Browsing by Subject "speech processing"

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  • Virtala, Paula; Partanen, Eino; Tervaniemi, Mari; Kujala, Teija (2018)
    To process complex stimuli like language, our auditory system must tolerate large acoustic variance, like speaker variability, and still be sensitive enough to discriminate between phonemes and to detect complex sound relationships in, e.g., prosodic cues. Our study determined discrimination of speech sounds in input mimicking natural speech variability, and detection of deviations in regular pitch relationships (rule violations) between speech sounds. We investigated the automaticity and the influence of attention and explicit awareness on these changes by recording the neurophysiological mismatch negativity (MMN) and P3a as well as task performance from 21 adults. The results showed neural discrimination of phonemes and rule violations as indicated by MMN and P3a, regardless of whether the sounds were attended or not, even when participants could not explicitly describe the rule. While small sample size precluded statistical analysis of some outcomes, we still found preliminary associations between the MMN amplitudes, task performance, and emerging explicit awareness of the rule. Our results highlight the automaticity of processing complex aspects of speech as a basis for the emerging conscious perception and explicit awareness of speech properties. While MMN operates at the implicit processing level, P3a appears to work at the borderline of implicit and explicit.
  • Renvall, Hanna; Seol, Jaeho; Tuominen, Riku; Sorger, Bettina; Riecke, Lars; Salmelin, Riitta (2021)
    Rapid recognition and categorization of sounds are essential for humans and animals alike, both for understanding and reacting to our surroundings and for daily communication and social interaction. For humans, perception of speech sounds is of crucial importance. In real life, this task is complicated by the presence of a multitude of meaningful non-speech sounds. The present behavioural, magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI) study was set out to address how attention to speech versus attention to natural non-speech sounds within complex auditory scenes influences cortical processing. The stimuli were superimpositions of spoken words and environmental sounds, with parametric variation of the speech-to-environmental sound intensity ratio. The participants' task was to detect a repetition in either the speech or the environmental sound. We found that specifically when participants attended to speech within the superimposed stimuli, higher speech-to-environmental sound ratios resulted in shorter sustained MEG responses and stronger BOLD fMRI signals especially in the left supratemporal auditory cortex and in improved behavioural performance. No such effects of speech-to-environmental sound ratio were observed when participants attended to the environmental sound part within the exact same stimuli. These findings suggest stronger saliency of speech compared with other meaningful sounds during processing of natural auditory scenes, likely linked to speech-specific top-down and bottom-up mechanisms activated during speech perception that are needed for tracking speech in real-life-like auditory environments.
  • Ylinen, Artturi (Helsingin yliopisto, 2019)
    The present study uses functional magnetic resonance imaging (fMRI) to study the neural basis of naturalistic audiovisual speech processing. The study focuses on how performing different speech-related tasks affects the neural activation observed in the brain. The neural basis of speech processing has been studied for long, but previous experiments have mainly employed paradigms with simple stimuli, such as single phonemes, syllables, and words, and unnatural tasks, such as discrimination and memory tasks. Moreover, current models of speech processing are mainly based on studies using auditory-only stimuli. In natural situations, however, the aim of speech processing is to understand the meaning of what is being said, speech is most often audiovisual, it is often selectively attended to, and the signal-to-noise ratio of the speech signal varies (due to, for example, background noise). The present study aims to study speech processing in a more naturalistic setting that takes into account the above-mentioned factors. Moreover, it compares a situation where speech is processed naturalistically to a situation where speech is processed sub-lexically, as is often done in studies of speech processing. The participants of the present study were 19 healthy adults. They were presented with audiovisual dialogues where two people discuss everyday matters. In the background of the videos, the voice of a third speaker was always present so that the participants had to selectively attend to the dialogues. The auditory and visual qualities of the dialogues were modulated on two different levels. The participants performed three different tasks: 1) A semantic task, during which the participants focused on the semantic content of the dialogues. 2) A phonological task, during which the participants focused on the phonological structure of the speech. 3) A visual task, during which the participants ignored the dialogues and focused on a fixation cross. Task-dependent effects in the neural activation were found in multiple regions of the brain. Selective attention to speech was found to activate regions in the temporal lobes and the left inferior frontal gyrus (LIFG), a result consistent with earlier research. The semantic task was found to activate areas associated with semantic and socio-cognitive processing more than either of the other tasks, while the phonological task activated the posterior portion of the LIFG as well as the ventral portion of the left premotor cortex. Interactions between the effects of task and audiovisual quality were also found in many brain areas. These results show that task-dependent effects in speech processing can be seen in widespread regions of the brain, and that audiovisual quality can modulate activation differentially during different tasks. These findings highlight the importance of using naturalistic stimuli and tasks in studies of speech processing.