Stroke recovery and sensory plasticity: common mechanisms?

Presentations in this symposium are considered in the context of mechanisms of sensory plasticity, particularly in the auditory system. The auditory nervous system has two discrete end organs that are separately vulnerable to clinical and experimental injury, and brainstem processing mechanisms that are highly specialized for temporal, spectral, and spatial coding. These include giant axo-somatic synapses and neurons with spatially segregated bipolar dendrites, each innervated exclusively from one ear.

Audiovisual cues and perceptual learning of spectrally distorted speech.

Two experiments investigate the effectiveness of audiovisual (AV) speech cues (cues derived from both seeing and hearing a talker speak) in facilitating perceptual learning of spectrally distorted speech. Speech was distorted through an eight channel noise-vocoder which shifted the spectral envelope of the speech signal to simulate the properties of a cochlear implant with a 6 mm place mismatch: Experiment I found that participants showed significantly greater improvement in perceiving noise-vocoded speech when training gave AV cues than when it gave auditory cues alone. Experiment 2 compared training with AV cues with training which gave written feedback.

Effects of dynamic-range compression on the spatial attributes of sounds in normal-hearing listeners.

Objectives: Dynamic-range compression is routinely used in bilaterally fitted hearing devices. The objective of this study was to investigate how compression applied independently at each ear affects spatial perception in normal-hearing listeners and to relate the effects to changes in binaural cues caused by the compression for different types of sound.

Design: A semantic-differential method was used to measure the spatial attributes of sounds.

Dynamic-range compression affects the lateral position of sounds.

Dynamic-range compression acting independently at each ear in a bilateral hearing-aid or cochlear-implant fitting can alter interaural level differences (ILDs) potentially affecting spatial perception. The influence of compression on the lateral position of sounds was studied in normal-hearing listeners using virtual acoustic stimuli. In a lateralization task, listeners indicated the leftmost and rightmost extents of the auditory event and reported whether they heard (1) a single, stationary image, (2) a moving/gradually broadening image, or (3) a split image.

Health-related quality of life before and after management in adults referred to otolaryngology: a prospective national study.

Objective: An assessment of the effect of otolaryngological management on the health-related quality of life of patients.

Design: Application of the Health Utilities Index mark 3 (HUI-3) before and after treatment; application of the Glasgow Benefit Inventory (GBI) after treatment.

Setting: Six otolaryngological departments around Scotland.

Lack of generalization of auditory learning in typically developing children.

Purpose: To understand the components of auditory learning in typically developing children by assessing generalization across stimuli, across modalities (i.e., hearing, vision), and to higher level language tasks.

What is the role of the medial olivocochlear system in speech-in-noise processing?

The medial olivocochlear (MOC) bundle reduces the gain of the cochlear amplifier through reflexive activation by sound. Physiological results indicate that MOC-induced reduction in cochlear gain can enhance the response to signals when presented in masking noise. Some previous studies suggest that this "antimasking" effect of the MOC system plays a role in speech-in-noise perception.

Undirected head movements of listeners with asymmetrical hearing impairment during a speech-in-noise task.

It has long been understood that the level of a sound at the ear is dependent on head orientation, but the way in which listeners move their heads during listening has remained largely unstudied. Given the task of understanding a speech signal in the presence of a simultaneous noise, listeners could potentially use head orientation to either maximize the level of the signal in their better ear, or to maximize the signal-to-noise ratio in their better ear. To establish what head orientation strategy listeners use in a speech comprehension task, we used an infrared motion-tracking system to measure the head movements of 36 listeners with large (>16 dB) differences in hearing threshold between their left and right ears.

Towards quantifying cochlear implant localization performance in complex acoustic environments.

Cochlear implant (CI) users frequently report listening difficulties in reverberant and noisy spaces. While it is common to assess speech understanding with implants in background noise, binaural hearing performance has rarely been quantified in the presence of other sources, although the binaural system is a major contributor to the robustness of speech understanding in noisy situations with normal hearing. Here, a pointing task was used to measure horizontal localization ability of a bilateral CI user in quiet and in a continuous diffuse noise interferer at a signal-to-noise ratio of 0 dB.

The speech intelligibility benefit of a unilateral wireless system for hearing-impaired adults.

Objective: This study measured the effects of two previously untested practical considerations-venting and transmission delays-on speech intelligibility in a simulated unilateral wireless system, where a target signal in background noise was transmitted wirelessly to the hearing-impaired (HI) listener.

Design: Speech reception thresholds (SRTs) relative to the signal-to-noise ratio (SNR) were measured by varying the surrounding babble noise level. The target signal was presented at 0° azimuth in the soundfield and unilaterally via an insert earphone, using open and closed fittings with simulated-wireless delays ranging between 0-160 ms.

Training speech-in-noise perception in mainstream school children.

Objective: Auditory training has been advocated as a management strategy for children with hearing, listening or language difficulties. Because poor speech-in-noise perception is commonly reported, previous research has focused on the use of complex (word/sentence) stimuli as auditory training material to improve sentence-in-noise perception. However, some evidence suggests that engagement with the training stimuli is more important than the type of stimuli used for training.

The effect of hearing impairment on localization dominance for single-word stimuli.

Localization dominance (one of the phenomena of the "precedence effect") was measured in a large number of normal-hearing and hearing-impaired individuals and related to self-reported difficulties in everyday listening. The stimuli (single words) were made-up of a "lead" followed 4 ms later by a equal-level "lag" from a different direction. The stimuli were presented from a circular ring of loudspeakers, either in quiet or in a background of spatially diffuse babble.

