Mansour and numerous partners across Google Research and the Cloud AI team for providing valuable insights. Thanks to John Guilyard for creating the animations in this post.

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Posted by Fuzhao Xue, Research Intern, and Mostafa Dehghani, Research Scientist, Google

Adaptive computation refers to the ability of a machine learning system to adjust its behavior in response to changes in the environment. While conventional neural networks have a fixed function and computation capacity, i.e., they spend the same number of FLOPs for processing different inputs, a model with adaptive and dynamic computation modulates the computational budget it dedicates to processing each input, depending on the complexity of the input.

Adaptive computation in neural networks is appealing for two key reasons. First, the mechanism that introduces adaptivity provides an inductive bias that can play a key role in solving some challenging tasks. For instance, enabling different numbers of computational steps for different inputs can be crucial in solving arithmetic problems that require modeling hierarchies of different depths. Second, it gives practitioners the ability to tune the cost of inference through greater flexibility offered by dynamic computation, as these models can be adjusted to spend more FLOPs processing a new input.

Neural networks can be made adaptive by using different functions or computation budgets for various inputs. A deep neural network can be thought of as a function that outputs a result based on both the input and its parameters. To implement adaptive function types, a subset of parameters are selectively activated based on the input, a process referred to as conditional computation. Adaptivity based on the function type has been explored in studies on mixture-of-experts, where the sparsely activated parameters for each input sample are determined through routing.

Another area of research in adaptive computation involves dynamic computation budgets. Unlike in standard neural networks, such as T5, GPT-3, PaLM, and ViT, whose computation budget is fixed for different samples, recent research has demonstrated that adaptive computation budgets can improve performance on tasks where transformers fall short. Many of these works achieve adaptivity by using dynamic depth to allocate the computation budget. For example, the Adaptive Computation Time (ACT) algorithm was proposed to provide an adaptive computational budget for recurrent neural networks. The Universal Transformer extends the ACT algorithm to transformers by making the computation budget dependent on the number of transformer layers used for each input example or token. Recent studies, like PonderNet, follow a similar approach while improving the dynamic halting mechanisms.

In the paper “Adaptive Computation with Elastic Input Sequence”, we introduce a new model that utilizes adaptive computation, called AdaTape. This model is a Transformer-based architecture that uses a dynamic set of tokens to create elastic input sequences, providing a unique perspective on adaptivity in comparison to previous works. AdaTape uses an adaptive tape reading mechanism to determine a varying number of tape tokens that are added to each input based on input’s complexity. AdaTape is very simple to implement, provides an effective knob to increase the accuracy when needed, but is also much more efficient compared to other adaptive baselines because it directly injects adaptivity into the input sequence instead of the model depth. Finally, Adatape offers better performance on standard tasks, like image classification, as well as algorithmic tasks, while maintaining a favorable quality and cost tradeoff.

Adaptive computation transformer with elastic input sequence

AdaTape uses both the adaptive function types and a dynamic computation budget. Specifically, for a batch of input sequences after tokenization (e.g., a linear projection of non-overlapping patches from an image in the vision transformer), AdaTape uses a vector representing each input to dynamically select a variable-sized sequence of tape tokens.

AdaTape uses a bank of tokens, called a “tape bank”, to store all the candidate tape tokens that interact with the model through the adaptive tape reading mechanism. We explore two different methods for creating the tape bank: an input-driven bank and a learnable bank.

The general idea of the input-driven bank is to extract a bank of tokens from the input while employing a different approach than the original model tokenizer for mapping the raw input to a sequence of input tokens. This enables dynamic, on-demand access to information from the input that is obtained using a different point of view, e.g., a different image resolution or a different level of abstraction.

