Dimitris Voudouris

Tactile sensitivity is temporally modulated on a moving limb. Yet the neural mechanisms underlying this modulation remain unclear. Using electroencephalography, we examined the temporal dynamics of early somatosensory processing during goal-directed reaching. Participants (self-reported gender, 22 females, 16 males) reached for their unseen left hand while a brief standard vibration of a fixed intensity was applied to the moving right index finger in different movement phases. After each reach, participants compared this standard with a second comparison vibration of varying intensity applied to the sternum and judged which vibration felt stronger. The same task was performed at rest to account for individual differences. Psychophysical results revealed an overall reduction in tactile sensitivity during reaching compared with rest, with sensitivity transiently improving around maximum speed relative to the late movement phase. Consistent with the psychophysical data, the short-latency P45 component, originating in the primary somatosensory cortex, was reduced during reaching but recovered around maximum speed. Lower P45 amplitudes were associated with lower tactile sensitivity measured behaviorally, suggesting that early somatosensory processing may contribute to the observed perceptual changes. This temporally specific modulation of tactile perception, together with corresponding changes at the early cortical stage, is consistent with the idea that the somatosensory system dynamically adapts its sensitivity to facilitate control of somatosensory-guided actions.

Younger and older adults rely on similar visuomotor strategies. However, older adults exhibit delayed hand movements. They appear to compensate for motor slowing by maintaining predictive gaze shifts until the moment of acting, whereas younger adults reduce their predictions around that moment. Both groups showed learning of predictive gaze strategies over the time of the experiment.

When reaching for an object, tactile sensations from a sleeve sliding over the moving arm are often ignored, yet the computational principles governing this phenomenon remain unclear. Prevailing accounts propose that the nervous system reduces tactile sensitivity by prioritizing internal predictions over somatosensory feedback, but a quantitative explanation of its temporal dynamics has been lacking. Here, we show that tactile suppression is a consequence of optimal state estimation during movement. Using optimal feedback control theory, we predicted how the nervous system should dynamically weight uncertain internal predictions against noisy somatosensory input. Human participants performed goal-directed reaching movements while vibrotactile stimuli were delivered at different time points. Suppression weakened when internal uncertainty about hand position increased, consistent with greater reliance on sensory feedback. These results identify tactile suppression as a consequence of continuous and dynamic uncertainty-dependent sensorimotor state estimation rather than fixed gating, revealing a normative principle governing tactile sensitivity during action.

Tactile sensitivity is temporally modulated on a moving limb. Yet the neural mechanisms underlying this modulation remain unclear. Using electroencephalography, we examined the temporal dynamics of early somatosensory processing during goal-directed reaching. Participants reached for their unseen left hand while brief vibrations were applied to the moving right index finger in different movement phases. Psychophysical results revealed overall tactile suppression during reaching, with sensitivity transiently improving around maximum speed relative to the late movement phase. Consistent with the psychophysical data, the short-latency P45 component, originating in primary somatosensory cortex, was suppressed during reaching but recovered around maximum speed. Reduced P45 amplitudes were associated with stronger tactile suppression measured behaviorally, linking early cortical activity to perceptual changes. This temporally specific modulation of tactile perception, associated with early cortical processing, suggests that the somatosensory system dynamically adapts its sensitivity to facilitate sensorimotor control in goal-directed actions. Significance statement: The perception of touch on a moving hand varies across different movement phases, depending on the importance of sensory feedback for guiding the hand. Our results show that these changes in perception are linked to modulations in early brain activity, indicating that the central nervous system adjusts touch processing in a time-sensitive manner at early processing stages to facilitate control of goal-directed actions.

Perception is shaped by both the physical properties of stimuli and their contextual presentation, often leading to systematic biases such as the central tendency effect, where perceptual judgments shift toward the average of the stimulus set. This study explored the central tendency bias in vibrotactile perception, an area that has received limited attention while also replicating its well-documented occurrence in color perception to validate previous findings. Using a within-subject design, participants (5 males, 15 females) completed color and vibrotactile discrimination tasks, each consisting of three blocks, which comprised systematically shifted stimulus sets. In an established virtual reality color task, stimuli ranged from yellow–green to blue–green, while in the vibrotactile task, stimuli varied in vibration intensity around a baseline distribution. As predicted, the point of subjective equality shifted toward the mean of the stimulus sets in both tasks, confirming the presence of a central tendency bias. These findings demonstrate that perception of both color and vibrotactile intensity is not determined solely based on the physical properties of the stimulus per se, but it is rather influenced by the distribution of the presented stimuli, underscoring the pervasive role of contextual factors in shaping sensory judgments.

