Evaluation of response methods for the localization of nearby objects.

Four response methods for indicating the perceived locations of nearby objects were evaluated: the direct-location (DL) method, where a response pointer is moved directly to the perceived location of the target; the large-head (LH) and small-head (SH) methods, where the pointer is moved to the target location relative to a full-scale or half-scale manikin head; and the verbal report (VR) method, where the spherical coordinates of the target location are indicated verbally. Measurements with a visual target indicated that the DL method was relatively unbiased and considerably more accurate than the other methods, which were all roughly equivalent. Correcting for bias improved accuracy in the LH, SH, and VR responses, but not to the level of the uncorrected DL responses. Replacing the visual target with an acoustic stimulus approximately doubled the errors with the DL response but indicated similar performance in the front and rear hemispheres. The results suggest that DL is the most appropriate response method for close-range localization experiments.

Auditory localization of nearby sources. II. Localization of a broadband source.

Although many researchers have examined auditory localization for relatively distant sound sources, little is known about the spatial perception of nearby sources. In the region within 1 m of a listener's head, defined as the "proximal region," the interaural level difference increases dramatically as the source approaches the head, while the interaural time delay is roughly independent of distance. An experiment has been performed to evaluate proximal-region localization performance. An auditory point source was moved to a random position within 1 m of the subject's head, and the subject responded by pointing to the perceived location of the sound with an electromagnetic position sensor. The overall angular error (17 degrees) was roughly comparable to previously measured results in distal-region experiments. Azimuth error increased slightly as the sound source approached the head, but elevation performance was essentially independent of source distance. Distance localization performance was generally better than has been reported in distal-region experiments and was strongly dependent on azimuth, with the stimulus-response correlation ranging from 0.85 to the side of the head to less than 0.4 in the median plane. The results suggest that the enlarged binaural difference cues found in the head-related transfer function (HRTF) for nearby sources are important to auditory distance perception in the proximal region.

Information transmission with a multifinger tactual display.

In this work, the tactual information transmission capabilities of a tactual display designed to provide stimulation along a continuum from kinesthetic movements to cutaneous vibrations are assessed. The display is capable of delivering arbitrary waveforms to three digits (thumb, index, and middle finger) within an amplitude range from absolute detection threshold to about 50 dB sensation level and a frequency range from dc to above 300 Hz. Stimulus sets were designed at each of three signal durations (125, 250, and 500 msec) by combining salient attributes, such as frequency (further divided into low, middle, and high regions), amplitude, direction of motion, and finger location. Estimated static information transfer (IT) was 6.5 bits at 500 msec, 6.4 bits at 250 msec, and 5.6 bits at 125 msec. Estimates of IT rate were derived from identification experiments in which the subject's task was to identify the middle stimulus in a sequence of three stimuli randomly selected from a given stimulus set. On the basis of the extrapolations from these IT measurements to continuous streams, the IT rate was estimated to be about 12 bits/sec, which is roughly the same as that achieved by Tadoma users in tactual speech communication.

Auditory localization of nearby sources. Head-related transfer functions.

Although researchers have long recognized the unique properties of the head-related transfer function (HRTF) for nearby sources (within 1 m of the listener's head), virtually all of the HRTF measurements described in the literature have focused on source locations 1 m or farther from the listener. In this study, HRTFs for sources at distances from 0.12 to 1 m were calculated using a rigid-sphere model of the head and measured using a Knowles Electronic Manikin for Acoustic Research (KEMAR) and an acoustic point source. Both the calculations and the measurements indicate that the interaural level difference (ILD) increases substantially for lateral sources as distance decreases below 1 m, even at low frequencies where the ILD is small for distant sources. In contrast, the interaural time delay (ITD) is roughly independent of distance even when the source is close. The KEMAR measurements indicate that the direction of the source relative to the outer ear plays an important role in determining the high-frequency response of the HRTF in the horizontal plane. However, the elevation-dependent characteristics of the HRTFs are not strongly dependent on distance, and the contribution of the pinna to the HRTF is independent of distance beyond a few centimeters from the ear. Overall, the results suggest that binaural cues play an important role in auditory distance perception for nearby sources.

Reception of Morse code through motional, vibrotactile, and auditory stimulation.

