Tanvi Thakkar

Evidence for a neural source of the precedence effect in sound localization

Normal-hearing human listeners and a variety of studied animal species localize sound sources accurately in reverberant environments by responding to the directional cues carried by the first-arriving sound rather than spurious cues carried by later-arriving reflections, which are not perceived discretely. This phenomenon is known as the precedence effect (PE) in sound localization. Despite decades of study, the biological basis of the PE remains unclear. Though the PE was once widely attributed to central processes such as synaptic inhibition in the auditory midbrain, a more recent hypothesis holds that the PE may arise essentially as a by-product of normal cochlear function. Here we evaluated the PE in a unique human patient population with demonstrated sensitivity to binaural information but without functional cochleae. Users of bilateral cochlear implants (CIs) were tested in a psychophysical task that assessed the number and location(s) of auditory images perceived for simulated source-echo (lead-lag) stimuli. A parallel experiment was conducted in a group of normal-hearing (NH) listeners. Key findings were as follows: 1) Subjects in both groups exhibited lead-lag fusion. 2) Fusion was marginally weaker in CI users than in NH listeners but could be augmented by systematically attenuating the amplitude of the lag stimulus to coarsely simulate adaptation observed in acoustically stimulated auditory nerve fibers. 3) Dominance of the lead in localization varied substantially among both NH and CI subjects but was evident in both groups. Taken together, data suggest that aspects of the PE can be elicited in CI users, who lack functional cochleae, thus suggesting that neural mechanisms are sufficient to produce the PE.

Internalized elevation perception of simple stimuli in cochlear-implant and normal-hearing listeners

In normal-hearing (NH) listeners, elevation perception is produced by the spectral cues imposed by the pinna, head, and torso. Elevation perception in cochlear-implant (CI) listeners appears to be non-existent; this may be a result of poorly encoded spectral cues. In this study, an analog of elevation perception was investigated by having 15 CI and 8 NH listeners report the intracranial location of spectrally simple signals (single-electrode or bandlimited acoustic stimuli, respectively) in both horizontal and vertical dimensions. Thirteen CI listeners and all of the NH listeners showed an association between place of stimulation (i.e., stimulus frequency) and perceived elevation, generally responding with higher elevations for more basal stimulation. This association persisted in the presence of a randomized temporal pitch, suggesting that listeners were not associating pitch with elevation. These data provide evidence that CI listeners might perceive changes in elevation if they were presented stimuli with sufficiently salient elevation cues.

Mixed stimulation rates to improve sensitivity of interaural timing differences in bilateral cochlear implant listeners

Normal hearing listeners extract small interaural time differences (ITDs) and interaural level differences (ILDs) to locate sounds and segregate targets from noise. Bilateral cochlear implant listeners show poor sensitivity to ITDs when using clinical processors. This is because common clinical stimulation approaches use high rates [∼1000 pulses per-second (pps)] for each electrode in order to provide good speech representation, but sensitivity to ITDs is best at low rates of stimulation (∼100-300 pps). Mixing rates of stimulation across the array is a potential solution. Here, ITD sensitivity for a number of mixed-rate configurations that were designed to preserve speech envelope cues using high-rate stimulation and spatial hearing using low rate stimulation was examined. Results showed that ITD sensitivity in mixed-rate configurations when only one low rate electrode was included generally yielded ITD thresholds comparable to a configuration with low rates only. Low rate stimulation at basal or middle regions on the electrode array yielded the best sensitivity to ITDs. This work provides critical evidence that supports the use of mixed-rate strategies for improving ITD sensitivity in bilateral cochlear implant users.

The role of onset cues for lateralization of an auditory object in bilateral cochlear implant listeners

In bilateral cochlear implant (BiCI) listeners, a common strategy for improving sensitivity to interaural timing differences (ITDs) is to introduce low rate stimulation in some channels. Previous work showed that BiCI listeners can lateralize a single sound with a constant ITD at every pulse of a low rate stimulus. However, in reverberant environments, ITDs for a target sound can change over a short period of time. Previously we have shown that BiCI listeners had difficulty using the ITD in the first pulse of a low-rate pulse train to lateralize a stimulus when the subsequent pulses had ITDs that varied over time. Hence, we hypothesized that a more salient onset is needed in order to restore ITD sensitivity in BiCI listeners when using low rate stimulation. In this study, we measured the number of pulses with a consistent ITD that is needed at the onset in order for BiCI listeners to reliably perceive one sound in the correct location. Results suggest that BiCI listeners need at least five pulses with con...

Restoring binaural and spatial hearing in cochlear implant users

Adults and children who receive bilateral cochlear implants (BiCIs) have the potential to benefit from the integration of inputs arriving at the brain from both ears. Several factors play a key role in determining if patients will demonstrate binaural sensitivity. We are exploring these factors using two experimental approaches. In the first approach, BiCI users receive pulsatile stimulation to specific pairs of electrodes using research processors that synchronize stimulation with fidelity. We vary stimulus parameters such as temporal fine structure and envelope cues, places of stimulation along the cochleae, and number of electrodes to find parameters that maximize sensitivity to interaural differences for each patient. We also investigate the role of the electrode-neuron interface which is affected by numerous factors including neural health. In a second stimulation approach, we use clinical speech processor to deliver binaural stimulation designed specifically for that patient based on their clinical MAP. In these studies, we are using both standard psychophysics and eye gaze paradigms to understand the underlying processing involved in binaural and spatial hearing. This combined approach is enabling us to design multi-channel multi-rate stimulation strategies aimed at restoring binaural sensitivity and preserving speech understanding.Adults and children who receive bilateral cochlear implants (BiCIs) have the potential to benefit from the integration of inputs arriving at the brain from both ears. Several factors play a key role in determining if patients will demonstrate binaural sensitivity. We are exploring these factors using two experimental approaches. In the first approach, BiCI users receive pulsatile stimulation to specific pairs of electrodes using research processors that synchronize stimulation with fidelity. We vary stimulus parameters such as temporal fine structure and envelope cues, places of stimulation along the cochleae, and number of electrodes to find parameters that maximize sensitivity to interaural differences for each patient. We also investigate the role of the electrode-neuron interface which is affected by numerous factors including neural health. In a second stimulation approach, we use clinical speech processor to deliver binaural stimulation designed specifically for that patient based on their clinical ...

