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Informational masking and stream segregation of psychophysical stimuli in bilateral cochlear implant
Wijetillake A. , van Hoesel R. , Cowan R.
The HEARing CRC, Melbourne, Australia, 2The University of Melbourne, Melbourne, Australia
The ability to perceive a target signal can often be degraded by the presence of an interfering signal that is said
to 'mask' the target. Masking can occur when competing signals have overlapping spectral content that compete
for representation at the auditory periphery (energetic masking, EM). It can also result from confusions at a
central processing level (informational masking, IM) that make it difficult to perceptually segregate target from
interferer (stream segregation). IM is often alleviated in normal hearing listeners if differences exist between
competing signals to 'cue' stream segregation. Ear-of-entry and temporal structure can provide two such cues.
The present study aimed to explore the potential of ear-of-entry and temporal cues to provide IM release and
promote stream segregation in bilateral cochlear implant (BiCI) users via direct stimulation of the electrodes in
each ear.
Target pulse-train bursts sequences were applied to a single mid-array electrode. Masker sequences consisted
of four-electrode stimuli with two electrodes located more apical, and two more basal than the target. Masker
components were spaced at least ±5 electrodes from the target to minimize EM and observed masking effects
are assumed to be due to IM The target was presented monotically while maskers were presented either
ipsilateral (Ipsi) or contralateral to the target (Contra), or to both ears diotically (Diotic). Target burst rate was
either matched to the masker, or halved such that target bursts coincided with every second masker burst. In
Part A of the study, target-in-masker detection thresholds were determined for each condition via an adaptive 2down-1-up procedure. Lower detection thresholds indicate reduced masking. Part B involved a subjective
lateralization task in which participants listened to a subset of stimuli from Part A, and indicated whether
competing signals were heard as a single sound or as two segregated sounds, and where those sounds were
heard intracranially.
Target detection thresholds were not significantly different across Ipsi, Contra and Diotic conditions, nor were
Ipsi thresholds different for matched and halved target rates. However, thresholds for the halved-rate Contra
condition were substantially lower than for the matched-rate Contra and halved-rate Ipsi conditions, and in fact
approached target thresholds in quiet. Outcomes from the lateralization task indicated that stimuli in the
matched-rate Contra condition were perceived as a single percept whose location depended on the relative
levels of competing stimuli. In contrast, target and masker in the halved-rate Contra condition were heard as
separate percepts situated in opposite hemifields near their respective ears-of-entry. The findings suggest that
while either ear-of-entry or temporal structure cues are alone insufficient to provide IM release and promote
stream segregation, the combination of the two cues can yield considerable benefit to CI users.