The potential for communication through the kinesthetic aspect of the tactual sense was examined in a series of experiments employing Morse code signals. Experienced and inexperienced Morse code operators were trained to identify Morse code signals that were delivered as sequences of motional stimulation through up-down displacements (roughly 10 mm) of the fingertip. Performance on this task was compared with that obtained for both vibrotactile and acoustic presentation of Morse code using a 200-Hz tone delivered either to the fingertip through a minishaker or diotically to the two ears under headphones. For all three modalities, the ability to receive Morse code was examined as a function of presentation rate for tasks including identification of single letters, random three-letter sequences, common words, and sentences. Equivalent word-rate measures (i.e., product of percent correct scores and stimulus presentation rate) were nearly twice as high for auditory presentation as for vibrotactile stimulation, which in turn was about 1.3 times that for motional stimulation. The experienced subjects outperformed the inexperienced subjects by amounts that increased with task complexity. For example, the former were able to receive sentences at 18 words/min with motional stimulation, whereas the latter, following 75 h of training, were unable to perform this task. The present results and those of other research with tactual communication systems are compared, particularly regarding estimates of information-transfer rates.

Information transmission with a multi-finger tactual display.

Motivated by the highly successful Tadoma method of speech communication, a multi-finger positional display (the TACTUATOR) was developed to study perception via the kinesthetic and vibrotactile aspects of the tactual sensory system of the hand. The information transmission capabilities with the TACTUATOR were assessed through a series of absolute identification experiments. An information transfer (IT) of 5.6 to 6.5 bits for stimulus durations of 125 to 500 msec was obtained in absolute-identification experiments with sets of signals derived by varying frequency, amplitude, and site of stimulation of multicomponent waveforms. An estimated IT rate of 12 bits/sec was obtained by sequencing three random stimuli and (a) having the subject identify only the middle stimulus and (b) extrapolating this IT to that for continuous streams. This IT rate is roughly the same as that achieved by Tadoma users in tactual speech communication.

Assessment of blood donor privacy during health history interviews.

BACKGROUND: Given the personal nature of health history interviews, it is important to provide donors with both visual and auditory privacy. Privacy is affected by variables such as background noise, the use of visual screens, and the loudness of the donor's voice. STUDY DESIGN AND METHODS: In Phase I of this study, an interview station and waiting area were simulated. To measure auditory privacy, a speech intelligibility test was given to subjects with and without the use of a free-standing privacy screen and masking noise device. Phase II was a field trial designed to evaluate screens and masking noise. Background noise was measured during each blood collection operation, and donors completed a survey. RESULTS: In Phase I, speech intelligibility test scores ranged from 78 to 5.1 percent, depending on the type of visual screen and the number of masking noise devices used. In Phase II, with the use of screens, 94 percent of donors rated visual privacy as "good to excellent," compared with 74 percent who did so when no screens were used. At many blood drives, the background noise level exceeded the level of the masking noise. CONCLUSION: The use of visual screens increases donors' perception of visual privacy. The use of masking noise is effective only when the health history interview is conducted under conditions of low background noise levels.

Effects of channel-to-electrode mappings on speech reception with the ineraid cochlear implant.

OBJECTIVE: Research was conducted to assess multichannel saliency with the Ineraid cochlear prosthesis. The goal was to determine whether tonotopically ordered stimulation benefits speech reception. DESIGN: In a single high-performing subject, changes in speech reception were studied during two dramatic alterations to the normal (tonotopic) mapping between the sound-processors's four filter channels and the intracochlear electrode array: In one alteration, the four filter outputs were summed and delivered to one electrode; this single-channel mapping was worn for 7 days. In another alteration, the four filter outputs were connected in reverse tonotopic order to four intracochlear electrodes; this reversed mapping was worn for 3 days. RESULTS: When using the implant in conjunction with speechreading, all three mappings provided large gains over speechreading alone on the recognition of words in sentences. When using the implant alone (without speechreading), tests of consonant and vowel recognition, and the recognition of words in isolation and in sentences all showed substantial decreases in performance across the three mapping conditions: normal > single-channel > reversed. The patterns of segmental confusions and the relations among scores on different tests were highly consistent. Finally, the performance with the altered mappings was, in some ways, remarkably good. With single-channel mapping, 15% word recognition was obtained, much less than the 54% obtained with the normal mapping, but demonstrative of some open-set speech reception. With the reversed mapping, high levels of consonantal voicing and manner information were received, indicating good reception of time-intensity cues, but open-set word recognition was near zero. CONCLUSIONS: Despite the extensive spread of current associated with monopolar intracochlear stimulation, the Ineraid electrode array affords a degree of perceptual selectivity that substantially aids speech reception.

