Sublexical cues affect degraded speech processing: insights from fMRI

In natural listening situations, speech perception is often impaired by degraded speech sounds arriving at the ear. Contextual speech information can improve the perception of degraded speech and modify neuronal responses elicited by degraded speech. However, most studies on context effects on neural responses to degraded speech confounded lexico-semantic and sublexical cues. Here, we used fMRI to investigate how prior sublexical speech (e.g. pseudowords cues) affects neural responses to degraded sublexical speech and hence its processing and recognition. Each trial consisted of three consecutively presented pseudowords, of which the first and third were identical and degraded. The second pseudoword was always presented in clear form and either matched or did not match the degraded pseudowords. Improved speech processing through sublexical processing was associated with BOLD activation increases in frontal, temporal, and parietal regions, including the primary auditory cortex (PAC), posterior superior temporal cortex, angular gyrus, supramarginal gyrus, middle temporal cortex, and somato-motor cortex. These brain regions are part of a speech processing network and are involved in lexico-semantic processing. To further investigate the adaptive changes in PAC, we conducted a bilateral region of interest analysis on PAC subregions. PAC ROIs showed bilaterally increased activation in the match condition compared with the mismatch condition. Our results show that the perception of unintelligible degraded speech is improved and the neuronal population response is enhanced after exposure to intact sublexical cues. Furthermore, our findings indicate that the processing of clear meaningless sublexical speech preceding degraded speech could enhance the activity in the brain regions that belong to the cortical speech processing network previously reported in studies investigating lexico-semantic speech.