The brainstem is not just a passive relay station for auditory information. Rather, it is a hub of ascending (ear to brain) sound processing and descending (brain to ear) modulation of the incoming signal.
The neural routes that connect the sensory organs and the brain run both ways. The afferent pathway sends information toward the brain and the efferent pathway sends information toward the sensory organs. Just as the brain tells a pianist’s fingers how to move, it exerts an influence all along the auditory pathway.
Until recently, people assumed that passively evoked ABRs (Auditory Brainstem Responses) reflected one-way processing—that of the afferent, ear-to-brain path. To a first approximation, that assumption is true for a stimulus such as a click. Because of the signature shape of the click-evoked response and the small amount of variability among individuals, a click ABR is a nearly infallible indicator of hearing sensitivity. A response present and at the right timing indicates normal hearing. A response with delayed timing suggests hearing loss. Is there no response at all? That indicates no hearing or perhaps a neuropathological condition. Afferent processing in the auditory system is only half of the equation. The downward-projecting efferent auditory system has a profound effect. Even activity in the hair cells of the cochlea—the peripheral extreme of the chain—is modulated by higher-level processing. A cABR thus represents a snapshot of both afferent and efferent processing; while still a faithful representation of afferent processing, it is modulated by the total of an individual’s experience with the evoking sound.
The auditory system is a two-way street. As sound travels along an afferent pathway from the cochlea to the auditory cortex, the signals are constantly being tuned by "downward" influences. Those influences travel along an efferent network whose paths originate not only in the auditory cortex (darkest shading) but also in nonauditory areas such as the limbic system and cognitive centers of memory and attention. They affect all hubs of auditory processing down to the cochlea, including structures in the auditory midbrain - the primary source of the scalp-recorded complex auditory brainstem response (cABR).