Haptic localizations for onset and offset of vibro-tactile stimuli are dissociated

External location of touch is constructed post-hoc based on limb choice

When humans indicate on which hand a tactile stimulus occurred, they often err when their hands are crossed. This finding seemingly supports the view that the automatically determined touch location in external space affects limb assignment: the crossed right hand is localized in left space, and this conflict presumably provokes hand assignment errors. Here, participants judged on which hand the first of two stimuli, presented during a bimanual movement, had occurred, and then indicated its external location by a reach-to-point movement. When participants incorrectly chose the hand stimulated second, they pointed to where that hand had been at the correct, first time point, though no stimulus had occurred at that location. This behavior suggests that stimulus localization depended on hand assignment, not vice versa. It is, thus, incompatible with the notion of automatic computation of external stimulus location upon occurrence. Instead, humans construct external touch location post-hoc and on demand.

Dynamic Displacement Vector Interacts with Tactile Localization

Locating a tactile stimulus on the body seems effortless and straightforward. However, the perceived location of a tactile stimulation can differ from its physical location [1, 2, 3]. Tactile mislocalizations can depend on the timing of successive stimulations [2, 4, 5], tactile motion mechanisms [6], or processes that “remap” stimuli from skin locations to external space coordinates [7, 8, 9, 10, 11]. We report six experiments demonstrating that the perception of tactile localization on a static body part is strongly affected by the displacement between the locations of two successive task-irrelevant actions. Participants moved their index finger between two keys. Each keypress triggered synchronous tactile stimulation at a randomized location on the immobilized wrist or forehead. Participants reported the location of the second tactile stimulation relative to the first. The direction of either active finger movements or passive finger displacements biased participants’ tactile orientation judgements (experiment 1). The effect generalized to tactile stimuli delivered to other body sites (experiment 2). Two successive keypresses, by different fingers at distinct locations, reproduced the effect (experiment 3). The effect remained even when the hand that moved was placed far from the tactile stimulation site (experiments 4 and 5). Temporal synchrony within 600 ms between the movement and tactile stimulations was necessary for the effect (experiment 6). Our results indicate that a dynamic displacement vector, defined as the location of one sensorimotor event relative to the one before, plays a strong role in structuring tactile spatial perception.

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Human tactile localization is biased by simultaneous finger displacement

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The shift between two successive events biases the relative localization of touches

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Both active and passive movements induce a bias, even if far from the touched site

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The bias effect is vectorially organized

The haptic and the visual flash-lag effect and the role of flash characteristics

When a short flash occurs in spatial alignment with a moving object, the moving object is seen ahead the stationary one. Similar to this visual "flash-lag effect" (FLE) it has been recently observed for the haptic sense that participants judge a moving hand to be ahead a stationary hand when judged at the moment of a short vibration ("haptic flash") that is applied when the two hands are spatially aligned. We further investigated the haptic FLE. First, we compared participants' performance in two isosensory visual or haptic conditions, in which moving object and flash were presented only in a single modality (visual: sphere and short color change, haptic: hand and vibration), and two bisensory conditions, in which the moving object was presented in both modalities (hand aligned with visible sphere), but the flash was presented only visually or only haptically. The experiment aimed to disentangle contributions of the flash's and the objects' modalities to the FLEs in haptics versus vision. We observed a FLE when the flash was visually displayed, both when the moving object was visual and visuo-haptic. Because the position of a visual flash, but not of an analogue haptic flash, is misjudged relative to a same visuo-haptic moving object, the difference between visual and haptic conditions can be fully attributed to characteristics of the flash. The second experiment confirmed that a haptic FLE can be observed depending on flash characteristics: the FLE increases with decreasing intensity of the flash (slightly modulated by flash duration), which had been previously observed for vision. These findings underline the high relevance of flash characteristics in different senses, and thus fit well with the temporal-sampling framework, where the flash triggers a high-level, supra-modal process of position judgement, the time point of which further depends on the processing time of the flash.

