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Tactile Aids Inspire Research
Tactile aids, which translate sound waves into vibrations that can be felt by the skin, have been used for decades by people with severe/profound hearing loss to enhance speech/language development and improve speechreading. Although multichannel cochlear implants have gained the lion's share of attention and usage over the years by demonstrating their superiority in word recognition and speech understanding without the need to speechread,1 tactile aids continue to inspire research and may be undergoing a technological renewal.
In 2006, Iranian researchers published a study on the use of tactile aids, along with rehabilitation and training, in patients within the Department of Otolaryngology at the Tehran University of Medical Sciences.2 They designed four educational stages--detection, beginning pattern perception, recognition of speech, and comprehension of words--to check the improvement of subjects who used one-, two- and seven-channel tactile aids.
Patients with the seven-channel tactile aids were able to successfully pass through all four stages, leading the researchers to conclude, "Tactile aids are well accepted by the patients with severe to profound sensorineural hearing loss who do not benefit from usual hearing aids."
Touch Replaces Hearing
According to a 2009 article cited on ScienceDaily, Virginia Commonwealth University School of Medicine researchers have discovered that adult animals with hearing loss actually re-route the sense of touch into the hearing parts of the brain.3 The team reported a phenomenon known as cross-modal plasticity in the auditory system of adult animals. Cross-modal plasticity refers to the replacement of a damaged sensory system by one of the remaining ones. In this case, the sense of hearing is replaced with touch.
"One often learns, anecdotally, that 'grandpa' simply turned off his hearing aid because it was confusing and no longer helped. Our study indicates that hearing deficits in adult animals result in a conversion of their brains' sound processing centers to respond to another sensory modality, making the interpretation of residual hearing even more difficult,' said principal investigator Alex Meredith, PhD, a professor the VCU department of Anatomy and Neurobiology.
Although the researchers emphasized the possible downside of such plasticity by stating, "These findings raise the possibility that even mild hearing loss in adult humans can have serious and perhaps progressive consequences," there is also a possible upside: if the sense of touch is enhanced by moving into the auditory areas of the brain within the temporal lobe, then tactile aids may become even more useful in speech/language development and speechreading.
Auditory-Tactile Link
An auditory-tactile link was noted in humans by researchers at Johns Hopkins University, who wrote in 2009: "Environmental oscillations impinging upon the ear are generally thought to be processed independently of oscillations impinging upon the skin. Here, we show that frequency channels are perceptually linked across audition and touch."4
In a series of psychophysical experiments, the researchers demonstrated that auditory stimuli interfere with tactile frequency perception in a systematic manner, specifically, "performance on a tactile-frequency discrimination task is impaired when an auditory distractor is presented with the tactile stimuli, but only if the frequencies of the auditory and tactile stimuli are similar." Such "crosstalk" between different sensory modalities demonstrates the importance of considering the interrelationships rather than the independence of fundamental sensory dimensions, such as hearing, vision and touch.
Visual Neurons in the Auditory Cortex
This hearing-touch association brings to mind an earlier 2008 study showing that visual neurons can modulate the firing of neurons in the auditory cortex "in a manner that depends upon stimulus efficacy and timing."5 According to researchers at the Max Planck Institute for Cybernetics, "These neurons thus meet the criteria for sensory integration and provide the auditory modality with multisensory contextual information about co-occurring environmental events," such as speech.
New Generation of Tactile Aids
In light of these findings, the 2009 report that researchers in The Massachusetts Institute of Technology's (MIT) Sensory Communication Group are working on says a new generation of tactile aids gains even more importance for "deaf people who rely on lip reading and can't use or can't afford cochlear implants. The cost of the device and the surgery make cochlear implants prohibitive for many people, especially in developing countries."6
"Most deaf people will not have access to that technology in our lifetime," says Ted Moallem, a graduate student working on the project. "Tactile devices can be several orders of magnitude cheaper that cochlear implants."
The software being developed could be compatible with current smart phones, allowing such devices to be transformed into "unobtrusive" tactile aids for the deaf.
The MIT team hopes to improve the devices by refining the acoustic signal processing system to provide tactile cues that are tailored to boost lip-reading performance.
"Anyone who has a smart phone already has much of what they would need to run the program," including a microphone, digital signal-processing capability and a rudimentary vibration system, says Moallem.
The MIT researchers are testing devices that have at least two vibration ranges, one for high-frequency sounds and one for low-frequency sounds. As part of their project, the researchers have done several studies on the frequency reception ability of the skin. The human ear can perceive frequencies up to 20,000 Hz, but for touch receptors in the skin, optimal frequencies are below 500 Hz.
Using a laboratory setup with a device that can provide distinct vibration patterns to three fingers simultaneously, Moallem has done preliminary studies of deaf people's ability to interpret the vibrations from tactile devices.
Audiologists will do well to follow the tactile aid's update for those patients with severe/profound hearing loss who can use and benefit from such devices.
References
1. Miyamoto, RT, Robbins, AM, Osberger, MJ, et al. (1995). Comparison of multichannel tactile aids and other multichannel cochlear implants in children with profound hearing impairments. American Journal of Otology, 16(1): 8-13.
2. Karimi-Yazdi, A, Sazgar, AA, Nadimi-Tehran, A, et al. (2006). Application and usage of tactile aid in Iran. Archives of Iranian Medicine, 9(4): 344-7.
3. Virginia Commonwealth University (2009, March 25). Auditory regions of brain concert to sense of touch, hearing loss study finds. ScienceDaily. Accessed only at www.sciencedaily.com/releases/2009/03/090324111600.htm
4. Yau, JM, Olenczak, JB, Dammann, JF, et al. (2009). Temporal frequency channels are linked across audition and touch. Current Biology, Mar 4 [Epub ahead of print).
5. Kayser, C, Petkov, CI, Logothetis, NK. (2008). Visual modulation of neurons in auditory cortex. Cerebral Cortex, 18(7): 1560-74.
6. Thomson, E. (27-Feb-2009). MIT: Using touch to help deaf people. Massachusetts Institute of Technology. Accessed online at www.eurekalert.org/pub_releases/2009-02/miot/mut022709.php.
Jess Dancer is professor emeritus of audiology at the University of Arkansas at Little Rock. Contact him at jedancer@ualr.edu regarding your experiences with tactile aids.
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