Effects of level on nonspectral frequency difference limens for electrical and acoustic stimuli

Abstract

The purpose of this experiment was to study the effects of stimulus level on discrimination of frequency as represented in the temporal waveforms of acoustic and electrical signals. The subjects were four nonhuman primates in which one ear had been deafened and implanted with an electrode array and the other ear was untreated. Frequency difference limens for 100 Hz electrical sinusoidal stimulation via a cochlear implant in the deafened ear were compared to those for 100 Hz sinusoidally amplitude-modulated white noise (SAM noise) acoustic stimuli to the normal-hearing contralateral ear. To correct for loudness cues, levels of the test stimuli were varied relative to the reference-stimulus level. The test-stimulus levels at which the percent responses were minimum were determined. These levels were used to measure the frequency difference limens. Frequency difference limens for the electrical stimuli decreased as a function of reference-stimulus level through most of the dynamic range, while those for the acoustic stimuli reached a minimum at 20 dB to 40 dB above threshold. For the electrical stimuli the slopes and relative positions of the frequency difference limen vs. level functions varied from subject to subject, and with changes in electrode configuration within a subject. These differences were related to threshold level and dynamic range. At higher levels of stimulation, frequency difference limens for acoustic and electrical stimuli fell in the same range. The slopes and relative positions of the frequency difference limen vs. level functions for electrical stimuli did not parallel those of level difference limen vs. level functions collected simultaneously from the same ears. The data suggest that nonspectral frequency discrimination may depend on the number of nerve fibers stimulated. With prostheses in cochleas with less than a full complement of auditory nerve fibers, the data suggest that stimulation level is an important variable influencing discriminability.

Current-level discrimination using bipolar and monopolar electrode configurations in cochlear implants

“…This result is consistent with previous observations in humans (Cohen et al, 2001;Nelson et al, 1996;Shannon, 1983;Dillier et al, 1983;Hochmair-Desoyer et al, 1981) and in animals (Pfingst et al, 1983;Pfingst, 1984;Pfingst and Rai, 1990;Morris and Pfingst, 2000). Flat Weber functions were observed in the present study in some listeners using the bipolar configuration.…”

“…Similar results were observed in animals (Pfingst et al, 1983;Pfingst, 1984;Pfingst and Rai, 1990; Morris and Pfingst, 2000). Results were consistent across studies despite the use of several different metrics.…”

Stimulus features affecting psychophysical detection thresholds for electrical stimulation of the cochlea. II: Frequency and interpulse interval

“…Optimal function of prostheses depends in part on stimuli being presented to the patient at the proper level (Hochmair and Høchmair'Desøyer' 1983). Performance on a number of discrimination tasks is influenced by the level at which the stimuli are presented relative to threshold (Pfingst et aL, 1983;Shannon, 1983Shannon, , 1989aPfingst and Rai, 1990). Presenting all stimuli at optimal levels depends in part on knowledge of the detection-threshold functions.…”

Electrical stimulation of the auditory nerve: Effects of pulse width on frequency discrimination

“…Changes in level can also affect perceptions of both loudness and pitch (Townshend et al, 1987). Discrimination of change~ in the frequency of an electrical signal can be affected by simultaneous small changes in level, and it is possible to find a level change for which the performance is minimum (Pfingst and Rush, 1987;Pfingst and Rai, 1990). Performance at this test-stimulus level is of interest because in normal environmental signals, frequency and level changes are often not correlated.…”