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-mu s/phase biphasic pulse trains delivered at 125 Hz in 300-ms bursts. DLs were measured with an adaptive three-alternativ e forced-choice procedure. Fixed-level psychometric functions were als o obtained in four subjects to validate the adaptive DLs. Relative int ensity DLs, specified as Weber fractions in decibels {10 log(Delta I/I )} for standards above absolute threshold, decreased as a power functi on of stimulus intensity relative to absolute threshold {Delta I/I=bet a(I/I-0)(alpha)} in the same manner as Weber fractions for normal acou stic stimulation reported in previous studies. Exponents (alpha) of th e power function for electric stimulation ranged from -0.4 to -3.2, on average, an order of magnitude larger than exponents for acoustic sti mulation, which range from -0.07 to -0.11. Normalization of stimulus i ntensity to the dynamic range of hearing resulted in Weber functions w ith similar negative slopes for electric and acoustic stimulation, cor responding to an 8-dB average improvement in Weber fractions across th e dynamic range. Sensitivity to intensity change {10 log beta} varied from -0.42 to -13.5 dB compared to +0.60 to -3.34 dB for acoustic stim ulation, but on average was better with electric stimulation than with acoustic stimulation. Psychometric functions for intensity discrimina tion yielded Weber fractions consistent with adaptive procedures and d ' was a linear function of Delta I. Variability among repeated Weber-f raction estimates was constant across dynamic subjects, were traced to the intensity resolution limits of individual implanted receiver/stim ulators. DLs could not be accurately described by constant amplitude c hanges, expressed as a percentage of dynamic range {Delta A(%DR)}. Web er fractions from prelingually deafened subjects were no better or wor se than those from postlingually deafened subjects. The cumulative num ber of discriminable intensity steps across the dynamic range of elect ric hearing ranged from as few as 6.6 to as many as 45.2. Physiologic factors that may determine important features of electric intensity di scrimination are discussed in the context of a simple, qualitative, ra te-based model. These factors include the lack of compressive cochlear preprocessing, the relative steepness of neural rate-intensity functi ons, and individual differences in patterns of neural survival. (C) 19 96 Acoustical Society of America.