Modulating speech tracking through brain state-dependent changes in audio loudness

<i>Objective.</i>To determine whether the perceptual intensity of speech signals-manipulated via loudness and dynamically adjusted through a brain state-dependent stimulation (BSDS) paradigm-modulates neural speech tracking and short-term memory.<i>Approach.</i>We implemented an EEG brain state-dependent design in which real-time variations in alpha power were used to modulate the loudness of pre-recorded digits during a task modelled on the digit span test. Speech tracking was quantified using lagged Gaussian copula mutual information (2-10 Hz), and behavioural performance was assessed through recall accuracy.<i>Main results.</i>Contrary to our initial hypothesis that higher loudness would enhance speech tracking and memory via bottom-up attention, digit recall accuracy was stable across loudness conditions. Speech tracking revealed an unexpected pattern: louder stimuli presented during high alpha power (low attention) elicited reduced tracking magnitudes and shorter peak latencies, whereas quieter stimuli delivered during low alpha power (high attention) produced stronger and more temporally extended tracking responses.<i>Significance.</i>These findings may suggest that internal attentional state, rather than external stimulus salience, plays a dominant role in shaping speech encoding. The study provides proof-of-concept evidence for BSDS in auditory paradigms, showing the importance of attentional fluctuations and stimulus loudness in determining the strength and timing of neural speech tracking, with implications for the design of adaptive speech-enhancement strategies.