Vibrotactile amplitude discrimination capacity parallels magnitude changes in somatosensory cortex and follows Weber's Law.

In this study, we investigated the changes in perceptual metrics of amplitude discrimination that were observed in ten healthy human subjects with increasing intensities of stimulation. The ability to perceive differences in vibrotactile amplitude changed systematically with increasing stimulus magnitude (i.e., followed Weber's Law) in a near linear fashion (R (2) = 0.9977), and the linear fit determined by the amplitude discrimination task predicted the subjects' detection thresholds. Additionally, the perceptual metrics correlated well with observations from a previously reported study in which measures of SI cortical activity in non-human primates (squirrel monkeys) evoked by different amplitudes of vibrotactile stimulation were obtained (Simons et al. in BMC Neurosci 6:43, 2005). Stimuli were delivered simultaneously to two different skin sites (D2 and D3), enabling a method for the relatively rapid acquisition of the data. Stability and robustness of the measure, its rapid acquisition, and its apparent relationship with responses previously observed in SI cortex, led to the conclusion that deviations from the baseline values observed in the obtained perceptual metric could provide a useful indicator of cerebral cortical health.

Effects of adaptation on the capacity to differentiate simultaneously delivered dual-site vibrotactile stimuli.

The capacity of 20 healthy adult subjects for detecting differences in the amplitude of two simultaneously delivered 25 Hz vibrotactile stimuli was assessed both in the absence and presence of prior exposure to different conditions of adapting stimulation. Results obtained from this study demonstrate that increasing durations of adapting stimulation at one of the two skin sites, in the range of 0.2 to 2.0 s, lead to a systematic and progressive decrease in a subject's ability to accurately discriminate between the two different amplitudes. Delivery of adapting stimuli to both of the sites of skin stimulation prior to simultaneous delivery of the test and standard stimuli, however, leads to an improvement in amplitude discrimination performance--a finding which is consistent with prior published psychophysical studies that demonstrate improvements in discriminatory capacity with much longer durations of adaptation. Striking parallels between the results obtained in this study and those reported in a prior study of the effects of vibrotactile adaptation on the optical response of squirrel monkey contralateral SI cortex to vibrotactile stimulation [Simons, S.B., Chiu, J., Favorov, O.V., Whitsel, B.L., Tommerdahl, M., 2007. Duration-dependent response of SI to vibrotactile stimulation in squirrel monkey. J Neurophysiol. 97, 2121-9, Simons, S.B., Tannan, V., Chiu, J., Favorov, O.V., Whitsel, B.L., Tommerdahl, M., 2005. Amplitude-dependency of response of SI cortex to flutter stimulation. BMC Neurosci. 6, 43] suggest that the perceptual effects detected in this study could be attributable to adaptation-induced alterations of SI response.

A portable tactile sensory diagnostic device.

Current methods for applying multi-site vibratory stimuli to the skin typically involve the use of two separate vibrotactile stimulators, which can lead to difficulty with positioning of stimuli and in ensuring that stimuli are delivered perfectly in phase at the same amplitude and frequency. Previously, we reported a two-point stimulator (TPS) that was developed in order to solve the problem of delivering two-point stimuli to the skin at variable distances between the sites of stimulation. Because of the success of the TPS, we designed and fabricated a new stimulator with four significant improvements over our original device. First, the device is portable, lightweight and can be used in a variety of non-laboratory settings. Second, the device consists of two independently controlled stimulators which allow delivery of stimuli simultaneously to two distinct skin sites with different amplitude, frequency and/or phase. Third, the device automatically detects the skin surface and thus allows for much better automated control of stimulus delivery. Fourth, the device is designed for rapid manufacture and, therefore, can be made readily available to other research (non-laboratory) settings. To demonstrate the device, a modified Bekesy tracking method was used to evaluate the simultaneous amplitude discrimination capacity of 20 subjects.

Vibrotactile adaptation fails to enhance spatial localization in adults with autism.

A recent study [Tannan, V., Tommerdahl, M., Whitsel, B.L., 2006. Vibrotactile adaptation enhances spatial localization. Brain Res. 1102(1), 109-116 (Aug 2)] showed that pre-exposure of a skin region to a 5 s 25 Hz flutter stimulus ("adaptation") results in an approximately 2-fold improvement in the ability of neurologically healthy human adults to localize mechanical stimulation delivered to the same skin region that received the adapting stimulation. Tannan et al. [Tannan, V., Tommerdahl, M., Whitsel, B.L., 2006. Vibrotactile adaptation enhances spatial localization. Brain Res. 1102(1), 109-116 (Aug 2)] proposed that tactile spatial discriminative performance is improved following adaptation because adaptation is accompanied by an increase in the spatial contrast in the response of contralateral primary somatosensory cortex (SI) to mechanical skin stimulation--an effect identified in previous imaging studies of SI cortex in anesthetized non-human primates [e.g., Simons, S.B., Tannan, V., Chiu, J., Favorov, O.V., Whitsel, B.L., Tommerdahl, M, 2005. Amplitude-dependency of response of SI cortex to flutter stimulation. BMC Neurosci. 6(1), 43 (Jun 21) ; Tommerdahl, M., Favorov, O.V., Whitsel, B.L., 2002. Optical imaging of intrinsic signals in somatosensory cortex. Behav. Brain Res. 135, 83-91; Whitsel, B.L., Favorov, O.V., Tommerdahl, M., Diamond, M., Juliano, S., Kelly, D., 1989. Dynamic processes govern the somatosensory cortical response to natural stimulation. In: Lund, J.S., (Ed.), Sensory Processing in the Mammalian Brain. Oxford Univ. Press, New York, 79-107]. In the experiments described in this report, a paradigm identical to that employed previously by Tannan et al. [Tannan, V., Tommerdahl, M., Whitsel, B.L., 2006. Vibrotactile adaptation enhances spatial localization. Brain Res. 1102(1), 109-116 (Aug 2)] was used to study adults with autism. The results demonstrate that although cutaneous localization performance of adults with autism is significantly better than the performance of control subjects when the period of adapting stimulation is short (i.e., 0.5 s), tactile spatial discriminative capacity remained unaltered in the same subjects when the duration of adapting stimulation was increased (to 5 s). Both the failure of prior history of tactile stimulation to alter tactile spatial localization in adults with autism, and the better-than-normal tactile localization performance of adults with autism when the period of adaptation is short are concluded to be attributable to the deficient cerebral cortical GABAergic inhibitory neurotransmission characteristic of this disorder.

