You are provided with the abstract of a scientific paper and the abstracts of the papers it references. Your task is to write a Related Work section for the main paper. This section should concisely summarize and integrate the key points from the cited abstracts, reflecting how they relate to the main paper's contributions and context. In your summary, use numerical citation markers like [1], [2], etc., to refer to the corresponding papers.

Main Abstract:
Ultrasound tongue imaging (UTI) provides a convenient way to visualize the vocal tract during speech production. UTI is increasingly being used for speech therapy, making it important to develop automatic methods to assist various time-consuming manual tasks currently performed by speech therapists. A key challenge is to generalize the automatic processing of ultrasound tongue images to previously unseen speakers. In this work, we investigate the classification of phonetic segments (tongue shapes) from raw ultrasound recordings under several training scenarios: speaker-dependent, multi-speaker, speaker-independent, and speaker-adapted. We observe that models underperform when applied to data from speakers not seen at training time. However, when provided with minimal additional speaker information, such as the mean ultrasound frame, the models generalize better to unseen speakers.

Cited Abstract(s):
[1]: 

[2]: This article addresses the problem of continuous speech recognition from visual information only, without exploiting any audio signal. Our approach combines a video camera and an ultrasound imaging system for monitoring simultaneously the speaker's lips and the movement of the tongue. We investigate the use of convolutional neural networks (CNN) to extract visual features directly from the raw ultrasound and video images. We propose different architectures among which a multimodal CNN processing jointly the two visual modalities. Combined with an HMM-GMM decoder, the CNN-based approach outperforms our previous baseline based on Principal Component Analysis. Importantly, the recognition accuracy is only 4 lower than the one obtained when decoding the audio signal, which makes it a good candidate for a practical visual speech recognition system.

[3]: Hidden Markov Model and Deep Neural Network-Hidden Markov Model speech recognition performance for a portable ultrasound + video multimodal silent speech interface is investigated using Discrete Cosine Transform and Deep Auto Encoder-based features with a range of dimensionalities. Experimental results show that the two types of features achieve similar Word Error Rate, but that the autoencoder features maintain good performance even for very low-dimension feature vectors, demonstrating potential as a very compact representation of the information in multimodal silent speech data. It is also shown for the first time that the Deep Network Markov approach, which has been demonstrated to be beneficial for acoustic speech recognition and for articulatory sensor-based silent speech, improves the silent speech recognition performance for video-based silent speech recognition as well.

[4]: This paper describes a machine learning approach for extracting automatically the tongue contour in ultrasound images. This method is developed in the context of visual articulatory biofeedback for speech therapy. The goal is to provide a speaker with an intuitive visualization of his her tongue movement, in real-time, and with minimum human intervention. Contrary to most widely used techniques based on active contours, the proposed method aims at exploiting the information of all image pixels to infer the tongue contour. For that purpose, a compact representation of each image is extracted using a PCA-based decomposition technique (named EigenTongue). Artificial neural networks are then used to convert the extracted visual features into control parameters of a PCA-based tongue contour model. The proposed method is evaluated on 9 speakers, using data recorded with the ultrasound probe hold manually (as in the targeted application). Speaker-dependent experiments demonstrated the effectiveness of the proposed method (with an average error of 1.3 mm when training from 80 manually annotated images), even when the tongue contour is poorly imaged. The performance was significantly lower in speaker-independent experiments (i.e. when estimating contours on an unknown speaker), likely due to anatomical differences across speakers.

[5]: The article describes a video-only speech recognition system for a “silent speech interface” application, using ultrasound and optical images of the voice organ. A one-hour audiovisual speech corpus was phonetically labeled using an automatic speech alignment procedure and robust visual feature extraction techniques. HMM-based stochastic models were estimated separately on the visual and acoustic corpus. The performance of the visual speech recognition system is compared to a traditional acoustic-based recognizer.

[6]: Tongue gestural target classification is of great interest to researchers in the speech production field. Recently, deep convolutional neural networks (CNN) have shown superiority to standard feature extraction techniques in a variety of domains. In this letter, both CNN-based speaker-dependent and speaker-independent tongue gestural target classification experiments are conducted to classify tongue gestures during natural speech production. The CNN-based method achieves state-of-the-art performance, even though no pre-training of the CNN (with the exception of a data augmentation preprocessing) was carried out.
Earlier work concerned with speech recognition from ultrasound data has mostly been focused on speaker-dependent systems [4]5 [3] [2] [1] . An exception is the work of [6] , which investigates the classification of tongue gestures from ultrasound data using convolutional neural networks. Some results are presented for a speaker-independent system, although the investigation is limited to two speakers generalizing to a third. [4] present a method for automatic tongue contour extraction from ultrasound data. The system is evaluated in a speaker-independent way by training on data from eight speakers and evaluating on a single held out speaker. In both of these studies, a large drop in accuracy was observed when using speaker-independent systems in comparison to speaker-dependent systems. Our investigation differs from previous work in that we focus on child speech while using a larger number of speakers (58 children). Additionally, we use cross-validation to evaluate the performance of speaker-independent systems across all speakers, rather than using a small held out subset.