Intensity perception. XIV. Intensity discrimination in listeners with sensorineural hearing loss.

Intensity discrimination of pulsed tones (also called level discrimination) was measured as a function of level in 13 listeners with sensorineural hearing impairment of primarily cochlear origin, one listener with a vestibular schwannoma, and six listeners with normal hearing. Measurements were also made in normal ears presented with masking noise spectrally shaped to produce audiograms similar to those of the cochlearly impaired listeners. For unilateral impairments, tests were made at the same frequency in the normal and impaired ears. For bilateral-sloping impairments, tests were made at different frequencies in the same ear. The normal listeners showed results similar to other data in the literature. The listener with a vestibular schwannoma showed greatly reduced intensity resolution, except at a few levels. For listeners with recruiting sensorineural impairments, the results are discussed according to the configuration of the impairment and are compared across configurations at equal SPL, equal SL, and equal loudness level. Listeners with increasing hearing losses at frequencies above the test frequency generally showed impaired resolution, especially at high levels, and less deviation from Weber's law than normal listeners. Listeners with decreasing hearing loss at frequencies above the test frequency showed nearly normal intensity-resolution functions. Whereas these trends are generally present, there are also large differences among individuals. Results obtained from normal listeners who were tested in the presence of masking noise indicate that elevated thresholds and reduced dynamic range account for some, but not all, of the effects of recruiting sensorineural impairment on intensity resolution.

On the potential of fixed arrays for hearing aids.

Microphone arrays with fixed (time-invariant) weights are directed at enhancing a desired signal from one direction (straight ahead) while attenuating spatially distributed interference and reverberation. Using the theory of sensitivity-constrained optimal beamforming [Cox et al., IEEE Trans. Acoust. Speech Sig. Process. ASSP-34, 393-398 (1986)], free-field arrays of head-sized extents were studied. The key parameters affecting array design and performance are the set of transfer functions from the target direction to each array microphone [H(f)] and the intermicrophone cross-spectral densities for isotropic noise [Szz(f)]. Design variables included the orientation of the array, the number, and [as motivated by Soede, Ph.D. thesis, Delft University of Technology (1990)] the directionality of the microphones within the array, and the complexity and robustness of the required processing. Performance was characterized by the broadband intelligibility-weighted directivity (gain against isotropic noise) and noise sensitivity (reflecting the array's sensitivity to uncorrelated noise, as well as device tolerances). For broadside orientation, a variety of arrays based on cardioid and hypercardioid microphones gave very similar performance. They can provide directivities of 7-8 dB with easily implemented weights (simple scalars). For endfire orientation, as Soede (1990) recognized, similar directivities result with weights based on analog gains and pure time delays. However, with weightings chosen independently for each frequency, directivities up to approximately 11 dB may be obtained, although the increased noise sensitivities of these arrays require practical evaluation. Because of sound diffraction, placement of arrays onto the head potentially impacts both their design and performance. In-situ measurements of H(f) and Szz(f) as well as simplified theoretical models are suggested to explore the optimization of head-mounted arrays.

Relations among different measures of speech reception in subjects using a cochlear implant.

