Prior experience shapes speech perception: a behavioural and neuroimaging perspective

Recent developments in neuroimaging and computational methods have allowed us to measure brain responses to naturalistic stimuli such as words, sentences, and narratives. This has increased our understanding of how the human brain process connected speech. However, it is still unclear how the brain grasps the meaning of acoustically distorted speech. Further, the question of why an interpretation of the same narrative can vary between individuals has remained unknown. This thesis illuminated these issues by studying how prior experience affects speech perception. The first studies of the thesis demonstrated that initially unintelligible, acoustically distorted speech stimuli can be rendered intelligible by presenting them after only a single exposure to their respective intact counterparts. The intelligibility of sentences increased more than that of words, and vowels remained unintelligible. The contrast between the magnetoencephalography response to the distorted vowels presented before versus after the intact vowels revealed enhanced source current density in the auditory cortex and surrounding areas at the latencies of 130–160 ms from stimulus onset. The corresponding contrast between the first and second presentations of distorted sentences in functional magnetic imaging (fMRI) revealed modulations in responses generated in the primary auditory cortex and surrounding areas as well as in several extralinguistic brain areas that have been associated with memory and executive functions in previous studies. The final study showed that when subjects share a cultural family background, and therefore presumably have also accumulated more similar experience throughout their lifetime, this is reflected in the interpretation and neural processing of spoken narratives. In sum, the results of this thesis show that prior experience can dramatically increase speech intelligibility in acoustically adverse conditions, and lexico-semantic information in long-term memory seems to be important in this process. The results provide further evidence that the activity in the auditory areas is not only modulated by auditory information but also by prior experience. Moreover, it seems that experience accumulated throughout the lifetime is reflected in speech processing. The results of this thesis increase knowledge of speech processing in acoustically suboptimal conditions. The results may also help to overcome potential challenges in mutual understanding between individuals from different cultural backgrounds.