Tonotopic Organization of the Human Auditory Cortex

Abstract

Neuromagnetic measurements of responses to auditory stimuli consisting of pure tones amplitude-modulated at a low frequency have been used to deduce the location of cortical activity. The evoked field source systematically increased in depth beneath the scalp with increasing frequency of the tone. The tonotopic progression can be described as a logarithmic mapping.

Relationship of transient and steady-state auditory evoked fields

“…The main sources of cortical auditory evoked magnetic fields are intracellular currents flowing tangentially to the skull, originating in similarly oriented pyramidal cells in the walls of the sylvian fissure. Magnetic studies of the human auditory cortex have demonstrated functional relationships similar to those observed earlier in invasive animal and human studies and have also provided new information about the functional organization of the human auditory cortex (Romani et al 1982;Pantev et al 1989), and about the physiology of auditory feature extraction (Hari 1990;Csepe et al 1992).…”

“…The main sources of cortical auditory evoked magnetic fields are intracellular currents flowing tangentially to the skull, originating in similarly oriented pyramidal cells in the walls of the sylvian fissure. Magnetic studies of the human auditory cortex have demonstrated functional relationships similar to those observed earlier in invasive animal and human studies and have also provided new information about the functional organization of the human auditory cortex (Romani et al 1982;Pantev et al 1989), and about the physiology of auditory feature extraction (Hari 1990;Csepe et al 1992).Responses evoked by stimuli which follow each other at sufficiently long interstimulus intervals (ISIs) for the auditory system to return (mostly or completely) to its initial state before the next stimulus occurs (Buchwald and Huang 1975) are called transient evoked responses. If the ISI is shortened to such an extent that the transient response to one stimulus has not died away before the next stimulus is delivered, the com- pound response that appears is generally referred to as a steady-state response (SSR) (Regan 1982) or steadystate field (SSF) in magnetic recording.…”

“…Auditory steady-state responses can be driven by different types of periodically presented short acoustic stimuli, with a maximum response energy for the electric SSR at stimulus repetition rates in the range 35-40 Hz (Galambos et al 1981). Magnetic steady-state fields (SSFs) have been detected using amplitude modulated tones (Romani et al 1982) and trains of clicks (Miikelii and Hari 1987) as stimuli.…”

Human Primary Auditory Cortex Follows the Shape of Heschl's Gyrus

“…Thus, it was expected that human tonotopic gradients were rotated compared to the macaque and would be found running along (or parallel to) HG rather then across it. This model was somewhat supported by MEG measurements (Romani et al, 1982) and chronic microelectrode recordings (Howard et al, 1996) placing high frequencies medially on HG and low frequencies laterally on HG (thus potentially accounting for the low-to-high map of A1, but not R). Those recording methods were limited by poor localization accuracy and by limited sampling, respectively.…”

Extracting separate responses to simultaneously presented continuous auditory stimuli: An auditory attention study.

“…The fact that the HIGH stream elicits smaller responses than the LOW stream may be due in part to the logarithmic nature of the tonotopic organization of auditory cortex [14] i.e. the higher the frequency the smaller the area of cortex devoted to it.…”