Speech word intelligibility

Previous research that looked into brain regions that respond to speech intelligibility compared intelligible speech with conditions that included nonspeech baselines by employing subtractive designs (Binder et al., 2000; Scott, Blank, Rosen, & Wise, 2000). One PET study, for instance, revealed that intelligible speech activates the anterior left superior temporal sulcus (aSTS) (Scott et al., 2000).

The superior temporal sulcus (STS) in the left hemisphere.

The study carefully matched stimuli for acoustic complexity, which differed in clarity. It demonstrated that the left posterior STS responded to auditory signals on a phonetic level regardless of whether these stimuli were intelligible or not (Scott et al., 2000). On the contrary, the anterolateral stream from the PAC showed a response to intelligible speech. Specifically, it was shown that anterior and ventral to the PAC, the STS responded to clear speech only. This study can therefore be considered to have shown that the response of the anterior section of the left STS evidently differs from the response of the posterior section of the left STS (Scott et al., 2000). This study is consistent with prior research that showed intelligible speech such as connected speech (e.g. stories) to activate anterior temporal lobe areas (Mazoyer et al., 1993; Schlosser, Aoyagi, Fulbright, Gore, & McCarthy, 1998). The findings in Scott et al.’s study (2000) are also in line with observations on patients with semantic dementia that was linked to loss of grey matter within the temporal lobe of the left hemisphere (Chan et al., 2001).

These findings were extended by a correlational fMRI study, in which acoustically degraded speech stimuli that differed in three distinct manners and in their extent of being intelligible were rated for intelligibility (Davis & Johnsrude, 2003). Brain regions that were identified to respond to intelligibility included, in addition to the bilateral anterior middle temporal gyrus, the left anterior hippocampus, the left inferior frontal gyrus and angular gyrus and the left pSTG (Davis & Johnsrude, 2003). It has been suggested that the involvement of the left anterior hippocampus represents a response to meaningful speech stimuli, which previously had been shown to be normally encoded and maintained in parts of the left medial temporal lobe (Strange, Otten, Josephs, Rugg, & Dolan, 2002). Activations of the left pSTS and the left angular gyrus related to intelligibility have been suggested to provide evidence of additional processing streams (Davis & Johnsrude, 2003).

Moreover, it was shown regarding brain regions that are sensitive to degraded speech that sentences heard in noisy conditions may be separated from noise by low-level auditory operations (Davis & Johnsrude, 2003). The observation of an especially marked response in regions close to the PAC that increased with degradation has been indicated to illustrate the augmented use of attentional resources to the masked speech stimulus (Davis & Johnsrude, 2003). The intelligibility of speech of degraded quality was also reported to modulate increased responses to degraded speech within the frontal operculum (Davis & Johnsrude, 2003). The observed elevated recruitment of attention during the perception of speech of degraded quality is in line with the suggestion that the perception of degraded speech requires more attention compared to clear speech (Rabbitt, 1990). Recent evidence for example implies that the extent to which listeners pay attention to speech determines both the comprehension of sentences that differ in speech clarity and the involvement of those brain regions that uphold speech processing (Wild, Davis, & Johnsrude, 2012). Specifically, attention was found to improve the processing of unclear speech in STS and the lIFG.

In addition to the finding of a path that is directed to the anterolateral temporal cortex involved in stimulus intelligibility (Scott et al., 2000), another PET study revealed a path towards the posterior superior temporal cortex specialised in processes involved in repetition (Wise et al., 2001). The results from both PET studies were confirmed by a further fMRI study (Narain et al., 2003). Using a passive language-listening task, the study indicated robustly left-lateralised activation for intelligible speech, including the posterior STG and the anterior STS (Narain et al., 2003). However, when directly compared to the study it is based on (Scott et al., 2000), the study by Narain et al (2003) showed a more intense posterior response compared to the anterior activation on the STS. This difference in results was attributed to the difference of the method used which resulted in an altered power of analysis. This was confirmed by a reanalysis of the original PET study data in Scott et al.’s study (Narain et al., 2003).

 More recent research has also found elevated spectral information in the speech signal to activate the anterior STS in both hemispheres, in addition to the IFG (Obleser, Wise, Alex Dresner, & Scott, 2007a).

It was shown that when speech clarity decreases under adverse listening conditions, intelligibility of speech is assisted through raised functional connectivity across auditory cortical areas that include the posterior cingulate cortex, the dorsolateral prefrontal cortex and the angular gyrus (Obleser et al., 2007a). It has been suggested that the reported functional connectivity between the angular gyrus and the left IFG is supported by links between these areas through the superior longitudinal fasciculus (Eisner, McGettigan, Faulkner, Rosen, & Scott, 2010; Frey, Campbell, Pike, & Petrides, 2008; Obleser et al., 2007a). It can therefore be said that the processing of intelligible speech is supported by higher-level cortical regions that are distant from the PAC in an unfavourable listening condition and that the functional connectivity between these areas is fortified when speech intelligibility is aided by semantic context (Obleser et al., 2007a).