Developing an Evidence-Based Military Auditory Fitness-for-Duty Standard Based on the 80-Word Modified Rhyme Test.

OBJECTIVES: One important function of military audiology is to conduct evaluations of service members (SMs) with hearing loss to ensure they are fit for deployment in dangerous operational environments. The objective of this study was to establish evidence-based auditory fitness-for-duty criteria based on speech-in-noise performance on the 80- and 160-word clinical versions of the Modified Rhyme Test (MRT 80 and MRT 160 ). DESIGN: Approximately 2400 SMs with various levels of hearing loss were recruited to complete the MRT 80 in conjunction with their annual hearing conservation evaluations. These SMs were also asked to perform one or more operationally-relevant listening tasks based on audio recordings made in highly realistic military training environments. The scores on these tests were compared to determine how well a proposed cutoff criterion for the MRT 80 was able to identify individuals who are hard of hearing with an exceptionally high risk of abnormally poor performance on operationally-relevant hearing tasks. RESULTS: The results show that a cutoff criterion that combines the percent correct score on two lists of the MRT 80 (i.e. MRT 160 ) with information about the better-ear threshold at 2 kHz is generally able to separate listeners with hearing loss into those who are likely to perform relatively well on operational listening tests and those who are likely to perform poorly on these tasks. This is consistent with current military acquisition standards, which identify the MRT as the preferred test for evaluating speech intelligibility for radios, headsets, and other communication equipment. It is also consistent with prior studies conducted in high-fidelity military simulations which have shown a significant correlation between MRT performance and operational outcomes. CONCLUSIONS: The proposed selection criteria, along with the new hearing profile standards that were recently adopted by the US Army, appear to provide an effective evidence-based methodology for identifying those SMs with hearing loss who are most at risk for poor performance on hearing-critical military tasks.

The Effect of Walking on Auditory Localization, Visual Discrimination, and Aurally Aided Visual Search.

OBJECTIVE: The present study was designed to examine the impact that walking has on performance in auditory localization, visual discrimination, and aurally aided visual search tasks. BACKGROUND: Auditory localization and visual search are critical skills that are frequently conducted by moving observers, but most laboratory studies of these tasks have been conducted on stationary listeners who were either seated or standing during stimulus presentation. METHOD: Thirty participants completed three different tasks while either standing still or while walking at a comfortable self-selected pace on a treadmill: (1) an auditory localization task, where they identified the perceived location of a target sound; (2) a visual discrimination task, where they identified a visual target presented at a known location directly in front of the listener; and (3) an aurally aided visual search task, where they identified a visual target that was presented in the presence of multiple visual distracters either in isolation or in conjunction with a spatially colocated auditory cue. RESULTS: Participants who were walking performed auditory localization and aurally aided visual search tasks significantly faster than those who were standing, with no loss in accuracy. CONCLUSION: The improved aurally aided visual search performance found in this experiment may be related to enhanced overall activation caused by walking. It is also possible that the slight head movements required may have provided auditory cues that enhanced localization accuracy. APPLICATION: The results have potential applications in virtual and augmented reality displays where audio cues might be presented to listeners while walking.

The Effects of Blast-Related Neurotrauma on Aurally Aided Visual Search While Standing and Walking.

Service members (SMs) who have suffered mild traumatic brain injury due to blast exposure (b/TBI) often report post-concussive symptoms consistent with auditory, visual, or vestibular impairments even when they score within the normal range on traditional clinical tests of sensory function. One possible explanation for this discrepancy is that patients who score in the low normal range in more than one sensory modality may be severely impaired in tasks that require multisensory integration. This study evaluated unimodal and multimodal sensory performance in SMs with b/TBI and healthy controls by having them conduct four tasks while walking or standing in an immersive virtual environment: an Auditory Localization task (AL) where they moved a cursor to the perceived location of a sound; a Visual Discrimination task (VD) where they distinguished between two visual targets; an Aurally-Aided Visual Search Task (AAVS) where they used an auditory cue to locate and identify a visual target hidden in a field of visual distractors; and a Visual-Only Visual Search task (VOVS) where they located and identified a visual target in a field of distractors with no auditory cue. The results show the b/TBI and healthy control groups performed equivalently in the AL and VD tasks, but that the b/TBI group responded roughly 15% slower in the AAVS task and 50% slower in the VOVS task. Walking had no effect on performance in the visual-only tasks, but both groups responded faster while walking in the AL and AAVS tasks without any reduction in accuracy.

A novel psychophysical method for estimating the time course of olfactory rapid adaptation in humans.

