Auditory systems rapidly process incoming acoustic information, subserving functions such as identifying sound sources, assessing source locations, and in some species, creating highly accurate spatial renderings of their environmental surroundings (echolocation). Humans, through non-verbal and spoken language faculties, use sound as an efficient medium to transmit large amounts of semantic information and to express nuanced emotions. These skills are supported by a network of anatomical structures between the cochleae and cortex. Even though the experiments described in the following chapters investigated neuronal responses of cortical origins, brief and general descriptions of certain major structures in the entire auditory pathway and their putative functions are given below as background.
The cochlea and auditory brainstem
Sound that enters the ears begins its transduction from physical phenomenon to biochemical/-electric signal at the tympanic membrane (ear drum). Oscillations of this membrane are transferred through an elegantly levered system of tiny bones (malleus, incus, and stapes) that in turn apply pressure to the primary sensory organ of the auditory system, the coiled and multi-chambered cochlea. Through a membrane called the oval window, these mechanical perturbations are transduced into fluid pressure waves. Along the length of the cochlea is a resonant structure called the basilar membrane (ibid). The basilar membrane resonates at different frequencies along its length; following a logarithmic tonotopic (by frequency) distribution, high frequency sounds resonate the base, or oval-window end of the cochlea, and low frequency sounds resonate the other end apex. Resonation of the basilar membrane causes small hair cells (possessing rows of hair-like stereocilia) to produce a fluid shear force between the reticular and tectorial membranes. These structures and others form the basis of a structure referred to as the Organ of Corti that houses the majority of the molecular machinery of the cochlea. The shearing forces of the hair cells cause neurotransmitters to be released, exciting the dendritic end of the cochlear nerve; its cell bodies reside in the spiral ganglion. Action potentials then proceed down the cochlear nerve to the cochlear nucleus of the brainstem.
The cochlear nuclei are anatomically subdivided broadly into dorsal and ventral sections; this division results in to quasi-distinct information processing streams. The ventral section (especially the anterior section, AVCN) primarily functions for sound localization, projecting onto the bilateral superior olivary complexes (SOC). Projections between the SOCs and the inferior colliculus form the lateral lemnisci (LL). The dorsal section (DCN) primarily extracts spectral and temporal acoustic information for source identification, sending most of its projections onto the contralateral inferior colliculus (IC). Information from these two streams seems to predominantly converge and integrate at the level of the inferior colliculi. The IC primarily projects to the medial geniculate body (MGB), a portion of the thalamus that is dominated by auditory functions. The ventral portion of the MGB is laminar and displays tonotopic organization similar to its primary source of input, the laminar central nucleus of the IC. The ventral MGB sends projections, via the acoustic radiations, to the transverse temporal gyri (Heschl’s gyri) in cortex, proposed locations for primary auditory cortices in humans. The dorsal and medial portions of the MGB (which are non-laminar) send projections to the planum temporale and other “higher-order” auditory cortical regions. The overall functional layout of the MGB is somewhat elusive (except for the ventral MGB) and it is highly influenced by the cortex through modulatory corticofugal inputs, perhaps owing to its purported role as an interface between the cortex and brainstem structures. Generally, the anatomical elements of the auditory brainstem show some well-defined functional organization. Notably, most of these structures possess tonotopic axes.
The primary auditory cortex in primates is purported to lie along the superior temporal plane; the cytoarchitectonic architectures of these cortices have been best described in the macaque. “Core” areas are usually defined in auditory cortices by their architectonic specificity, predominant thalamic input from the ventral MGB, and highly defined functional topographies; tonotopic representations in these regions usually display very systematic organization. Surrounding these core regions are belt regions, forming a ring around the central core. Parabelt regions form another band of anatomically distinct regions that are located laterally to the core/belt structures. Further from the core, tonotopic organization becomes less organized, likely reflecting more specialized computations of auditory signals such as the processing of sound-source identity and vocalization features. Additionally, belt and parabelt regions further project laterally into the superior temporal sulci (STS) and other temporal lobe structures as well as into frontal regions.
The auditory cortex in humans has similarly been described in architectonic and functional studies. Anatomically, the human cortical locations of primary auditory, or core regions, likely occur along the Heschl’s gyri. Belt and parabelts regions are likely to be found along the middle aspects of the STG, planum temporale, and into the STS. Functionally, numerous attempts have been made to describe tonotopically-defined core regions in human auditory cortices. Generally, mirror-symmetric tonotopic maps can be revealed on or near Heschl’s gyri, oftentimes posterior to the gyri. Woods et al. have performed very detailed functional analyses of the proposed auditory cortical fields in humans in an attempt to identify core-belt-parabelt organization and tuning similar to that seen in macaque models.
The elucidation of the functions of auditory cortex and related regions has often paralleled work and theory accomplished in the visual faculties. Similar to the visual system, two pathways for information were derived for the auditory system: an anterior-ventral pathway for object identification (“what” pathways) and a posterior-dorsal pathway for sound localization and action (“where” and/or “how” pathways). The anteriorventral “what” pathway is thought to house cortical processing pathways that analyze the fine acoustic structure of incoming auditory stimuli, allowing for very precise identification of sound sources; in cases of living sound sources, the sex, health, emotional state, and even intent can be surmised by close analyses of their respective vocal signals.