Humans typically combine eye and head movements to visually track and interact with moving objects. If the object’s trajectory can be predicted, humans can shift gaze to the anticipated object’s future location. For instance, when hitting a moving ball, people predictively look at the ball’s bouncing location, allowing them to overcome sensorimotor delays that would otherwise limit visual information uptake from that position. Such delays become pronounced with healthy aging, which might drive a behavioral shift towards stronger reliance on predictive gaze allocation to future positions of interest during visuomotor tasks. To investigate this, we examined gaze and hand movements of older and younger adults who played an iPad-based version of the game ‘Pong’ against an automated opponent. Participants moved a paddle of two different sizes with their finger to intercept a ball moving in two speeds and bouncing off the side walls. We hypothesized that aging would lead to earlier gaze shifts to positions of interest, such as to the final wall bounce location before the ball moved toward the participants’ paddle, and to their own paddle around the moment of interception. As expected, both age groups intercepted fewer balls when using smaller paddles. Contrary to our hypothesis, older participants looked later than younger adults to the final wall bounce. However, they shifted their gaze earlier to their own paddle before interception, especially at the beginning of the experiment. This suggests that aging enhances predictive gaze shifts related to the immediate action. While gaze allocation patterns largely overlap between older and younger adults, aging specifically leads to adaptive eye movements just before interacting with the environment.

People generally look at positions that are important for their current actions, such as objects they intend to grasp. What if there are obstacles on their path to such objects? We asked participants to walk into a room and pour the contents of a cup placed on a table into another cup elsewhere on the table. There were two small obstacles on the floor between the door and the table. There was a third obstacle on the table near the target cup. Participants often looked at the items on the table from the beginning, but, as they approached and entered the room, they often looked at the floor near the obstacles, although there was nothing particularly informative to see there. Thus they primarily relied on peripheral vision and memory of where they had seen obstacles to avoid knocking over the obstacles. As they approached the table, they mainly looked at the object that they intended to grasp and the obstacle near it. We conclude that people mainly look at positions at which they plan to physically interact with the environment, rather than at items that constrain such interactions.

Healthy aging comes with poorer sensory processing and pronounced sensorimotor delays that can impair behavior. Here, we demonstrate that older adults rely on predictive gaze allocation during interception, which enables them to maintain high motor performance. Specifically, older adults made earlier predictive gaze shifts to future interception locations compared with younger adults, while maintaining high interception performance. We suggest that gaze allocation is adaptable in aging to maintain high performance despite age-related sensorimotor limitations.

Tactile sensitivity drops in a moving than static limb due to a combination of central, predictive mechanisms and peripheral effects. This suppression is dynamically modulated during movement, as shown during upper-limb actions, yet little is known about its implication during complex lower-limb movements. We investigated tactile sensitivity during naturalistic kicking by delivering vibrotactile probe stimuli to the balancing and kicking feet at different movement phases. In Experiment 1, participants kicked a suspended ball while tactile sensitivity was probed at movement onset , mid- swing , ball contact , and after -contact. Results revealed distinct modulation patterns in each foot. When transitioning from bipedal to unipedal stance, tactile processing at the balancing foot was particularly suppressed but at the kicking foot it improved, suggesting concurrent modulation across the two legs depending on their motor function. Tactile sensitivity remained rather invariant at other time points, but was strongly suppressed on the kicking foot at the moment of ball contact. The strength of this suppression correlated with kicking speed, which could reflect either stronger predictive control or stronger peripheral processes that mask the vibrotactile probe. To test these, a new set of participants held their foot still while a ball collided with it at high or low speed. Suppression was greater with faster ball contacts, revealing that peripheral processes can modulate tactile processing. These findings show that lower-limb tactile sensitivity during goal-directed leg movements can be concurrently modulated across the legs, presumably reflecting an interplay between central sensorimotor processes guiding the movement and peripheral processes affecting sensitivity. Significance Statement: Tactile sensitivity is known to fluctuate during movement, but little is understood about how it is tuned during complex lower-limb actions. Using a naturalistic ball-kicking task, we reveal distinct modulation patterns in the balancing and kicking feet, showing that postural and guiding demands dynamically shape tactile sensitivity. We further demonstrate that the strength of tactile modulation is influenced by peripheral processes, such as tactile masking. These findings highlight that lower-limb tactile processing can be flexibly and concurrently modulated in the two legs during state transitions that impose different sensorimotor demands for complex natural behavior.