The impact of asymmetric rates on interaural time difference lateralization and auditory object formation in bilateral cochlear implant and normal hearing listeners

Normal hearing (NH) listeners are able to accurately identify and locate sound sources using auditory object formation (AOF) and interaural time differences (ITDs). Temporal cues can further facilitate AOF and ITD sensitivity: this includes within- and across-ear stimulus’ rate, envelope-, and onset-symmetry. Bilateral cochlear implant (BiCI) listeners are not guaranteed to receive symmetrical or complementing temporal information across ears. Cochlear-Nucleus devices undergo “peak-picking” where stimulation of electrodes could yield asymmetric rates across the ears, interrupting good AOF and ITD sensitivity. We investigated the impact of asymmetric rates and ITDs, on AOF and ITD lateralization. BiCI and NH listeners were presented with diotic and dichotic pulsatile stimulus rates, combined with an ITD. Rate was fixed in one ear, and varied in the contralateral ear. In a single interval, six-alternative forced-choice task, listeners reported where and how many sounds they heard. We hypothesized that with ...

Pitch magnitude estimation can predict across-ear pitch comparisons in cochlear-implant users

Bilateral cochlear implants provide auditory input in both ears, but sensitivity to interaural time differences (ITDs) varies across individuals. One factor affecting ITD sensitivity is place-of-stimulation mismatch across ears. Currently there is no fast method for estimating mismatch. Since pitch varies depending on place-of-stimulation, our laboratory uses two pitch-matching methods to estimate matched place-of-stimulation across ears: pitch magnitude estimation (PME) and direct pitch comparison (DPC). PME involves rating pitch from 1 (lowest) to 100 (highest) for even numbered electrodes in each ear. DPC involves comparing the pitch of one electrode against six electrodes in the opposite ear, yielding an estimate of place-of-stimulation mismatch at one electrode. DPC predicts interaural electrode pairs with good ITD sensitivity, but is relatively slow because pairs are chosen one at a time. PME is a much faster, global estimate of the relative difference of interaural electrodes, but it is uncertain w...

The impact of cochlear implantation on spatial hearing and listening effort

We studied the potential benefits of bilateral hearing in cochlear implants (CI) users, and in patients with single-sided deafness (SSD) who receive a CI in the deaf ear and have normal hearing in the other ear. We hypothesized that listening effort, measured with pupil dilation, can reveal benefits of bilateral hearing that may not be consistently observed when localization or spatial release from masking (SRM) are measured. Result from 12 bilateral CI users showed reduction in listening effort with bilateral hearing compared to the poor ear or better ear alone. In patients with SSD, benefits of adding a CI to a normal hearing ear can emerge over a protracted period of a year or longer. In addition, for at least some of the subjects bilateral hearing (adding a CI to the normal hearing ear) produced release from listening effort even in conditions where SRM was not observed. That is, speech intelligibility did not always improve with spatial separation of target and competing speech, but pupil dilation wa...

Restoring sensitivity to interaural timing differences in bilateral cochlear implant listeners using multi-electrode stimulation

Identification and discrimination of sound sources in complex auditory environments is facilitated for normal-hearing listeners from access to interaural time differences (ITDs). For patients fitted with bilateral cochlear implants (BiCI), binaural sensitivity is harder to achieve due to several factors, including asynchronous interaural processing. BiCI listeners have shown good ITD sensitivity with low rates of electrical stimulation; however, low-rate delivery of ITD cues is unrealistic for the high-rate pulsatile stimulation required to achieve good speech understanding. One solution is to present a mix of high- and low-rate stimulation on different electrodes, preserving both speech recognition and sound localization ability. In the present study, a binaural benefit was observed in a mixed-rate strategy under direct electrical stimulation: ITD sensitivity was measured in a two-alternative forced-choice discrimination task, using seven multi-electrode conditions. We hypothesized that by introducing lo...

The precedence effect: Exploring the build-up of echo thresholds in cochlear-implant listeners

In reverberant environments, listeners rely on early arriving spatial cues to accurately localize sound sources, a phenomenon known as the precedence effect (PE). In deaf individuals fitted with bilateral cochlear implants (BiCIs), this effect is diminished, ostensibly due to the fact that clinical processers do not preserve binaural cues. We have recently demonstrated that BiCI listeners do exhibit aspects of the PE similar to normal-hearing (NH) listeners when binaural stimulation is restored using synchronized research devices. Here, we consider whether BiCI users also demonstrate an aspect of the PE known as “buildup”—enhancement of the PE after a repeated stimulus. BiCI users with demonstrated sensitivity to interaural time differences (ITDs) were tested using dichotic electrical pulses (±500µs ITD in opposing “lead” and “lag” pulse pairs, with lead-lag delays of 1–64 ms). On each trial, listeners indicated (1) whether one or two locations were perceived (to assess “fusion”) and (2) the location perc...