In some cases, tokenization in a different level of abstraction is not possible, thus an input-driven tape bank is not feasible, such as when it’s difficult to further split each node in a graph transformer. To address this issue, AdaTape offers a more general approach for generating the tape bank by using a set of trainable vectors as tape tokens. This approach is referred to as the learnable bank and can be viewed as an embedding layer where the model can dynamically retrieve tokens based on the complexity of the input example. The learnable bank enables AdaTape to generate a more flexible tape bank, providing it with the ability to dynamically adjust its computation budget based on the complexity of each input example, e.g., more complex examples retrieve more tokens from the bank, which let the model not only use the knowledge stored in the bank, but also spend more FLOPs processing it, since the input is now larger.

Finally, the selected tape tokens are appended to the original input and fed to the following transformer layers. For each transformer layer, the same multi-head attention is used across all input and tape tokens. However, two different feed-forward networks (FFN) are used: one for all tokens from the original input and the other for all tape tokens. We observed slightly better quality by using separate feed-forward networks for input and tape tokens.

An overview of AdaTape. For different samples, we pick a variable number of different tokens from the tape bank. The tape bank can be driven from input, e.g., by extracting some extra fine-grained information or it can be a set of trainable vectors. Adaptive tape reading is used to recursively select different sequences of tape tokens, with variable lengths, for different inputs. These tokens are then simply appended to inputs and fed to the transformer encoder.

AdaTape provides helpful inductive bias

We evaluate AdaTape on parity, a very challenging task for the standard Transformer, to study the effect of inductive biases in AdaTape. With the parity task, given a sequence 1s, 0s, and -1s, the model has to predict the evenness or oddness of the number of 1s in the sequence. Parity is the simplest non-counter-free or periodic regular language, but perhaps surprisingly, the task is unsolvable by the standard Transformer.

Evaluation on the parity task. The standard Transformer and Universal Transformer were unable to perform this task, both showing performance at the level of a random guessing baseline.

Despite being evaluated on short, simple sequences, both the standard Transformer and Universal Transformers were unable to perform the parity task as they are unable to maintain a counter within the model. However, AdaTape outperforms all baselines, as it incorporates a lightweight recurrence within its input selection mechanism, providing an inductive bias that enables the implicit maintenance of a counter, which is not possible in standard Transformers.

Evaluation on image classification

We also evaluate AdaTape on the image classification task. To do so, we trained AdaTape on ImageNet-1K from scratch. The figure below shows the accuracy of AdaTape and the baseline methods, including A-ViT, and the Universal Transformer ViT (UViT and U2T) versus their speed (measured as number of images, processed by each code, per second). In terms of quality and cost tradeoff, AdaTape performs much better than the alternative adaptive transformer baselines. In terms of efficiency, larger AdaTape models (in terms of parameter count) are faster than smaller baselines. Such results are consistent with the finding from previous work that shows that the adaptive model depth architectures are not well suited for many accelerators, like the TPU.

We evaluate AdaTape by training on ImageNet from scratch. For A-ViT, we not only report their results from the paper but also re-implement A-ViT by training from scratch, i.e., A-ViT(Ours).

A study of AdaTape’s behavior

In addition to its performance on the parity task and ImageNet-1K, we also evaluated the token selection behavior of AdaTape with an input-driven bank on the JFT-300M validation set. To better understand the model’s behavior, we visualized the token selection results on the input-driven bank as heatmaps, where lighter colors mean that position is more frequently selected. The heatmaps reveal that AdaTape more frequently picks the central patches. This aligns with our prior knowledge, as central patches are typically more informative — especially in the context of datasets with natural images, where the main object is in the middle of the image. This result highlights the intelligence of AdaTape, as it can effectively identify and prioritize more informative patches to improve its performance.

We visualize the tape token selection heatmap of AdaTape-B/32 (left) and AdaTape-B/16 (right). The hotter / lighter color means the patch at this position is more frequently selected.

Conclusion

AdaTape is characterized by elastic sequence lengths generated by the adaptive tape reading mechanism. This also introduces a new inductive bias that enables AdaTape to have the potential to solve tasks that are challenging for both standard transformers and existing adaptive transformers. By conducting comprehensive experiments on image recognition benchmarks, we demonstrate that AdaTape outperforms standard transformers and adaptive architecture transformers when computation is held constant.