A comprehensive set of speech reception measures were obtained in a group of about 20 postlingually deafened adult users of the Ineraid multichannel cochlear implant. The measures included audio, visual, and audiovisual recognition of words embedded in two types of sentences (with differing degrees of difficulty) and audio-only recognition of isolated monosyllabic words, consonant identification (12 alternatives, /Ca/), and vowel identification (8 alternatives, /bVt/). For most implantees, the audiovisual gains in the sentence tests were very high. Quantitative relations among audio-only scores were assessed using power-law transformations suggested by Boothroyd and Nittrouer [J. Acoust. Soc. Am. 84, 101-114 (1988)] that can account for the benefit of sentence context (via a factor k) and the relation between word and phoneme recognition (via a factor j). Across the broad range of performance that existed among the subjects, substantial order was observed among measures of speech reception along the continuum from recognition of words in sentences, words in isolation, speech segments, and the retrieval of underlying phonetic features. Correlations exceeded 0.85 among direct and sentence-derived measures of isolated word recognition as well as among direct and word-derived measures of segmental recognition. Results from a variety of other studies involving presentation of limited auditory signals, single-channel and multichannel implants, and tactual systems revealed a similar pattern among word recognition, overall consonant identification performance, and consonantal feature recruitment. Finally, improving the reception of consonantal place cues was identified as key to producing the greatest potential gains in speech reception.

Analytic study of the Tadoma method: improving performance through the use of supplementary tactual displays.

Although results obtained with the Tadoma method of speechreading have set a new standard for tactual speech communication, they are nevertheless inferior to those obtained in the normal auditory domain. Speech reception through Tadoma is comparable to that of normal-hearing subjects listening to speech under adverse conditions corresponding to a speech-to-noise ratio of roughly 0 dB. The goal of the current study was to demonstrate improvements to speech reception through Tadoma through the use of supplementary tactual information, thus leading to a new standard of performance in the tactual domain. Three supplementary tactual displays were investigated: (a) an articulatory-based display of tongue contact with the hard palate; (b) a multichannel display of the short-term speech spectrum; and (c) tactual reception of Cued Speech. The ability of laboratory-trained subjects to discriminate pairs of speech segments that are highly confused through Tadoma was studied for each of these augmental displays. Generally, discrimination tests were conducted for Tadoma alone, the supplementary display alone, and Tadoma combined with the supplementary tactual display. The results indicated that the tongue-palate contact display was an effective supplement to Tadoma for improving discrimination of consonants, but that neither the tongue-palate contact display nor the short-term spectral display was highly effective in improving vowel discriminability. For both vowel and consonant stimulus pairs, discriminability was nearly perfect for the tactual reception of the manual cues associated with Cued Speech. Further experiments on the identification of speech segments were conducted for Tadoma combined with Cued Speech. The observed data for both discrimination and identification experiments are compared with the predictions of models of integration of information from separate sources.

Better speech recognition with cochlear implants.

HIGH levels of speech recognition have been achieved with a new sound processing strategy for multielectrode cochlear implants. A cochlear implant system consists of one or more implanted electrodes for direct electrical activation of the auditory nerve, an external speech processor that transforms a microphone input into stimuli for each electrode, and a transcutaneous (rf-link) or percutaneous (direct) connection between the processor and the electrodes. We report here the comparison of the new strategy and a standard clinical processor. The standard compressed analogue (CA) processor presented analogue waveforms simultaneously to all electrodes, whereas the new continuous interleaved sampling (CIS) strategy presented brief pulses to each electrode in a nonoverlapping sequence. Seven experienced implant users, selected for their excellent performance with the CA processor, participated as subjects. The new strategy produced large improvements in the scores of speech reception tests for all subjects. These results have important implications for the treatment of deafness and for minimal representations of speech at the auditory periphery.

Robustness of an adaptive beamforming method for hearing aids.

We describe the results of computer simulations of a multimicrophone adaptive-beamforming system as a noise reduction device for hearing aids. Of particular concern was the system's sensitivity to violations of the underlying assumption that the target signal is identical at the microphones. Two- and four-microphone versions of the system were tested in simulated anechoic and modestly-reverberant environments with one and two jammers, and with deviations from the assumed straight-ahead target direction. Also examined were the effects of input target-to-jammer ratio and adaptive-filter length. Generally, although the noise-reduction performance of the system is degraded by target misalignment and modest reverberation, the system still provides positive advantage at input target-to-jammer ratios up to about 0 dB. This is in contrast to the degrading target-cancellation effect that the system can have when the equal-target assumption is violated and the input target-to-jammer ratio is greater than zero.

Analysis of a synthetic Tadoma system as a multidimensional tactile display.

