Effects of pulse rate and electrode array design on intensity discrimination in cochlear implant users

The effects of pulse rate on intensity discrimination were evaluated in 14 subjects with Clarion C-I cochlear implants. Subjects had a standard [Clarion spiral electrode array (SPRL group)] or perimodiolar electrode array [Clarion HiFocus electrode array with electrode positioning system (HF+EPS group)]. Weber fractions for intensity discrimination [Wf_dB = 10 log ΔI/I] were evaluated at five levels over dynamic range at each of three pulse rates (200, 1625 and 6500 pps) using monopolar stimulation. Weber fractions were smaller for 200 pps stimuli than for 1625 or 6500 pps stimuli in both groups. Weber fractions were significantly smaller for SPRL subjects (mean WfdB = -9.1 dB) than for HF+EPS subjects (mean WfdB = -6.7 dB). Intensity difference limens (DLs) expressed as a percentage of dynamic range (DR) (ΔI_%DR = ΔI/DR_dB*100) did not vary systematically with pulse rate in either group. Larger intensity DLs combined with smaller dynamic ranges led to fewer intensity steps over the dynamic range for HF + EPS subjects (average 9 steps) compared to SPRL subjects (average 23 steps). The observed effects of pulse rate and electrode array design may stem primarily from an inverse relationship between absolute current amplitude and the size of intensity DLs. The combination of smaller dynamic ranges and larger Weber fractions in HF+ EPS subjects could be the result of increased variability of neural outputs in these subjects.