Ensemble Machine Learning Model for Inner Speech Recognition: A Subject-Specific Investigation

Inner speech recognition has gained enormous interest in recent years due to its applications in rehabilitation, developing assistive technology, and cognitive assessment. However, since language and speech productions are a complex process, for which identifying speech components has remained a challenging task. Different approaches were taken previously to reach this goal, but new approaches remain to be explored. Also, a subject-oriented analysis is necessary to understand the underlying brain dynamics during inner speech production, which can bring novel methods to neurological research. A publicly available dataset, “Thinking Out Loud Dataset,” has been used to develop a Machine Learning (ML)-based technique to classify inner speech using 128-channel surface Electroencephalography (EEG) signals. The dataset is collected on a Spanish cohort of ten subjects while uttering four words (“Arriba,” “Abajo,” “Derecha,” and “Izquierda”) by each participant. Statistical methods were employed to detect and remove motion artifacts from the signals. A large number (191 per channel) of time-, frequency- and time-frequency-domain features were extracted. Eight feature selection algorithms are explored, and the best feature selection technique is selected for subsequent evaluations. The performance of six ML algorithms is evaluated, and an ensemble model is proposed. Deep Learning models are also explored, and the results are compared with the classical ML approach. The proposed ensemble model, by stacking the five best logistic regression models, generated a promising result with an overall accuracy of 81.13% and an F1 score of 81.12% in the classification of four inner speech words using surface EEG signals.