Animals and particularly humans can sense touch via several different modalities such as two-point discrimination touch, flutter, pressure, and vibration to name a few. Vibration is an oscillatory, mechanical waveform that can be periodic like a pendulum on a grandfather clock or random such as tires moving on a dirt road. Either form will produce a wave that alternates between increasing and decreasing in length, which can be felt by the sense of vibration.
Derived from a Greek word meaning to “shake,” pallesthesia is the medical term for the sensation of mechanical vibration on or near the body. Vibrations can be conducted by the bones in the middle ear for the sense of hearing, which in turn results in movement of the hair cells in the cochlea. Additionally, vibrations can be detected by touch receptors in the skin and by nerves near bones, which are able to conduct vibrations. The sensory pathway that is used to detect vibration sensation is the dorsal columns/medial lemniscus system, which transmits the signal from the periphery to the spinal cord, brainstem, and sensory cortices of the brain.
Testing Vibration Sense in a Neurological Exam
Health care providers, particularly neurologists, will test vibration sense to determine (1) hearing ability, (2) deafness, (3) general exteroception ability, and (4) peripheral neuropathies of the extremities in patients. A tuning fork, a metal instrument with two prongs of equal length and a handle, is used to test vibration sensations.
To determine hearing ability, the neurologist will hit the prongs against his or her hand to cause the fork to vibrate and then place the fork next to the person’s ear or on the bones of the skull behind the ear. This is to test the two categories of deafness—conductive and sensorineural. Conductive hearing loss is the result of a disruption in sound wave conduction within the outer and middle ear. Sensorineural deafness, on the other hand, is the result of damage or injury to the cochlea or the auditory nerve that prevents the transduction of the electrical impulse created by sound waves. A combination of both conductive and sensorineural hearing loss is called mixed deafness. Deafness can be unilateral, affecting one ear, or bilateral, affecting both ears. Tuning fork tests, such as the Weber’s test and the Rinne’s test, are used to distinguish between the type of hearing loss, the extent of the loss, as well as the differentiation between unilateral and bilateral deafness.
To determine general exteroception and nerve conduction abilities of the body, the health care provider will place a vibrating tuning fork first on the sternum of the patient and then on each extremity (arms and legs). Placing the tuning fork on the sternum allows the person to identify the sensation and understand what he or she should feel. To test the lower body, the vibrating fork is placed on the ball of the individual’s right or left big toe. If the patient can feel the vibration, then it suggests that nerve conduction from the furthest distance of the leg is working normally. If the patient cannot feel the vibration, then the health care provider will move the vibrating tuning fork to the next bony prominence: the malleolus of the ankle, the tibial shaft, the tibial tuberosity, and then the anterior iliac crest. The lack of vibration sense can tell the neurologist where nerve damage or peripheral neuropathies are located on the leg. The test is repeated on the other lower extremity and then on each upper extremity, starting with the pads of the fingers and then moving superiorly to the shoulder.
Jennifer L. Hellier
See also: Discriminative Touch; Exteroception; Interoception; Senses of Animals; Touch
Dougherty, Patrick, & Chieyeko Tsuchitani. (2015). Somatosensory pathways. In John H. Byrne (Ed.), Neuroscience Online, an electronic textbook for the neurosciences (Chap. 4). Retrieved from http://neuroscience.uth.tmc.edu/s2/chapter04.html
Swenson, Rand. (2006). Chapter 7: Somatosensory Systems. In Review of clinical and functional neuroscience. Dartmouth College. Retrieved from https://www.dartmouth.edu/~rswenson/NeuroSci/chapter_7A.html