Stimulus-dependent effects on tactile spatial acuity.

BACKGROUND: Previous studies have shown that spatio-tactile acuity is influenced by the clarity of the cortical response in primary somatosensory cortex (SI). Stimulus characteristics such as frequency, amplitude, and location of tactile stimuli presented to the skin have been shown to have a significant effect on the response in SI. The present study observes the effect of changing stimulus parameters of 25 Hz sinusoidal vertical skin displacement stimulation ("flutter") on a human subject's ability to discriminate between two adjacent or near-adjacent skin sites. Based on results obtained from recent neurophysiological studies of the SI response to different conditions of vibrotactile stimulation, we predicted that the addition of 200 Hz vibration to the same site that a two-point flutter stimulus was delivered on the skin would improve a subject's spatio-tactile acuity over that measured with flutter alone. Additionally, similar neurophysiological studies predict that the presence of either a 25 Hz flutter or 200 Hz vibration stimulus on the unattended hand (on the opposite side of the body from the site of two-point limen testing - the condition of bilateral stimulation - which has been shown to evoke less SI cortical activity than the contralateral-only stimulus condition) would decrease a subject's ability to discriminate between two points on the skin. RESULTS: A Bekesy tracking method was employed to track a subject's ability to discriminate between two-point stimuli delivered to the skin. The distance between the two points of stimulation was varied on a trial-by-trial basis, and several different stimulus conditions were examined: (1) The "control" condition, in which 25 Hz flutter stimuli were delivered simultaneously to the two points on the skin of the attended hand, (2) the "complex" condition, in which a combination of 25 Hz flutter and 200 Hz vibration stimuli were delivered to the two points on the attended hand, and (3) a "bilateral" condition, in which 25 Hz flutter was delivered to the two points on the attended hand and a second stimulus (either flutter or vibration) was delivered to the unattended hand. The two-point limen was reduced (i.e., spatial acuity was improved) under the complex stimulus condition when compared to the control stimulus condition. Specifically, whereas adding vibration to the unilateral two-point flutter stimulus improved spatial acuity by 20 to 25%, the two-point limen was not significantly affected by substantial changes in stimulus amplitude (between 100 - 200 microm). In contrast, simultaneous stimulation of the unattended hand (contralateral to the attended site), impaired spatial acuity by 20% with flutter stimulation and by 30% with vibration stimulation. CONCLUSION: It was found that the addition of 200 Hz vibration to a two-point 25 Hz flutter stimulus significantly improved a subject's ability to discriminate between two points on the skin. Since previous studies showed that 200 Hz vibration preferentially evokes activity in cortical area SII and reduces or inhibits the spatial extent of activity in SI in the same hemisphere, the findings in this paper raise the possibility that although SI activity plays a major role in two-point discrimination on the skin, influences relayed to SI from SII in the same hemisphere may contribute importantly to SI's ability to differentially respond to stimuli applied to closely spaced skin points on the same side of the body midline.

A novel device for delivering two-site vibrotactile stimuli to the skin.

Current methods for applying two-site vibration stimuli to the skin typically involve the use of two separate vibrotactile stimulators, which can lead to difficulty with positioning of stimuli and in ensuring that stimuli are delivered perfectly in phase at the same amplitude and frequency. This report describes the Two-Point Stimulator (TPS) that was developed in order to deliver two-point stimuli to the skin at variable distances between the sites of stimulation on a trial-by-trial basis. The apparatus attaches to a vibrotactile stimulator, modifying it from the standard single probe tip to two probe tips. Each of the two probe tips can be independently positioned to set the tip-to-tip spacing. Both points of the TPS are driven by the single vibrotactile stimulator and distances between the two sites can be varied on a trial-by-trial basis. To test the device, a modified Bekesy tracking method was developed and used for two-point limen testing under stimulus conditions of varying amplitude and frequency. Data collected were consistent with previously published reports, suggesting that one possible use of the device would be to provide a means for improved measures of spatio-tactile acuity.