Understanding the link between somatosensory temporal discrimination and movement execution in healthy subjects

The somatosensory temporal discrimination threshold (STDT) is the shortest interval at which an individual recognizes paired stimuli as separate in time. We investigated whether and how voluntary movement modulates STDT in healthy subjects. In 17 healthy participants, we tested STDT during voluntary index‐finger abductions at several time‐points after movement onset and during motor preparation. We then tested whether voluntary movement‐induced STDT changes were specific for the body segment moved, depended on movement kinematics, on the type of movement or on the intensity for delivering paired electrical stimuli for STDT. To understand the mechanisms underlying STDT modulation, we also tested STDT during motor imagery and after delivering repetitive transcranial magnetic stimulation to elicit excitability changes in the primary somatosensory cortex (S1). When tested on the moving hand at movement onset and up to 200 msec thereafter, STDT values increased from baseline, but during motor preparation remained unchanged. STDT values changed significantly during fast and slow index‐finger movements and also, though less, during passive index‐finger abductions, whereas during tonic index‐finger abductions they remained unchanged. STDT also remained unchanged when tested in body parts other than those engaged in movement and during imagined movement. Nor did testing STDT at increased intensity influence movement‐induced STDT changes. The cTBS‐induced S1 cortical changes left movement‐induced STDT changes unaffected. Our findings suggest that movement execution in healthy subjects may alter STDT processing.

Afferent motor feedback determines the perceived location of tactile stimuli in the external space presented to the moving arm

People make systematic errors when localizing a brief tactile stimulus in the external space presented on the index finger while moving the arm. Although these errors likely arise in the spatiotemporal integration of the tactile input and information about arm position, the underlying arm position information used in this process is not known. In this study, we tested the contributions of afferent proprioceptive feedback and predictive arm position signals by comparing localization errors during passive vs. active arm movements. In the active trials, participants were instructed to localize a tactile stimulus in the external space that was presented to the index finger near the time of a self-generated arm movement. In the passive trials, each of the active trials was passively replayed in randomized order, using a robotic device. Our results provide evidence that the localization error patterns of the passive trials are similar to the active trials and, moreover, did not lag but rather led the active trials, which suggests that proprioceptive feedback makes an important contribution to tactile localization. To further test which kinematic property of this afferent feedback signal drives the underlying computations, we examined the localization errors with movements that had differently skewed velocity profiles but overall the same displacement. This revealed a difference in the localization patterns, which we explain by a probabilistic model in which temporal uncertainty about the stimulus is converted into a spatial likelihood, depending on the actual velocity of the arm rather than involving an efferent, preprogrammed movement.NEW & NOTEWORTHY We show that proprioceptive feedback of arm motion rather than efferent motor signals contributes to tactile localization during an arm movement. Data further show that localization errors depend on arm velocity, not displacement per se, suggesting that instantaneous velocity feedback plays a role in the underlying computations. Model simulation using Bayesian inference suggests that these errors depend not only on spatial but also on temporal uncertainties of sensory and motor signals.

Auditory-Somatosensory Temporal Sensitivity Improves When the Somatosensory Event Is Caused by Voluntary Body Movement

When we actively interact with the environment, it is crucial that we perceive a precise temporal relationship between our own actions and sensory effects to guide our body movements. Thus, we hypothesized that voluntary movements improve perceptual sensitivity to the temporal disparity between auditory and movement-related somatosensory events compared to when they are delivered passively to sensory receptors. In the voluntary condition, participants voluntarily tapped a button, and a noise burst was presented at various onset asynchronies relative to the button press. The participants made either “sound-first” or “touch-first” responses. We found that the performance of temporal order judgment (TOJ) in the voluntary condition (as indexed by the just noticeable difference (JND)) was significantly better (M = 42.5 ms ± 3.8 SEM) than that when their finger was passively stimulated (passive condition: M = 66.8 ms ± 6.3 SEM). We further examined whether the performance improvement with voluntary action can be attributed to the prediction of the timing of the stimulation from sensory cues (sensory-based prediction), kinesthetic cues contained in voluntary action, and/or to the prediction of stimulation timing from the efference copy of the motor command (motor-based prediction). When three noise bursts were presented before the target burst with regular intervals (predictable condition) and when the participant’s finger was moved passively to press the button (involuntary condition), the TOJ performance was not improved from that in the passive condition. These results suggest that the improvement in sensitivity to temporal disparity between somatosensory and auditory events caused by the voluntary action cannot be attributed to sensory-based prediction and kinesthetic cues. Rather, the prediction from the efference copy of the motor command would be crucial for improving the temporal sensitivity.

Active Manual Movement Improves Directional Perception of Illusory Force

Active touch sensing is known to facilitate the discrimination or recognition of the spatial properties of an object from the movement of tactile sensors on the skin and by integrating proprioceptive feedback about hand positions or motor commands related to ongoing hand movements. On the other hand, several studies have reported that tactile processing is suppressed by hand movement. Thus, it is unclear whether or not the active exploration of force direction by using hand or arm movement improves the perception of the force direction. Here, we show that active manual movement in both the rotational and translational directions enhances the precise perception of the force direction. To make it possible to move a hand in space without any physical constraints, we have adopted a method of inducing the sensation of illusory force by asymmetric vibration. We found that the precision of the perceived force direction was significantly better when the shoulder is rotated medially and laterally. We also found that directional errors supplied by the motor response of the perceived force were smaller than those resulting from perceptual judgments between visual and haptic directional stimuli. These results demonstrate that active manual movement boosts the precision of the perceived direction of an illusory force.

