Outline 2.1 The Auditory What (“Ventral”) Stream
- Definition
- Funktion
- lateralization differences
- Anatomie
- Rolls Group 1
To-Do’s
- check wording
- check sources
2.1 The Auditory What (“Ventral”) Stream
The auditory ‘what’-stream represents the more conceptually stable part of the dual-stream architecture, mostly tasked with transformation from auditory signals to semantic representations. The ‘what’-stream’s role in mapping sounds to meaning remains widely accepted as fundamental. This chapter discusses existing literature about the auditory ventral ‘what’-stream.
2.1.1 Grouping by Rolls (2022)
Throughout this thesis, I will refer to the groups proposed by Rolls et al (2022), that identified three large-scale networks using effective connectome analysis. These groups provide an anatomical reference for the regions discussed throughout the thesis. Their Group 1 consists of inferior STS regions (STSva, STSvp), anterior inferior temporal cortex (TE1a), temporal pole (TGd) and parietal PGi, aligning closely with the ventral stream described here. This group has effective connectivity (EC) directed toward Broca’s areas (44, 45, 47s), receiving input from high-level areas, has EC with memory-related regions, with the frontal pole and EC directed to parts of Group 2. This group includes a large frontal system with regions 44, 45, 47l, SFL and 55b, also including inferior temporal region TGv. This group has robust EC with superior (STSda, STSdp, STGa) and inferior regions as STSvp and the Peri-Sylvian Language Area (PSL). Finally, Group 3 is centered around superior temporal lobe and the temporoparietal junction with areas A5, STGa, STSda, STSdp, PSL, STV and TPOJ1. They show EC with some visual and auditory areas (Belt regions, A1, A4, A5), also with the frontal operculum and has strong EC directed towards 44 and 45.
2.1.2 Functional Definition
The primary objective is auditory object identification - the process of isolation of what is being heard (e.g. specific words, a dog’s bark or a piano) from a chaotic acoustic environment.
It operates as a sound-to-meaning starting with feature-extraction in the primary auditory cortex, which responds well to pure tones (Rauschecker & Afsahi (2023) - Journal of Comparative Neurology). Top-down attention in the ‘what’-pathway is mediated by the IFJ, mainly the anterior part. The IFJ is located at the junction of the inferior precentral sulcus (iPCS) and the inferior frontal sulcus (IFS), corresponding to BA6, BA8 and BA44 (Bedini & Baldauf (2021)). The IFJ is subdivided into an anterior and a posterior part:
- anterior Inferior Frontal Junction (IFJa): Unlike the FEF, the IFJa processes non-spatial, object-based information and is part of the Frontoparietal Network (FPN) and functions as a mediator of top-down, feature-based attention, particularly in language-related networks (Ji et al., 2019, as cited in Bedini & Baldauf, 2021). It is also involved in working memory, and may encode information at a higher level of abstraction than the FEF. It can modulate DAN activity according to current task demands and toggle between DAN and VAN activity. Critically, IFJa receives EC from Groups 2 and 3 (Rolls et al., 2022), but not from Group 1 - suggesting that IFJa is preferentially accessed by frontal language output regions and superior temporal auditory areas, but not by the inferior STS network.
- posterior Inferior Frontal Junction (IFJp - Control Seed): While the majority of neuroimaging literature treats the IFJ as a monolithic unit, recent structural and functional evidence suggests a clear dissociation between its anterior (IFJa) and posterior (IFJp) subdivisions (Bedini (2023) - Brain Structure). IFJp acts as a core node of the multiple-demand system, activated by generalized executive tasks (e.g., reasoning, math). We included IFJp specifically as a control region to validate the target specificity of the IFJa towards the auditory what-pathway.
2.1.3 Anatomical Trajectory
The anteroventral auditory ‘what’-stream originates in the Herschl’s Gyrus, extending through the Superior Temporal Gyrus (STGa), and along the Superior Temporal Sulcus (STS), Middle and Inferior Temporal Gyrus, to the anterior inferior frontal gyrus, the Broca’s areas 44, 45 and 47l (Ahveninen et al., 2006; Frühholz, 2015). Additionally, Soyuhos & Baldauf (2023) demonstrated that anterior IFJ has strong power coupling with the temporal cortex in delta and gamma frequencies.
2.1.4 Differences in Lateralization
The auditory ‘what-stream shows a left hemispheric dominance for language-related processing. Rolls (2022) - NeuroImage reports stronger left-hemispheric EC across what-stream regions STS, temporal pole, PGi and inferior frontal regions 44, 45 and 47l. The right hemisphere, however, still plays a complementary role in emotional and prosodic aspects of speech, activating right STG and IFG (Frühholz (2015) - NeuroImage), suggesting a division of labor in which the left hemisphere dominates semantic processing and the right contributes to the affective dimension of auditory objects.