Cortical inactivation by cooling in small animals.

Reversible inactivation of the cortex by surface cooling is a powerful method for studying the function of a particular area. Implanted cooling cryoloops have been used to study the role of individual cortical areas in auditory processing of awake-behaving cats. Cryoloops have also been used in rodents for reversible inactivation of the cortex, but recently there has been a concern that the cryoloop may also cool non-cortical structures either directly or via the perfusion of blood, cooled as it passed close to the cooling loop.

Different representations of tooth chatter and purr call in guinea pig auditory cortex.

Multielectrode arrays were used to compare responses to tooth chatter and purr calls from all eight areas of the auditory cortex in anaesthetized guinea pigs. These calls have different behavioural contexts: males emit tooth chatters in aggressive encounters and the purr call during courtship behaviour. Of the two core areas, the primary auditory cortex responded better to both signals than the dorsocaudal core area.

Reexamining the evidence for a pitch-sensitive region: a human fMRI study using iterated ripple noise.

Human neuroimaging studies have identified a region of auditory cortex, lateral Heschl's gyrus (HG), that shows a greater response to iterated ripple noise (IRN) than to a Gaussian noise control. Based in part on results using IRN as a pitch-evoking stimulus, it has been argued that lateral HG is a general "pitch center." However, IRN contains slowly varying spectrotemporal modulations, unrelated to pitch, that are not found in the control stimulus.

Training sound localization in normal hearing listeners with and without a unilateral ear plug.

Surprisingly little is known about the ability of adult human listeners to learn to localize sounds in the free field. In this study, we presented broadband noise bursts at 24 equally spaced locations in a 360° horizontal plane in both normal-hearing conditions and when listeners were fitted with a unilateral earplug. Localization improvement was found over the initial four training sessions, prior to plug insertion which produced an immediate and profound impairment in localization, particularly on the side of the plug.

Dimension-specific attention directs learning and listening on auditory training tasks.

The relative contributions of bottom-up versus top-down sensory inputs to auditory learning are not well established. In our experiment, listeners were instructed to perform either a frequency discrimination (FD) task ("FD-train group") or an intensity discrimination (ID) task ("ID-train group") during training on a set of physically identical tones that were impossible to discriminate consistently above chance, allowing us to vary top-down attention whilst keeping bottom-up inputs fixed. A third, control group did not receive any training.

Mapping feature-sensitivity and attentional modulation in human auditory cortex with functional magnetic resonance imaging.

Feature-specific enhancement refers to the process by which selectively attending to a particular stimulus feature specifically increases the response in the same region of the brain that codes that stimulus property. Whereas there are many demonstrations of this mechanism in the visual system, the evidence is less clear in the auditory system. The present functional magnetic resonance imaging (fMRI) study examined this process for two complex sound features, namely frequency modulation (FM) and spatial motion.

Re-examining the relationship between audiometric profile and tinnitus pitch.

Objective: We explored the relationship between audiogram shape and tinnitus pitch to answer questions arising from neurophysiological models of tinnitus: 'Is the dominant tinnitus pitch associated with the edge of hearing loss?' and 'Is such a relationship more robust in people with narrow tinnitus bandwidth or steep sloping hearing loss?'

Design: A broken-stick fitting objectively quantified slope, degree and edge of hearing loss up to 16 kHz. Tinnitus pitch was characterized up to 12 kHz. We used correlation and multiple regression analyses for examining relationships with many potentially predictive audiometric variables.

Neuronal spike-train responses in the presence of threshold noise.

The variability of neuronal firing has been an intense topic of study for many years. From a modelling perspective it has often been studied in conductance based spiking models with the use of additive or multiplicative noise terms to represent channel fluctuations or the stochastic nature of neurotransmitter release. Here we propose an alternative approach using a simple leaky integrate-and-fire model with a noisy threshold.

Age differences in the purr call distinguished by units in the adult guinea pig primary auditory cortex.

Many communication calls contain information about the physical characteristics of the calling animal. During maturation of the guinea pig purr call the pitch becomes lower as the fundamental frequency progressively decreases from 476 to 261 Hz on average. Neurons in the primary auditory cortex (AI) often respond strongly to the purr and we postulated that some of them are capable of distinguishing between purr calls of different pitch.

Development of auditory processing in 6- to 11-yr-old children.

Objectives: The aim of this study is to provide developmental standards on a variety of temporal, spectral, and binaural psychoacoustic (auditory processing [AP]) tests in typically developing children, including immediate and delayed retest reliability, and comparisons between single listener performance on different tests. This study also informs choices on the selection of tests for clinical evaluation of hearing and listening (e.g.

Forward suppression in the auditory cortex is frequency-specific.

We investigated how physiologically observed forward suppression interacts with stimulus frequency in neuronal responses in the guinea pig auditory cortex. The temporal order and frequency proximity of sounds influence both their perception and neuronal responses. Psychophysically, preceding sounds (conditioners) can make successive sounds (probes) harder to hear.

Indications for temporal fine structure contribution to co-modulation masking release.

The contribution of temporal fine structure (TFS) information to co-modulation masking release (CMR) was examined by comparing CMR obtained with unprocessed or vocoded stimuli. Tone thresholds were measured in the presence of a sinusoidally amplitude-modulated on-frequency band (OFB) of noise and zero, two, or four flanking bands (FBs) of noise whose envelopes were either co- or anti-modulated with the OFB envelope. Vocoding replaced the TFS of the tone and masker with unrelated TFS of noise or sinusoidal carriers.