Acknowledgments

One of the authors of this post, Mostafa Dehghani, is now at Google DeepMind.

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Posted by Greg Corrado, Head of Health AI, Google Research, and Yossi Matias, VP, Engineering and Research, Google Research

Medicine is an inherently multimodal discipline. When providing care, clinicians routinely interpret data from a wide range of modalities including medical images, clinical notes, lab tests, electronic health records, genomics, and more. Over the last decade or so, AI systems have achieved expert-level performance on specific tasks within specific modalities — some AI systems processing CT scans, while others analyzing high magnification pathology slides, and still others hunting for rare genetic variations. The inputs to these systems tend to be complex data such as images, and they typically provide structured outputs, whether in the form of discrete grades or dense image segmentation masks. In parallel, the capacities and capabilities of large language models (LLMs) have become so advanced that they have demonstrated comprehension and expertise in medical knowledge by both interpreting and responding in plain language. But how do we bring these capabilities together to build medical AI systems that can leverage information from all these sources?

In today’s blog post, we outline a spectrum of approaches to bringing multimodal capabilities to LLMs and share some exciting results on the tractability of building multimodal medical LLMs, as described in three recent research papers. The papers, in turn, outline how to introduce de novo modalities to an LLM, how to graft a state-of-the-art medical imaging foundation model onto a conversational LLM, and first steps towards building a truly generalist multimodal medical AI system. If successfully matured, multimodal medical LLMs might serve as the basis of new assistive technologies spanning professional medicine, medical research, and consumer applications. As with our prior work, we emphasize the need for careful evaluation of these technologies in collaboration with the medical community and healthcare ecosystem.

A spectrum of approaches

Several methods for building multimodal LLMs have been proposed in recent months [1, 2, 3], and no doubt new methods will continue to emerge for some time. For the purpose of understanding the opportunities to bring new modalities to medical AI systems, we’ll consider three broadly defined approaches: tool use, model grafting, and generalist systems.

The spectrum of approaches to building multimodal LLMs range from having the LLM use existing tools or models, to leveraging domain-specific components with an adapter, to joint modeling of a multimodal model.

Tool use

In the tool use approach, one central medical LLM outsources analysis of data in various modalities to a set of software subsystems independently optimized for those tasks: the tools. The common mnemonic example of tool use is teaching an LLM to use a calculator rather than do arithmetic on its own. In the medical space, a medical LLM faced with a chest X-ray could forward that image to a radiology AI system and integrate that response. This could be accomplished via application programming interfaces (APIs) offered by subsystems, or more fancifully, two medical AI systems with different specializations engaging in a conversation.

This approach has some important benefits. It allows maximum flexibility and independence between subsystems, enabling health systems to mix and match products between tech providers based on validated performance characteristics of subsystems. Moreover, human-readable communication channels between subsystems maximize auditability and debuggability. That said, getting the communication right between independent subsystems can be tricky, narrowing the information transfer, or exposing a risk of miscommunication and information loss.

Model grafting

A more integrated approach would be to take a neural network specialized for each relevant domain, and adapt it to plug directly into the LLM — grafting the visual model onto the core reasoning agent. In contrast to tool use where the specific tool(s) used are determined by the LLM, in model grafting the researchers may choose to use, refine, or develop specific models during development. In two recent papers from Google Research, we show that this is in fact feasible. Neural LLMs typically process text by first mapping words into a vector embedding space. Both papers build on the idea of mapping data from a new modality into the input word embedding space already familiar to the LLM. The first paper, “Multimodal LLMs for health grounded in individual-specific data”, shows that asthma risk prediction in the UK Biobank can be improved if we first train a neural network classifier to interpret spirograms (a modality used to assess breathing ability) and then adapt the output of that network to serve as input into the LLM.

