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Postural Control
Noise-Enhanced Human Sensorimotor Function ©DIGITAL STOCK
Using Stochastic Resonance to Enhance Somatosensation and Improve Performance of the Human Balance Control System
JAMES J. COLLINS, ATTILA A. PRIPLATA, DENISE C. GRAVELLE, JAMES NIEMI, JASON HARRY, AND LEWIS A. LIPSITZ
S
omatosensory feedback is an important component of the balance control system. Older adults, patients with diabetic neuropathy, and patients with stroke exhibit a marked deficit in the perception of cutaneous and proprioceptive stimuli (e.g., see [1] and references therein). Such changes to the somatosensory system, which have been associated with increases in mechanoreceptor detection thresholds, may predispose individuals to falls, which are the most common cause of morbidity and mortality among older persons. Accordingly, there is a pressing need to develop bioengineering techniques and devices that improve sensorimotor function in older adults and patients with sensory deficits. Recently, it has been shown that noise can enhance the detection and transmission of weak signals in certain nonlinear systems, via a mechanism known as stochastic resonance (SR). The phenomenon of SR, which is counterintuitive given that noise has traditionally been viewed as a detriment to signal detection and transmission, is based on the concept that the flow of information through a system can be maximized by the presence of a particular, nonzero level of noise. SR was originally proposed in the context of global climate modeling as a possible explanation for the periodic recurrences of the Earth’s ice ages [2], [3]. Since then, SR-type dynamics have been demonstrated in a variety of physical and biological systems [4]-[10], including rat cutaneous afferents [11] and human muscle spindles [12]. In this article, we review our work on using input noise (mechanical and electrical, respectively) to enhance somatosensation in humans [1], [13]-[16] and improve the performance of the human balance control system [17], [18]. We also discuss bioengineering applications and future directions for SR-based techniques and devices. Noise-Enhanced Human Somatosensory Function Mechanical Input Noise
As a first effort in this area, we designed a study to examine the effects of input noise on tactile sensation in humans. Specifically, we were interested in studying noise-mediated changes in the perception of subthreshold tactile stimuli. We hypothesized that the ability of an individual to detect a subthreshold tactile stimulus can be significantly enhanced by the presence of a particular, nonzero level of noise [13], [14]. 76
IEEE ENGINEERING IN MEDICINE AND BIOLOGY MAGAZINE
To test this hypothesis, we conducted psychophysical experiments on ten healthy young subjects (age 18-31 years, mean 25 years). Local indentations were applied to the tip of each subject’s right middle digit using a cylindrical probe (Figure 1). The protocol consisted of the presentation of: a) a subthreshold mechanical stimulus plus mechanical noise, or b) no mechanical stimulus plus mechanical noise (Figure 1). The subjects were instructed to indicate when they detected a stimulus. Each trial consisted of 20 presentations, which were equally and randomly distributed between “stimulus” and “no stimulus.” The interpresentation interval was 5 s. The intensity of the input noise was held constant for each trial and varied between trials. Seven to nine different noise intensity levels were included in the protocol. Two trials were conducted for each noise level. The presentation order of the different noise levels was randomized. To characterize SR-type behavior, we used a measure, %correct, that quantifies the percentage of trials for which a subject correctly identified the presentation of “stimulus” or “no stimulus.” The %correct should, on average, be 50 for a protocol involving a subthreshold stimulus and an equal number of “stimulus” and “no stimulus” presentations. On the other hand, this measure should be near 100 for a protocol with test stimuli that are well above the detection threshold. Nine of the ten subjects we examined exhibited clear SR-type behavior: as input noise intensity increased, the %correct increased significantly to a peak (p
