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Developing a system for testing stochastic resonance effects on tactile sensitivity with LabVIEW

Kimberly-Clark is a medical, professional, and consumer products company best known for its disposable consumer products, which are an essential part of everyday life.


The company is also a major player in the clinical market, delivering products such as surgical gloves, drapes, digestive health products, and more. Its professional products include eye, hand, ear, and dust protection, as well as high-performance Jackson safety auto-diming welding helmets.

Additionally, Kimberly-Clark is conducting numerous research projects to explore innovative ways to improve the quality of life of its consumers. One such research project requires applying an electrical noise signal to a subject in a controlled manner to quantify its effect on nerve sensitivity.

Kimberly-Clark was challenged with creating a system to precisely and repeatedly control the amount of electrical stimuli applied to a subject during stochastic resonance testing while reducing systematic data collection errors.

Using an isolated bipolar constant current stimulator, an NI USB-6221 BNC multifunction data acquisition (DAQ) module, and the NI LabVIEW graphical programming environment to develop a system that controls the output of the stimulator and automates the stochastic resonance testing process.

Because it used LabVIEW to control the process, the Kimberly-Clark team was able to apply electrical stimuli, and timing, rather than a manual solution, resulting in test conditions that were consistent and repeatable.

As people age, the effectiveness of their various sensory systems diminish. Solutions to assist people with age-triggered losses in visual and auditory sensory modality are quite prevalent.

However, solutions to overcome age-triggered tactile sensory losses are less prevalent. Tactile sensory loss can impair a person’s quality of life in many ways. Loss of tactile sensitivity in the hands may diminish the ability to carry out manual tasks such as writing or pushing a button. Vibrotactile sensory loss in the big toe can lead to an increased tendency for the elderly to lose their balance and fall. Approximately one-third of adults in the U.S. aged 65 and older fall at least once each year, and 10 to 15% of these falls result in serious injury or fracture.

Numerous studies have shown that sub-threshold electrical or mechanical noise stimulation may help people overcome functional difficulties due to age-related tactile sensory loss.1-4 Noise stimulation, alternatively called stochastic resonance, energizes tactile sensory neurons in the skin so that they are predisposed to fire in response to environmental stimuli. By increasing the sensitivity of mechanoreceptors, clinicians can effectively boost sensory information in a fashion that is concordant with normal functions. As indicated in several articles, this technology has been used in clinical studies to investigate its effectiveness in reducing threshold values of tactile sensation. Both mechanical and electrical noise stimulation have been used in these studies.

Our team needed a system to deliver sub-threshold electrical noise stimulation to a test subject. The system requirements included:

• Control of all instrumentation in the stochastic resonance system
• Automation of the experiment flow to aid the researcher in efficiently and effectively carrying out experiments
• Collection of pertinent experimental data outputs
• Portability so the system could be used at home on a laptop or desktop

The key components in the stochastic resonance system are the bipolar constant current stimulator, an NI USB-6221, and LabVIEW. We used a -10 to 10 V analog waveform input supplied by an analog port on the NI USB-6221 to control the current stimulator output.
Kimberly-Clark designed the system so that one researcher can apply physical stimuli and control electrical stimuli to the subject.

The LabVIEW software automatically controls electrical stimuli while prompting the researcher for proper mechanical stimuli. The company used built-in LabVIEW math and waveform functions to send a Gaussian noise signal with specific peak amplitude to the stimulator, which results in the subject receiving a current stimulus matching the wave shape and amplitude of the control signal. For additional safety, stimulator feedback is monitored by the analog input port on the DAQ module to ensure the stimulator output stays below a predetermined maximum peak.

The company selected the NI USB-6221 BNC DAQ module for this system because of the analog I/O features, acquisition and control performance, USB flexibility, ease of installation, and its integration with LabVIEW. The built-in tools in LabVIEW for waveform generation, waveform analysis, communication with DAQ modules, and other common functions saved considerable development time because the programmer could concentrate on higher-level test process flow.

Waveform construction was a critical step in the testing process for two reasons. First, it is important to prevent any unintended exposure to dc currents because this may be interpreted as a false positive by the test subject. Human perception of a dc current is different than that of an ac current. Second, because of inherent heightened sensitivity to abrupt changes in electrical stimulation, ramped non-periodic waveforms ‘softened’ the electrical stimulus application and removal.

Before testing can begin, Kimberly-Clark performed a two-part process to establish the subject’s threshold of detection (TD). The first part presents a white noise electrical stimulus at a low value that is gradually ramped up in amplitude until the subject indicates they can feel the stimulus. After a few iterations of this process, an average TD is established.

During the process of ramping up the applied electrical stimulus level, the tester must load the DAQ module with new waveforms. When a running waveform is stopped to load and restart a new waveform, the stimulator output appeared to stick at some random dc level, presumably the last buffer value read. This is normal because it takes a small amount of time to replace the waveform already in the DAQ modules memory. The DAQ module just outputs whatever value was present when the task was stopped to load the new waveform into memory.

Alternatively, the company used the waveform regeneration mode found in the NI-DAQmx Write class property node to load a new waveform even as the NI USB-6221 was executing the current waveform. An additional benefit of this method is that the noise waveform appeared to ramp from the old to new value. Therefore, the output transitions smoothly between levels so the subject is never exposed to a dc current.

The next part of the TD process requires the subject to indicate whether or not they feel a one-second burst of electrical stimuli during a given period simply labeled as red or blue time. The level is set above and below the TD value obtained in the previous process to fine tune the subjects actual TD. The waveforms in this case are created and then multiplied by a unit waveform with ramps on the front and back end to ensure they do not start and end abruptly.

The team used the LabVIEW Advanced Signal Processing toolkit, which included waveform generation, filtering, and measurement tools, to program and fine tune these specialized waveform generating algorithms to happen in hours, rather than days.

Collecting extensive experiment data requires a consistent, repeatable process. Our LabVIEW solution is efficient and saves time by automating some of the mundane control functions for the researcher, so the researcher can concentrate on the proper application of the physical stimuli.

The use of USB-based equipment from NI means that a dedicated computer does not have to be assigned to the systems, adding flexibility, portability, and easier system upgrades. In addition, because we used LabVIEW to control the process, applied electrical stimuli, and timing, rather than a manual solution, the test conditions are consistent and repeatable
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References:
1. “Enhancing Tactile Sensation in Older Adults with Electrical Noise Stimulation”, Dhruv et al (2002), Neuroreport, 13(5)
2. “Noise-Enhanced Vibrotactile Sensitivity in Older Adults, Patients with Stroke, and Patients with Diabetic Neuropathy”, Liu et al (2002), Arch Phys Med Rehabil, vol 38
3. “Noise-Enhanced Human Balance Control”, Priplata et al (2002), Physical Review Letters, 89(23)
4. “Touch Noise Increases Vibrotactile Sensitivity in Old and Young”, Wells et al (2005), Psychological Science, 16(4)
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