Tonotopy in the auditory system begins at the cochlea, the small snail-like structure in the inner ear that sends information about sound to the brain.
However, binaural fusion in the superior olivary complex onward adds significant amounts of information encoded in the signal strength of each ganglion.
[1] The earliest evidence for tonotopic organization in auditory cortex was indicated by Vladimir E. Larionov in an 1899 paper entitled "On the musical centers of the brain", which suggested that lesions in an S-shaped trajectory resulted in failure to respond to tones of different frequencies.
[5] More recently, advances in technology have allowed researchers to map the tonotopic organization in healthy human subjects using electroencephalographic (EEG) and magnetoencephalographic (MEG) data.
While most human studies agree on the existence of a tonotopic gradient map in which low frequencies are represented laterally and high frequencies are represented medially around Heschl's gyrus, a more detailed map in human auditory cortex is not yet firmly established due to methodological limitations[6] Tonotopic organization in the cochlea forms throughout pre- and post-natal development through a series of changes that occur in response to auditory stimuli.
[7] Research suggests that the pre-natal establishment of tonotopic organization is partially guided by synaptic reorganization; however, more recent studies have shown that the early changes and refinements occur at both the circuit and subcellular levels.
[8] In mammals, after the inner ear is otherwise fully developed, the tonotopic map is then reorganized in order to accommodate higher and more specific frequencies.
[10] Further experiments have demonstrated a conserved role of Sonic Hedgehog emanating from the notochord and floor plate in establishing tonotopic organization during early development.
[13] This pressure wave travels along the BM of the cochlea until it reaches an area that corresponds to its maximum vibration frequency; this is then coded as pitch.
[13] Audio frequency, otherwise known as the pitch, is currently the only characteristic of sound that is known with certainty to be topographically mapped in the central nervous system.
However, other characteristics may form similar maps in the cortex such as sound intensity,[18][19] tuning bandwidth,[20] or modulation rate,[21][22][23] but these have not been as well studied.
[30] These frequency shifts in response to environmental stimuli have been shown to improve performance in perceptual behavior tasks in adult mice that were tone-reared during auditory critical period.
[36] It has been shown that subsets of the tonotopic map in A1 can be held in a plastic state indefinitely by exposing the rats to white noise consisting of frequencies within a particular range determined by the experimenter.
[37] A recent toxicity study showed that in-utero and postnatal exposure to polychlorinated biphenyl (PCB) altered overall primary auditory cortex (A1) organization, including tonotopy and A1 topography.
[38] Studies in mature A1 have focused on neuromodulatory influences and have found that direct and indirect vagus nerve stimulation, which triggers neuromodulator release, promotes adult auditory plasticity.
[40] Furthermore, behavioral training using rewarding or aversive stimuli, commonly known to engage cholinergic afferents and 5-HT3AR cells, has also been shown to alter and shift adult tonotopic maps.