Localization of primary auditory cortex in humans by magnetoencephalography

Area of Science:

• Neuroscience
• Auditory Neuroscience
• Biophysics

Background:

• The primary auditory cortex (PAC) is activated early by auditory stimuli, contributing to auditory evoked fields (AEFs).
• However, a universally detectable AEF component specific to PAC remains elusive.
• Identifying reliable neural correlates of PAC activity is crucial for understanding auditory processing.

Purpose of the Study:

• To identify a genuine correlate of primary auditory cortex (PAC) activity within auditory evoked fields (AEFs).
• To investigate the neural origin of the dP20m component in human auditory processing.

Main Methods:

• Recorded auditory evoked fields (AEFs) using a 37-channel axial gradiometer system in nine normal-hearing subjects.
• Presented brief click stimuli to the right ear over two independent sessions with a high number of trials (>8500 per session).
• Analyzed the first temporal derivative of the magnetic field, specifically the dP20m peak, and localized its dipole source.

Main Results:

• A peak in the first temporal derivative of the magnetic field at approximately 20 ms (dP20m) was identified as a reliable correlate of PAC activity.
• Dipole localization of dP20m showed high reproducibility between sessions, with coordinates differing by only a few millimeters.
• Coregistration with MRI suggested the dP20m originates near the retroinsular part of Heschl's gyrus, a region associated with PAC.
• Evidence indicates that dP20m and the P30m component arise from different cortical sources, with P30m receiving input from multiple areas.

Conclusions:

• The dP20m represents a genuine and reliable neural correlate of primary auditory cortex (PAC) activity.
• This finding offers a new tool for investigating early auditory processing in humans.
• The dP20m provides a more specific marker for PAC activity compared to the broader P30m component.