The Tadoma method is a means of speech reception based on tactile monitoring of the articulatory process. A "synthetic" Tadoma system, involving an artificial face with six facial actions, has been developed as a first-order approximation to the natural Tadoma system. Experiments were conducted to explore the information-transmission characteristics of the synthetic Tadoma system in terms of the four facial movements it incorporates: upper lip in-out, lower lip in-out, lower lip up-down, and jaw up-down movements. Discrimination experiments showed that the just-noticeable difference associated with each movement is about 9% of the reference displacement. One-dimensional (1-D) absolute identification experiments produced, on the average, 1.6 bits of information transfer. Four dimensional (4-D) identification experiments produced information transfers in the range of 3-4 bits. Of the four dimensions considered, performance on the lower lip up-down movement was most affected, and performance on the jaw up-down movement was least affected, by simultaneous roving movements on the other dimensions. As a result of the interaction among the movement channels, the sum of the 1-D information transfers exceeds the 4-D information transfer. However, the sum of the 1-D information transfers obtained from tests with roving parameters is approximately equal to the 4-D information transfer (possibly exemplifying a "generalized information-transfer additivity law"). In general, both the discrimination and identification results appear unexceptional and, hence, the reception of facial movement information by itself does not appear to account for the extraordinary success of the Tadoma method.

Manual discrimination and identification of length by the finger-span method.

Experiments were conducted on length resolution for objects held between the thumb and fore-finger. The just noticeable difference in length measured in discrimination experiments is roughly 1 mm for reference lengths of 10 to 20 mm. It increases monotonically with reference length but violates Weber's law. Also, it decreases when the subject is permitted to maintain a constant finger span between trials; however, it tends to increase when the nondominant hand is used. As would be expected from studies of other stimulus dimensions in other sense modalities, resolution is considerably poorer in identification experiments than in discrimination experiments. For stimulus sets that cover a broad range (90 mm), the total information transfer is roughly 2 bits; for those that cover a relatively small range (18 mm), it is roughly 1 bit. The data are analyzed and interpreted using analysis techniques and models that have been used previously in studies of audition (e.g., Durlach & Braida, 1969).

Ear canal acoustic distortion at 2f1-f2 from human ears: relation to other emissions and perceived combination tones.

Two aspects of the intermodulation distortion product at 2f1-f2 generated by normal human ears and measured acoustically in the ear canal were studied: (1) its relation to tone-evoked and spontaneous otoacoustic emissions, and (2) its relation to the perceived combination tone at the same frequency. With regard to (1), substantial differences among ears in the detectability of emissions were observed; ears tended to exhibit all or none of the emission types that were sought. Within ears possessing emissions, the magnitudes of tone-evoked emissions and acoustic distortion showed a similar dependence on frequency. With regard to (2), a three-primary-tone stimulus was employed to ask whether the ear canal acoustic distortion tone is canceled under the same stimulus conditions that produce perceptual cancellation. Simultaneous cancellation of perceptual and acoustic distortion was produced rarely. Results are interpreted qualitatively with a model in which primary tones produce distortion at their interaction region within the cochlea; this distortion propagates to the distortion-frequency place where it mediates perception. This same distortion wave produces emission components at additional locations, including the primary-tone interaction region, which sum vectorially to mediate the emitted acoustic distortion product.

Preliminary results of speech-reception tests obtained with the synthetic Tadoma system.

In the Tadoma method of speech reception used by some deaf-blind individuals, speech is understood by placing a hand on the face of the talker and feeling certain mechanical actions of the face associated with speech production. The synthetic Tadoma system is a computer-driven artificial face that simulates these mechanical actions. This paper reports some preliminary data on the discrimination of nonsense syllables with the synthetic system. Although further work is required to produce an accurate simulation, the results suggest that such a goal is indeed achievable.

Multidimensional tactile displays: identification of vibratory intensity, frequency, and contactor area.

Experiments were conducted to determine the ability of subjects to identify vibrotactile stimuli presented to the distal pad of the middle finger. The stimulus sets varied along one or more of the following dimensions: intensity of vibration, frequency of vibration, and contactor area. Identification performance was measured by information transfer. One-dimensional stimulus sets produced values in the range 1-2 bits and, for most subjects, three-dimensional sets produced values in the range 4-5 bits. Of the three dimensions considered, performance on the intensity variable was most affected, and performance on contactor area least affected, by simultaneous variations in the other dimensions.