The frequency selectivity of the normal and pathological human cochlea

Area of Science:

• Auditory Neuroscience
• Otoacoustic Emissions
• Hearing Physiology

Background:

• Auditory nerve (AP) tuning curves characterize the frequency selectivity of auditory nerve fibers.
• Previous research has established differences in tuning between single auditory nerve fibers and AP recordings.
• Sensorineural hearing loss (SNHL) impacts cochlear function and frequency resolution.

Purpose of the Study:

• To measure human auditory nerve tuning curves using transtympanic electrocochleography.
• To compare tuning curve sharpness in individuals with normal hearing versus those with SNHL.
• To estimate normal human cochlear fiber tuning based on guinea pig data.

Main Methods:

• Transtympanic electrocochleography was used to record human AP tuning curves.
• Simultaneous and forward masking paradigms were employed.
• Tuning curve sharpness was quantified using Q10 dB values.

Main Results:

• Normal hearing subjects showed average Q10 dB values of 2.3 (2 kHz), 3.6 (4 kHz), and 4.7 (8 kHz).
• Patients with SNHL (threshold elevations >40 dB) exhibited less sharply tuned curves (Q10 dB 1-2 at 2/4 kHz, 1-2.3 at 8 kHz).
• Guinea pig data indicated cochlear fiber tuning is 1.8 times sharper than AP tuning, suggesting estimated human cochlear fiber Q10 dB values of 4.2 (2 kHz), 6.5 (4 kHz), and 8.5 (8 kHz).

Conclusions:

• Human auditory nerve tuning curves are significantly less sharp in individuals with cochlear SNHL.
• The findings provide quantitative evidence of reduced frequency selectivity in SNHL.
• Estimated human cochlear fiber tuning suggests a higher degree of frequency resolution than directly measured AP tuning.