Relations between tactile sensitivity of the finger, arm, and cheek skin over the lifespan showing decline only on the finger.

Touch sensitivity generally declines with age, contributing to loss of manual dexterity and tactile function. We investigated how touch changes over the lifespan, using different tests and on three body sites. We used a classical test of force detection sensitivity, where calibrated monofilaments were applied passively to the right index finger pad, forearm, and cheek. In addition, at the index, we used an active touch spatial discrimination task, developed by our group. Spatial discrimination was estimated through participants' ability to evaluate the distance between parallel bands printed on acrylic plates. Data were collected from 96 healthy women, aged 20-75 years. Force detection and tactile spatial discrimination on the index deteriorated significantly with age; however, no change was found for tactile detection on the forearm or cheek. Tactile detection on the cheek remained remarkably highly sensitive throughout life. There was a significant positive relationship between force detection and spatial discrimination on the index. Further, force detection on the forearm was significantly associated with detection on the index and cheek. Our results suggest a decrease in touch perception with age on the index finger pad, yet a preservation of tactile sensitivity in hairy skin. This opens discussion about the impact of daily activities upon the glabrous hand skin and on the function of hairs in tactile sensitivity. We highlight the need for new methods in evaluating tactile sensitivity on hairy skin.

Applying cosmetic oil with added aromatic compounds improves tactile sensitivity and skin properties.

Tactile sensitivity generally decreases with aging and is associated with impairments in skin properties. Products that hydrate the skin can combat touch deficits and aromatic compounds have been shown to improve skin mechanical properties. Thus, we tested a base cosmetic oil against a perfumed oil, applied to the skin of females aged 40-60 years, on tactile sensitivity and skin properties after repeated application. Tactile detection thresholds were assessed using calibrated monofilaments applied at the index finger, palm, forearm, and cheek. Spatial discrimination on the finger was assessed using pairs of plates with different inter-band spaces. These tests were performed before and after 1 month of base or perfumed oil use. We found that tactile detection thresholds and spatial discrimination improved only in perfumed oil group. A complementary immunohistological study using human skin was conducted to estimate the expression of olfactory receptor OR2A4 and elastic fiber length. Further, the expression of OR2A4 intensity and the length of elastic fibers increased significantly with oil application, where larger effects were seen with the perfumed oil. We conclude that the application of a perfumed oil may be of additional benefit and could repair, and even prevent, tactile decline with aging by ameliorating skin condition.

Passive Proprioceptive Training Alters the Sensitivity of Muscle Spindles to Imposed Movements.

Humans rely on precise proprioceptive feedback from our muscles, which is important in both the acquisition and execution of movements, to perform daily activities. Somatosensory input from the body shapes motor learning through central processes, as demonstrated for tasks using the arm, under active (self-generated) and passive conditions. Presently, we investigated whether passive movement training of the ankle increased proprioceptive acuity (psychophysical experiment) and whether it changed the peripheral proprioceptive afferent signal (microneurography experiment). In the psychophysical experiment, the ankle of 32 healthy human participants was moved passively using pairs of ramp-and-hold movements in different directions. In a pretraining test, participants made judgements about the movement direction in a two-alternative forced choice paradigm. Participants then underwent passive movement training, but only half were cued for learning, where a reference position was signaled by a sound and the participant had to learn to recognize this position; they then completed a post-training test. In a paradigm using the same setup, nine healthy participants underwent microneurography recordings of Ia muscle afferents from the peroneal nerve, where all were cued during training. In the psychophysical experiment, proprioceptive acuity improved with training only in the cued group. In the microneurography experiment, we found that muscle afferent firing was modulated, via an increase in the dynamic index, after training. We suggest that changes in muscle afferent input from the periphery can contribute to and support central perceptual and motor learning, as shown under passive conditions using ankle movements, which may be exploited for movement rehabilitation.

Emotions can alter kinesthetic acuity.

Kinesthesia, the perception of our own body movements, relies on the integration of proprioceptive information arising mostly from muscle spindles, which are sensory receptors in skeletal muscles. We recently showed that emotions alter the proprioceptive messages from such muscle afferents, making them more sensitive to muscle lengthening when participants were listening sad music. Presently, we investigated whether these changes in proprioceptive feedback relating to emotional state may affect the perception of limb movements. Kinesthetic acuity was tested in 20 healthy, young adults by imposing ramp-and-hold movements that consisted of either plantar flexion or dorsiflexion movements of the ankle at 0.04°/s, or no movement. These were imposed during four emotional conditions (listening to neutral, sad, or happy music, or no music). The participants were asked to relax and focus on music (or nothing), and then they shifted their focus to the direction of an incoming movement. Once this had finished, they were asked its direction. Muscle activity, heart rate, and electrodermal activity were recorded during each trial, and after each music condition the participants rated the emotion felt on a visual analog scale. The rating of the emotional content of the music corroborated with changes in physiological measures. Kinesthetic acuity was also affected by the emotional state and found to be larger during the sad condition, as compared to the no music or neutral conditions. We conclude that emotion can shape our perception of movements, which we show here where feeling sadness significantly increase our kinesthetic acuity, this may be functionally relevant for the preparation of appropriate behavioral responses.