Intensity discrimination as a function of stimulus level with electric stimulation

Difference limens (DLs) for changes in electric current were measured from multiple electrodes in each of eight cochlear-implanted subjects. Stimuli were 200-μs/phase biphasic pulse trains delivered at 125 Hz in 300- ms bursts. DLs were measured with an adaptive three-alternative forced- choice procedure. Fixed-level psychometric functions were also obtained in four subjects to validate the adaptive DLs. Relative intensity DLs, specified as Weber fractions in decibels {10 log(ΔI/I)} for standards above absolute threshold, decreased as a power function of stimulus intensity relative to absolute threshold {ΔI/I-β(I/I₀)(α)} in the same manner as Weber fractions for normal acoustic stimulation reported in previous studies. Exponents (α) of the power function for electric stimulation ranged from - 0.4 to -3.2, on average, an order of magnitude larger than exponents for acoustic stimulation, which range from 0.07 to -0.11. Normalization of stimulus intensity to the dynamic range of hearing resulted in Weber functions with similar negative slopes for electric and acoustic stimulation, corresponding to an 8-dB average improvement in Weber fractions across the dynamic range. Sensitivity to intensity change {10 log β} varied from 0.42 to - 13.5 dB compared to +0.60 to -3.34 dB for acoustic stimulation, but on average was better with electric stimulation than with acoustic stimulation. Psychometric functions for intensity discrimination yielded Weber fractions consistent with adaptive procedures and d' was a linear function of ΔI. Variability among repeated Weber-fraction estimates was constant across dynamic range. Relatively constant Weber fractions across all or part of the dynamic range, observed in some subjects, were traced to the intensity resolution limits of individual implanted receiver/stimulators. DLs could not be accurately described by constant amplitude changes, expressed as a percentage of dynamic range {ΔA(%DR)}. Weber fractions from prelingually deafened subjects were no better or worse than those from postlingually deafened subjects. The cumulative number of discriminable intensity steps across the dynamic range of electric hearing ranged from as few as 6.6 to as many as 45.2. Physiologic factors that may determine important features of electric intensity discrimination are discussed in the context of a simple, qualitative, rate-based model. These factors include the lack of compressive cochlear preprocessing, the relative steepness of neural rate-intensity functions, and individual differences in patterns of neural survival.