Forward-masked spatial tuning curves in cochlear implant users

Forward-masked psychophysical spatial tuning curves (fmSTCs) were measured in twelve cochlear-implant subjects, six using bipolar stimulation (Nucleus devices) and six using monopolar stimulation (Clarion devices). fmSTCs were measured at several probe levels on a middle electrode using a fixed-level probe stimulus and variable-level maskers. The average fmSTC slopes obtained in subjects using bipolar stimulation ( 3.7 dB ∕ mm ) were approximately three times steeper than average slopes obtained in subjects using monopolar stimulation ( 1.2 dB ∕ mm ) . Average spatial bandwidths were about half as wide for subjects with bipolar stimulation ( 2.6 mm ) than for subjects with monopolar stimulation ( 4.6 mm ) . None of the tuning curve characteristics changed significantly with probe level. fmSTCs replotted in terms of acoustic frequency, using Greenwood's [ J. Acoust. Soc. Am. 33 , 1344-1356 ( 1961 ) ] frequency-to-place equation, were compared with forward-masked psychophysical tuning curves obtained previously from normal-hearing and hearing-impaired acoustic listeners. The average tuning characteristics of fmSTCs in electric hearing were similar to the broad tuning observed in normal-hearing and hearing-impaired acoustic listeners at high stimulus levels. This suggests that spatial tuning is not the primary factor limiting speech perception in many cochlear implant users.