Spoken word perception

Previous neuroimaging research (Binder et al., 2000; Scott & Johnsrude, 2003) showed that listening to spoken words activates auditory regions within the anterolateral STG and STS, which are linked to belt and parabelt areas. This observation corresponds to findings of research that compared the perception of consonant-vowel syllables and vowels to that of white noise and pure tones, indicating that activations responsive to phonetic perception occur in the STS (Jӓncke, Wüstenberg, Scheich, & Heinze, 2002).

While the passive presentation of words activates the STG bilaterally (Binder, Swanson, Hammeke, & Sabsevitz, 2008a; Petersen, Fox, Posner, Mintun, & Raichle, 1988; Wise et al., 1991), the comparison of the perception of spoken words to that of silence results in activation associated with speech within the posterior part of the STS/STG together with the left inferior frontal gyrus (lIFG) (e.g. Binder et al., 1994b; Petersen et al., 1988; Wise et al., 2001). For instance, both the perception and the recovery of individual words through memory have been linked to the left posterior superior temporal sulcus (pSTS) within the left superior temporal cortex (Wise et al., 2001). It has therefore been implied that the pSTS might link the perception of words with the long-standing representations of known words stored in memory (Wise et al., 2001). The pSTS/STG and lIFG have also been implicated with speech-related activations in studies that compared syllables to noise (Zatorre et al., 1992), words with reversed speech (Price et al., 1996a) and in research, which involved participants’ completion of phonological monitoring tasks (Demonet et al., 1992).

The left inferior frontal gyrus (lIFG)  (in orange) encompassing the pars orbitalis, pars triangualris, and the pars opercularis, with the last two constituting Broca's area (Brodmann areas 44 and 45).

More recently, speech processing was observed to elicit a premotor response that was correlated with improved perceptual performance (Callan, Callan, Gamez, Sato, & Kawato, 2010; Osnes, Hugdahl, & Specht, 2011). Nonetheless, a lack of activity in the left premotor cortex in response to articulatory complexity during perceptual processing of speech was concluded to imply that during speech perception, the left premotor cortex might be active only to a certain degree (Tremblay & Small, 2011a). Therefore it can be said that together with the IFG, left posterior temporal areas might be part of a network for phonological processing of perceived speech (Zatorre et al., 1992), which is contributed to by premotor and frontoparietal areas for articulatory processing (Tremblay & Small, 2011a).

References:

Binder, J.R., Frost, J. A., Hammeke, T.A., Bellgowan, P.S.F., Springer, J.A., Kaufman, J.N. &Posing, E.T. (2000). Human temporal lobe activation by speech sounds and non-speech sounds. Cerebral Cortex, 10, 512-28.

Binder, J.R., Rao, S.M., Hammeke, T.A., Frost, J.A., Bandettini, P.A., & Hyde, J.S. (1994b). Effects of stimulus rate on signal response during functional magnetic resonance imaging of auditory cortex. Brain Research Cognitive Brain Research, 2, 31-38.

Binder, J.R., Swanson, S.J., Hammeke, T.A., & Sabsevitz, D.S. (2008a). A comparison of five fMRI protocols for mapping speech comprehension systems, Epilepsia, 49, 1980-1997.

Callan, D., Callan, A., Gamez, M., Sato, M.A., &Kawato, M. (2010). Premotor cortex mediates perceptual performance. Neuroimage, 51, 844-858.

Demonet, J.-F., Chollet, F., Ramsay, A., Cardebat, D., Nespoulous, J.-L., Wise, R., Rascol, A., & Frackowiak, R. (1992). The anatomy of phonological and semantic processing in normal subjects. Brain, 115, 1753-1768. 

Jӓncke, L., Wüstenberg, T., Scheich, H., & Heinze, H.-J. (2002). Phonetic perception and the temporal cortex. NeuroImage, 15, 733-746.

Osnes, B., Hugdahl, K., Specht, K. (2011). Effective connectivity analysis demonstrates involvement of premotor cortex during speech perception. Neuroimage, 54, 2437-2445.

Petersen, S.E., Fox, P.T., Posner, M.I., Mintun, M, Raichle, M.E. (1988). Positron emission tomographic studies of the cortical anatomy of single-word processing. Nature, London, 331, 585-589.

Price, C. J., Wise, R., Warburton, E.A., Moore, C.J., Howard, D., Patterson, K., Frackowiak, R.S., &Friston, K.J. (1996a). Hearing and saying. The functional neuroanatomy of auditory word processing, Brain, 119 (Pt3), 919-931.

Scott, S.K., & Johnsrude, I.S. (2003). The neuroanatomical and functional organization of speech perception. Trends in Neuroscience, 26, 100-107.

Tremblay, P., Small, V.L. (2011a). On the context dependent nature of the contribution of the ventral premotor cortex to speech perception. Neuroimage, 57, 1561-1571.

Wise, R.J., Chollet, F., Hadar, U., Friston, K., Hoffner, E., & Frackowiak, R. (1991). Distribution of cortical neural networks involved in word comprehension and word retrieval. Brain, 114, 1803-1817.

