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.