The second paper, “ELIXR: Towards a general purpose X-ray artificial intelligence system through alignment of large language models and radiology vision encoders”, takes this same tack, but applies it to full-scale image encoder models in radiology. Starting with a foundation model for understanding chest X-rays, already shown to be a good basis for building a variety of classifiers in this modality, this paper describes training a lightweight medical information adapter that re-expresses the top layer output of the foundation model as a series of tokens in the LLM’s input embeddings space. Despite fine-tuning neither the visual encoder nor the language model, the resulting system displays capabilities it wasn’t trained for, including semantic search and visual question answering.

Our approach to grafting a model works by training a medical information adapter that maps the output of an existing or refined image encoder into an LLM-understandable form.

Model grafting has a number of advantages. It uses relatively modest computational resources to train the adapter layers but allows the LLM to build on existing highly-optimized and validated models in each data domain. The modularization of the problem into encoder, adapter, and LLM components can also facilitate testing and debugging of individual software components when developing and deploying such a system. The corresponding disadvantages are that the communication between the specialist encoder and the LLM is no longer human readable (being a series of high dimensional vectors), and the grafting procedure requires building a new adapter for not just every domain-specific encoder, but also every revision of each of those encoders.

Generalist systems

The most radical approach to multimodal medical AI is to build one integrated, fully generalist system natively capable of absorbing information from all sources. In our third paper in this area, “Towards Generalist Biomedical AI”, rather than having separate encoders and adapters for each data modality, we build on PaLM-E, a recently published multimodal model that is itself a combination of a single LLM (PaLM) and a single vision encoder (ViT). In this set up, text and tabular data modalities are covered by the LLM text encoder, but now all other data are treated as an image and fed to the vision encoder.

Med-PaLM M is a large multimodal generative model that flexibly encodes and interprets biomedical data including clinical language, imaging, and genomics with the same model weights.

We specialize PaLM-E to the medical domain by fine-tuning the complete set of model parameters on medical datasets described in the paper. The resulting generalist medical AI system is a multimodal version of Med-PaLM that we call Med-PaLM M. The flexible multimodal sequence-to-sequence architecture allows us to interleave various types of multimodal biomedical information in a single interaction. To the best of our knowledge, it is the first demonstration of a single unified model that can interpret multimodal biomedical data and handle a diverse range of tasks using the same set of model weights across all tasks (detailed evaluations in the paper).

This generalist-system approach to multimodality is both the most ambitious and simultaneously most elegant of the approaches we describe. In principle, this direct approach maximizes flexibility and information transfer between modalities. With no APIs to maintain compatibility across and no proliferation of adapter layers, the generalist approach has arguably the simplest design. But that same elegance is also the source of some of its disadvantages. Computational costs are often higher, and with a unitary vision encoder serving a wide range of modalities, domain specialization or system debuggability could suffer.

The reality of multimodal medical AI

To make the most of AI in medicine, we’ll need to combine the strength of expert systems trained with predictive AI with the flexibility made possible through generative AI. Which approach (or combination of approaches) will be most useful in the field depends on a multitude of as-yet unassessed factors. Is the flexibility and simplicity of a generalist model more valuable than the modularity of model grafting or tool use? Which approach gives the highest quality results for a specific real-world use case? Is the preferred approach different for supporting medical research or medical education vs. augmenting medical practice? Answering these questions will require ongoing rigorous empirical research and continued direct collaboration with healthcare providers, medical institutions, government entities, and healthcare industry partners broadly. We look forward to finding the answers together.

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Posted by Eleni Triantafillou, Research Scientist, and Malik Boudiaf, Student Researcher, Google

Deep learning has recently made tremendous progress in a wide range of problems and applications, but models often fail unpredictably when deployed in unseen domains or distributions. Source-free domain adaptation (SFDA) is an area of research that aims to design methods for adapting a pre-trained model (trained on a “source domain”) to a new “target domain”, using only unlabeled data from the latter.

