Excerpt from this neuroscience article. Basically, what this article says is that we know now from research of the last ten years or so that the auditory system is not a fixed system. It responds to changes in the auditory environment. Our hearing is not hard-wired like sound level meters. Our hearing is highly adaptable.
Contrary to traditional views that early stages of sensory processing are not plastic, new studies discussed in this article have established that the auditory brainstem is dynamic. The plethora of intrinsic and synaptic plasticity mechanisms observed in the auditory brainstem in combination with the noninvasive methods of assessing auditory brainstem function in humans provides a platform for relating subcortical auditory processing to higher-order sensory and cognitive tasks involving speech and music. Therefore, we suggest that the auditory brainstem offers an ideal model to study the mechanisms and functions of nontraditional aspects of sensory processing, such as synaptic and intrinsic plasticity and recurrent feedback from higher levels of processing.
The past 10 years of research have revealed how timing information is fed through auditory brainstem pathways. This research has provided insight into how sounds are localized by vertebrates, but much less is known about how these pathways adapt to ongoing sensory activity and how they contribute to the perception and interpretation of environmental sounds, including speech, under normal and pathological conditions. Therein lies the exciting future of revealing the role of plasticity observed in the auditory brainstem.
Whether activity-dependent changes are initiated/modulated in a topdown fashion, as predicted by the RHT [Reverse Hierarchy Theory, Ahissar and Hochstein, 2004, ed.] through the efferent, corticofugal system linking the cortex and the auditory brainstem; through local mechanisms of adaptation to the acoustic properties of the input (Dean et al., 2005); or through an interaction of afferent and efferent mechanisms is a challenge for future research to resolve.