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(DeCasper and Prescott, 1984). Newborns also have cry melodies reminiscent of their maternal language (Mampe et al., 2009).
Because of their specificity, for example, of the mater- nal language, all of these abilities are not likely to be hardwired by genetic programs. Consequently, these experiments point to a fundamental effect of the fetus being exposed to the mother’s voice and suggest that complex auditory processing is possible in humans before term birth. Moreover, the selectivity of the responses to the maternal voices indicate that a significant amount of circuit plasticity has occurred in the auditory pathway before birth to create neuronal circuits that allow the developing brain to distinguish the mother’s voice from other voices. What has been unclear, however, is how such early sound exposure shapes the auditory pathway and which neurons and circuits are being influenced.
Functional Organization of the
Auditory System
The auditory system is organized in a hierarchical manner
starting from the conversion of sound into neural impulses in the ear up to the complex analysis or the evoked neuro- nal activity patterns in central brain structures. Sound is transmitted through the ear canal and middle ear and then enters the inner ear, the cochlea, where it is converted into neural activity. Sound-evoked neural activity then propa- gates through a series of different brainstem and midbrain structures before reaching the auditory thalamus (medial geniculate body [MGB]) and finally the auditory cortex (ACtx; Figure 1A) (Budinger and Kanold, 2018). The
ACtx is a key sound-processing region for many higher order processes such as the processing of complex stimuli like speech and music (Wang, 2018). The ACtx itself is composed of six layers of morphologically different neu- rons that are highly interconnected and are thought to perform the hierarchical processing of sounds (Budinger and Kanold, 2018).
One of the hallmarks of the functional organization of sensory cortices in the adult is the orderly organiza- tion of neurons responding to sensory features such as sound frequency across the cortical surface in that they form “maps” of the sensory space (Kaas, 2000). In the auditory system, cells respond selectively to a particular sound frequency and the orderly organization means that neighboring cells share frequency selectivity and that
there is an orderly progression of frequency preference across the cortex (Schreiner and Winer, 2007).
Thus cells preferring low-frequency sounds (Figure 1A, blue) are located at one end of the ACtx, whereas cells that prefer high-frequency sounds (Figure 1A, red) are located at the other end of the ACtx, with cells that prefer midfrequency sounds (Figure 1A, green) in-between.
The resulting map of sound frequency is called a “tono- topic map,” and the orderly organization is thought to be important for normal brain function (Kaas, 1997). The tonotopic organization of the auditory system originates in the cochlea and requires precisely ordered projections
Figure 1. A: hierarchical processing of sound from cochlea to auditory cortex (ACtx). The cochlea transduces sounds into neural impulses that are relayed to the auditory cortex via brainstem nuclei and the auditory thalamus (medial geniculate body [MGB]). Different parts of the cochlea respond selectively to different sound frequencies (colors). The orderly frequency map is preserved up to the ACtx. The ACtx contains different interconnected layers. Inputs to the ACtx from the MGB arrive in layer 4. B: sequential generation of cortical layers. Subplate neurons and cells in the marginal zone (MZ) are born before the permanent cortical layers. Newborn neurons in the ventricular zone (VZ; purple) migrate radially to their target layer and differentiate.
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