Designing adaptation methods for deep models is an important area of research. While the increasing scale of models and training datasets has been a key ingredient to their success, a negative consequence of this trend is that training such models is increasingly computationally expensive, out of reach for certain practitioners and also harmful for the environment. One avenue to mitigate this issue is through designing techniques that can leverage and reuse already trained models for tackling new tasks or generalizing to new domains. Indeed, adapting models to new tasks is widely studied under the umbrella of transfer learning.

SFDA is a particularly practical area of this research because several real-world applications where adaptation is desired suffer from the unavailability of labeled examples from the target domain. In fact, SFDA is enjoying increasing attention [1, 2, 3, 4]. However, albeit motivated by ambitious goals, most SFDA research is grounded in a very narrow framework, considering simple distribution shifts in image classification tasks.

In a significant departure from that trend, we turn our attention to the field of bioacoustics, where naturally-occurring distribution shifts are ubiquitous, often characterized by insufficient target labeled data, and represent an obstacle for practitioners. Studying SFDA in this application can, therefore, not only inform the academic community about the generalizability of existing methods and identify open research directions, but can also directly benefit practitioners in the field and aid in addressing one of the biggest challenges of our century: biodiversity preservation.

In this post, we announce “In Search for a Generalizable Method for Source-Free Domain Adaptation”, appearing at ICML 2023. We show that state-of-the-art SFDA methods can underperform or even collapse when confronted with realistic distribution shifts in bioacoustics. Furthermore, existing methods perform differently relative to each other than observed in vision benchmarks, and surprisingly, sometimes perform worse than no adaptation at all. We also propose NOTELA, a new simple method that outperforms existing methods on these shifts while exhibiting strong performance on a range of vision datasets. Overall, we conclude that evaluating SFDA methods (only) on the commonly-used datasets and distribution shifts leaves us with a myopic view of their relative performance and generalizability. To live up to their promise, SFDA methods need to be tested on a wider range of distribution shifts, and we advocate for considering naturally-occurring ones that can benefit high-impact applications.

Distribution shifts in bioacoustics

Naturally-occurring distribution shifts are ubiquitous in bioacoustics. The largest labeled dataset for bird songs is Xeno-Canto (XC), a collection of user-contributed recordings of wild birds from across the world. Recordings in XC are “focal”: they target an individual captured in natural conditions, where the song of the identified bird is at the foreground. For continuous monitoring and tracking purposes, though, practitioners are often more interested in identifying birds in passive recordings (“soundscapes”), obtained through omnidirectional microphones. This is a well-documented problem that recent work shows is very challenging. Inspired by this realistic application, we study SFDA in bioacoustics using a bird species classifier that was pre-trained on XC as the source model, and several “soundscapes” coming from different geographical locations — Sierra Nevada (S. Nevada); Powdermill Nature Reserve, Pennsylvania, USA; Hawai’i; Caples Watershed, California, USA; Sapsucker Woods, New York, USA (SSW); and Colombia — as our target domains.

This shift from the focalized to the passive domain is substantial: the recordings in the latter often feature much lower signal-to-noise ratio, several birds vocalizing at once, and significant distractors and environmental noise, like rain or wind. In addition, different soundscapes originate from different geographical locations, inducing extreme label shifts since a very small portion of the species in XC will appear in a given location. Moreover, as is common in real-world data, both the source and target domains are significantly class imbalanced, because some species are significantly more common than others. In addition, we consider a multi-label classification problem since there may be several birds identified within each recording, a significant departure from the standard single-label image classification scenario where SFDA is typically studied.

Illustration of the “focal → soundscapes” shift. In the focalized domain, recordings are typically composed of a single bird vocalization in the foreground, captured with high signal-to-noise ratio (SNR), though there may be other birds vocalizing in the background. On the other hand, soundscapes contain recordings from omnidirectional microphones and can be composed of multiple birds vocalizing simultaneously, as well as environmental noises from insects, rain, cars, planes, etc.