Wise, R.J., Scott, S.K., Blank, S.C., Mummery, C.J., Murphy, K., & Warburton, E.A. (2001). Separate neural subsystems within 'Wernicke's area. Brain, 124, 83-95.

Zatorre, R.J., Evans, A.C., Meyer, E., & Giede, A. (1992). Lateralization of phonetic and pitch discrimination in speech processing. Science, 256, 846-849.

Hierarchical structure and multiple parallel streams within the human auditory system

The study of human auditory processing has been aided to a large extent by research on the auditory system of non-human primates since invasive methods on humans cannot usually be carried out apart from infrequent cases as in neurosurgical treatments (Howard et al., 2000; Kaas & Hackett, 1998; Pandya & Sanides, 1972; Rauschecker, 1998b). According to anatomical evidence, the primate auditory cortex contains cortical auditory regions that are three distinct levels of processing including the auditory core, ‘belt’ and ‘parabelt’ areas (Kaas & Hackett, 1998). The auditory core consists of three primary cortical fields, which are cytoarchitectonically distinct in structure and which receive compact parallel information from the thalamus. These cortical fields send projections to the ‘belt’ area that surrounds them (Hackett, Stephniewska, & Kaas, 1998a; Kaas & Hackett, 1998; Kaas & Hackett, 2000; Pandya, 1995; Rauschecker, 1998b). These cortical fields link to those of the lateral ‘parabelt’ (Kaas & Hackett, 1998). Cortical fields within the ‘belt’ and ‘parabelt’ are considered to send projections to the frontal, parietal and also temporal lobes (Kaas & Hackett, 2000).

The three main auditory cortical regions: core area (with the primary auditory receiving area A1), the belt area, and the parabelt area. Arrows show how signals travel from core,  to belt, to parabelt (from Kaas, Hackett, &Tram, 1999). 

The function specific character of the core and belt areas reflects the tonotopic nature of the auditory cortex (Rauschecker, 1995). Accordingly, while neurons within cortical fields of the auditory core show responsiveness to pure tones, neurons in the lateral ‘belt’ area respond selectively to different bandwidths of noise (Rauschecker, 1995). In contrast, neurons within the parabelt region respond to intricate sounds such as expressions that are particular to a species (Kosaki, Hashikawa, He, & Jones, 1997; Rauschecker, Tian, & Hauser, 1995).

The communication among the auditory core, belt and parabelt is considered to occur in sequence with most information being serially conveyed from the auditory core through the belt area to the parabelt area (Kaas & Hackett, 2000). Although neurons in both belt and parabelt areas receive thalamic inputs, belt neurons appear to rely on core inputs and parabelt neurons rely on belt inputs for auditory activation (Kaas & Hackett, 1998). As the intricacy of processing demands increases, at each processing level, the communication of cortical fields with remote fields is facilitated through corticocortical networks (Kaas & Hackett, 1998). The hierarchical character of the auditory core, belt and parabelt, based on their connections and response characteristics, upholds three distinct levels of processing in the auditory system (Kaas & Hackett, 1998, 2000; Pandya, 1995; Rauschecker, 1998b).


Recent imaging studies of the human brain have confirmed the idea of a comparable hierarchical formation within the human auditory system. Accordingly, the PAC within the HG has been considered to be similar to the auditory core regions in primates (Morosan et al., 2001). The PAC includes three distinct cytoarchitectonic areas and is situated on the dorsal exterior of the STG, being mainly concealed in the Sylvian fissure. Similar to the primate auditory core, the human PAC has a tonotopic organization and demonstrates responses to pure tones and band pass noise (Formisano et al., 2003; Wessinger et al., 2001).

Tonotopic organization within the primary auditory cortex: the frequency spectrum of sound being mapped across the surface of the brain as neurons at distinct locations are sensitive to distinct frequencies (from http://www.sltinfo.com/speech-perception/). 

The notion of multiple levels of processing occurring in parallel within the human auditory system has also been substantiated through human neuroimaging studies (Bruegge & Reale, 1985; Zatorre, Evans, Meyer, & Gjedde, 1992). Accordingly, while high-level linguistic processing such as semantic and syntactic processing was linked to activated frontal and temporal areas (Peelle, Johnsrude, & Davis, 2010), low-level processing of acoustic speech shapes occurred within the locality of the PAC (Davis & Johnsrude, 2003).

Moreover, the discovery of an anterior and a posterior auditory processing stream in primates, with the anterolateral auditory association cortex being involved in articulations particular to a species, is comparable to the observation of anterior and posterior streams in the human auditory system (Kaas & Hackett, 1999; Rauschecker, 1998a; Romanski et al., 1999; Upadhyay et al., 2008). Specifically, by employing structural connectivity, one study could distinguish between a route going from the posterior HG to the posterior STG and a route that runs from the anterior HG to the anterior STG, with communication between these routes most likely to be facilitated through the STS (Upadhyay et al., 2008). As a consequence it can be said that although humans and primates differ in the complexity of their brains, a great deal of information about the human auditory system has been inferred from the study of the auditory organization of primates.