In this presentation, we describe a novel method for estimating the onset time course of psychophysical odor adaptation in human observers. The method employs stimulus conditions derived from an analogous stimulus paradigm in audition. To test this procedure, we used liquid-dilution olfactometry to estimate 2-bottle discrimination thresholds for brief (600 ms) presentations of vanilla odor; 17 volunteers (14 females; ages 18-24) served as participants. The adapting odorant concentration for each participant was set relative to baseline threshold for the 600-ms target alone (i.e., the same level relative to each participant's threshold). To characterize the adaptation-onset time course, we compared thresholds for targets presented simultaneously with the adapting stimulus as a function of the relative delay between the onset of the adapting stimulus and onset of the target. As predicted from the analogous auditory studies, thresholds for the target stimulus increased in an orderly manner with increases in adaptation-to-target onset delay (i.e., as the adaptation process progressively decreased sensitivity). Initial increases in threshold were consistently observed for the briefest onset delays of 50-100 ms. An onset time constant was estimated at 319 ms by fitting a 2-component exponential to the mean group function. Adaptation magnitude was dependent on the level of adapting odorant, relative to threshold. When thresholds were measured in one participant with a different, unrelated target odorant, cineole, there was no effect of the vanilla-adapting stimulus on threshold. The results suggest that olfactory rapid adaptation is measurable psychophysically within 50-200 ms after odor onset, values consistent with physiological measures of adaptation in olfactory receptor neurons. This novel stimulus paradigm offers a powerful psychophysical tool to study both odor adaptation and stimulus interactions at the olfactory periphery.

High-resolution in situ imaging of cochlear implant electrode arrays in cat temporal bones using Tuned Aperture Computed Tomography (TACT).

OBJECTIVE: To determine the suitability of Tuned Aperture Computed Tomography (TACT) to generate high-resolution images of intracochlear electrode arrays, in situ, with sufficient anatomic and electrode detail to relate the location of individual electrode contacts to important anatomic landmarks in cat cadaveric temporal bones. The ultimate objective is to develop an imaging technology whereby variations in electrode location, relative to the target neural tissues, can be accurately determined and related to variations in performance with the cochlear implant. DESIGN: Cat temporal bones were implanted with an experimental scala tympani electrode array and an external fiducial landmark. A series of conventional 2D digital radiographs were collected from a variety of x-ray source projection angles and served as for generation of 3D volume renderings using the TACT software toolbox. The 3D renderings were then reoriented and resliced interactively to view the cochlear and electrode features of interest. RESULTS: Significant electrode and anatomical details could be visualized including the course of the electrode wires (<40 microm diameter), the location of all electrode contacts and the outline of the scala tympani. CONCLUSIONS: TACT generates high-resolution 3D images from 2D conventional radiographs. With TACT, the 3D renderings can be interactively reoriented and resectioned to permit visualization of any cochlear or electrode feature. In the present study, this aspect of TACT affords the opportunity to view of the location of each electrode contact relative to the adjacent cochlear features, such as the scalar walls. Because TACT uses conventional radiographic images to generate the volume renderings, the quality and resolution of the resulting 2D images do not suffer from artifacts characteristic of CT. These findings suggest that TACT may be a powerful tool for understanding the contribution of electrode placement to perceptual performance with the cochlear implant.

Optimization of TACT imaging protocols for in situ visualization of cochlear electrode arrays in cat temporal bones.

OBJECTIVE: To explore the effect of the number of two-dimensional (2D) images and x-ray projection angles on the resolution of reconstructed three-dimensional (3D) volumes of intracochlear electrode arrays in cadaveric cat temporal bones using Tuned Aperture Computed Tomography (TACT). DESIGN: Multiple 2D radiographs (basis images (BI)) of implanted cadaveric cat temporal bones were acquired using a range of projection angles, and imported into the TACT workbench. 3D volumes were reconstructed using varying numbers of BIs. Contrast resolution in the image was determined by comparing the contrast ratio (using maximum and minimum grayscale values) in specified anatomic areas of interest. RESULTS: Systematically increasing the number of BIs used in the reconstruction process resulted in a systematic increase in contrast resolution. Likewise, increasing the range of effective projection angles, as also the number of such angles used in the TACT computation also increased the contrast resolution of the resulting images. CONCLUSIONS: Precise determination of the location of cochlear implant electrodes in situ is critical to understanding the factors influencing efficacy of electrical stimulation of the deaf ear. Renderings generated with the TACT algorithm produce 3D images permitting visualization of implant electrode features and anatomic details with resolution sufficient to accurately localize electrode contacts within scala tympani. The quality of resulting images, evaluated as a function of image contrast, improved with a larger number of BIs in the reconstruction. Wider projection angles also improved image detail in addition to generating thinner slices. Any loss in contrast was compensated for by the number of BIs. TACT can thus be optimized to provide useful data to help characterize the location of intracochlear electrode arrays.