The auditory evoked magnetic fields to very high frequency tones

Area of Science:

• Neuroscience
• Auditory Neuroscience
• Magnetoencephalography

Background:

• Auditory evoked magnetic fields (AEFs) provide insights into human auditory processing.
• Previous studies have primarily focused on frequencies within the typical human hearing range.
• Investigating responses to very high frequencies and ultrasound is crucial for understanding the limits of auditory perception.

Purpose of the Study:

• To systematically investigate auditory evoked magnetic fields (AEFs) in response to pure tones from 4000 Hz to 40,000 Hz, including ultrasound.
• To determine the characteristics of the N1m response, including amplitude, latency, and equivalent current dipole (ECD) location, across different frequencies.
• To explore interhemispheric differences in auditory processing of high-frequency sounds.

Main Methods:

• Recorded AEFs in 12 subjects using magnetoencephalography under binaural listening conditions.
• Presented auditory stimuli including pure tones (4000 Hz to 14,000 Hz) and ultrasound (20,000 Hz, 40,000 Hz), along with a click sound.
• Analyzed the N1m component of AEFs, focusing on its amplitude, peak latency, and the location and orientation of its equivalent current dipole (ECD).

Main Results:

• N1m responses were detected for tones up to 12,000 Hz in all subjects, and up to 14,000 Hz in most subjects.
• No significant N1m responses were identified for ultrasound frequencies (≥20,000 Hz).
• N1m amplitude decreased significantly for tones above 8000 Hz, with a tendency for longer latency and more medial/posterior ECD locations at higher frequencies.
• The right hemisphere showed significantly larger N1m amplitudes and more anterior/medial ECDs for all tested frequencies, indicating right hemisphere priority.

Conclusions:

• The human auditory cortex exhibits tonotopic organization up to the upper limit of the audible range.
• Directly air-conducted ultrasound does not elicit detectable N1m responses, suggesting it falls outside the functional range of the primary auditory cortex.
• The right auditory cortex plays a dominant role in processing very high-frequency sounds, as evidenced by larger AEF amplitudes.