Intensity discrimination as a function of stimulus level with electric stimulation

Area of Science:

• Auditory Neuroscience
• Biomedical Engineering
• Signal Processing

Background:

• Cochlear implants bypass damaged auditory structures to directly stimulate the auditory nerve.
• Electric current intensity is a crucial parameter for speech perception in cochlear implant users.
• Understanding electric difference limens (DLs) is vital for optimizing cochlear implant performance.

Purpose of the Study:

• To measure and characterize difference limens (DLs) for electric current changes in cochlear implant (CI) users.
• To compare electric intensity discrimination with normal acoustic hearing.
• To investigate factors influencing electric intensity discrimination.

Main Methods:

• Adaptive three-alternative forced-choice procedure used to measure DLs in eight CI subjects.
• Stimuli: 200-microseconds/phase biphasic pulse trains at 125 Hz.
• Fixed-level psychometric functions obtained for validation; Weber fractions (relative DLs) calculated.

Main Results:

• Electric DLs, as Weber fractions, decreased with stimulus intensity following a power function, similar to acoustic hearing.
• Exponents for electric stimulation were significantly steeper (average -0.4 to -3.2) than for acoustic stimulation (-0.07 to -0.11).
• Electric hearing showed an average 8-dB improvement in Weber fractions across the dynamic range and greater sensitivity to intensity change on average.

Conclusions:

• Electric intensity discrimination in CI users follows similar principles to acoustic hearing but with enhanced steepness.
• The dynamic range of electric hearing allows for a significant number of discriminable intensity steps (6.6 to 45.2).
• Factors like lack of cochlear preprocessing and neural survival influence electric intensity discrimination.