Audio files		Focal domain		Soundscape domain1
Spectogram images

State-of-the-art SFDA models perform poorly on bioacoustics shifts

As a starting point, we benchmark six state-of-the-art SFDA methods on our bioacoustics benchmark, and compare them to the non-adapted baseline (the source model). Our findings are surprising: without exception, existing methods are unable to consistently outperform the source model on all target domains. In fact, they often underperform it significantly.

As an example, Tent, a recent method, aims to make models produce confident predictions for each example by reducing the uncertainty of the model’s output probabilities. While Tent performs well in various tasks, it doesn’t work effectively for our bioacoustics task. In the single-label scenario, minimizing entropy forces the model to choose a single class for each example confidently. However, in our multi-label scenario, there’s no such constraint that any class should be selected as being present. Combined with significant distribution shifts, this can cause the model to collapse, leading to zero probabilities for all classes. Other benchmarked methods like SHOT, AdaBN, Tent, NRC, DUST and Pseudo-Labelling, which are strong baselines for standard SFDA benchmarks, also struggle with this bioacoustics task.

Evolution of the test mean average precision (mAP), a standard metric for multilabel classification, throughout the adaptation procedure on the six soundscape datasets. We benchmark our proposed NOTELA and Dropout Student (see below), as well as SHOT, AdaBN, Tent, NRC, DUST and Pseudo-Labelling. Aside from NOTELA, all other methods fail to consistently improve the source model.

Introducing NOisy student TEacher with Laplacian Adjustment (NOTELA)

Nonetheless, a surprisingly positive result stands out: the less celebrated Noisy Student principle appears promising. This unsupervised approach encourages the model to reconstruct its own predictions on some target dataset, but under the application of random noise. While noise may be introduced through various channels, we strive for simplicity and use model dropout as the only noise source: we therefore refer to this approach as Dropout Student (DS). In a nutshell, it encourages the model to limit the influence of individual neurons (or filters) when making predictions on a specific target dataset.

DS, while effective, faces a model collapse issue on various target domains. We hypothesize this happens because the source model initially lacks confidence in those target domains. We propose improving DS stability by using the feature space directly as an auxiliary source of truth. NOTELA does this by encouraging similar pseudo-labels for nearby points in the feature space, inspired by NRC’s method and Laplacian regularization. This simple approach is visualized below, and consistently and significantly outperforms the source model in both audio and visual tasks.

NOTELA in action. The audio recordings are forwarded through the full model to obtain a first set of predictions, which are then refined through Laplacian regularization, a form of post-processing based on clustering nearby points. Finally, the refined predictions are used as targets for the noisy model to reconstruct.

Conclusion

The standard artificial image classification benchmarks have inadvertently limited our understanding of the true generalizability and robustness of SFDA methods. We advocate for broadening the scope and adopt a new assessment framework that incorporates naturally-occurring distribution shifts from bioacoustics. We also hope that NOTELA serves as a robust baseline to facilitate research in that direction. NOTELA’s strong performance perhaps points to two factors that can lead to developing more generalizable models: first, developing methods with an eye towards harder problems and second, favoring simple modeling principles. However, there is still future work to be done to pinpoint and comprehend existing methods’ failure modes on harder problems. We believe that our research represents a significant step in this direction, serving as a foundation for designing SFDA methods with greater generalizability.

Acknowledgements

One of the authors of this post, Eleni Triantafillou, is now at Google DeepMind. We are posting this blog post on behalf of the authors of the NOTELA paper: Malik Boudiaf, Tom Denton, Bart van Merriënboer, Vincent Dumoulin*, Eleni Triantafillou* (where * denotes equal contribution). We thank our co-authors for the hard work on this paper and the rest of the Perch team for their support and feedback.

¹Note that in this audio clip, the bird song is very faint; a common property in soundscape recordings where bird calls aren’t at the “foreground”. ^↩

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