The buzz-lag effect

In the flash-lag illusion, a brief visual flash and a moving object presented at the same location appear to be offset with the flash trailing the moving object. A considerable amount of studies investigated the visual flash-lag effect, and flash-lag-like effects have also been observed in audition, and cross-modally between vision and audition. In the present study, we investigate whether a similar effect can also be observed when using only haptic stimuli. A fast vibration (or buzz, lasting less than 20 ms) was applied to the moving finger of the observers and employed as a "haptic flash." Participants performed a two-alternative forced-choice (2AFC) task where they had to judge whether the moving finger was located to the right or to the left of the stationary finger at the time of the buzz. We used two different movement velocities (Slow and Fast conditions). We found that the moving finger was systematically misperceived to be ahead of the stationary finger when the two were physically aligned. This result can be interpreted as a purely haptic analogue of the flash-lag effect, which we refer to as "buzz-lag effect." The buzz-lag effect can be well accounted for by the temporal-sampling explanation of flash-lag-like effects.

Apparent time interval of visual stimuli is compressed during fast hand movement

The influence of body movements on visual time perception is receiving increased attention. Past studies showed apparent expansion of visual time before and after the execution of hand movements and apparent compression of visual time during the execution of eye movements. Here we examined whether the estimation of sub-second time intervals between visual events is expanded, compressed, or unaffected during the execution of hand movements. The results show that hand movements, at least the fast ones, reduced the apparent time interval between visual events. A control experiment indicated that the apparent time compression was not produced by the participants’ involuntary eye movements during the hand movements. These results, together with earlier findings, suggest hand movement can change apparent visual time either in a compressive way or in an expansive way, depending on the relative timing between the hand movement and visual stimulus.

Motor commands induce time compression for tactile stimuli

Saccades cause compression of visual space around the saccadic target, and also a compression of time, both phenomena thought to be related to the problem of maintaining saccadic stability (Morrone et al., 2005; Burr and Morrone, 2011). Interestingly, similar phenomena occur at the time of hand movements, when tactile stimuli are systematically mislocalized in the direction of the movement (Dassonville, 1995; Watanabe et al., 2009). In this study, we measured whether hand movements also cause an alteration of the perceived timing of tactile signals. Human participants compared the temporal separation between two pairs of tactile taps while moving their right hand in response to an auditory cue. The first pair of tactile taps was presented at variable times with respect to movement with a fixed onset asynchrony of 150 ms. Two seconds after test presentation, when the hand was stationary, the second pair of taps was delivered with a variable temporal separation. Tactile stimuli could be delivered to either the right moving or left stationary hand. When the tactile stimuli were presented to the motor effector just before and during movement, their perceived temporal separation was reduced. The time compression was effector-specific, as perceived time was veridical for the left stationary hand. The results indicate that time intervals are compressed around the time of hand movements. As for vision, the mislocalizations of time and space for touch stimuli may be consequences of a mechanism attempting to achieve perceptual stability during tactile exploration of objects, suggesting common strategies within different sensorimotor systems.

Spatiotemporal integration for tactile localization during arm movements: a probabilistic approach

It has been shown that people make systematic errors in the localization of a brief tactile stimulus that is delivered to the index finger while they are making an arm movement. Here we modeled these spatial errors with a probabilistic approach, assuming that they follow from temporal uncertainty about the occurrence of the stimulus. In the model, this temporal uncertainty converts into a spatial likelihood about the external stimulus location, depending on arm velocity. We tested the prediction of the model that the localization errors depend on arm velocity. Participants (n = 8) were instructed to localize a tactile stimulus that was presented to their index finger while they were making either slow- or fast-targeted arm movements. Our results confirm the model's prediction that participants make larger localization errors when making faster arm movements. The model, which was used to fit the errors for both slow and fast arm movements simultaneously, accounted very well for all the characteristics of these data with temporal uncertainty in stimulus processing as the only free parameter. We conclude that spatial errors in dynamic tactile perception stem from the temporal precision with which tactile inputs are processed.

Predicting future sensorimotor states influences current temporal decision making

Accurate motor execution is achieved by estimating future sensory states via a forward model of limb dynamics. In the current experiment, we probed the time course over which state estimation evolves during movement planning by combining a bimanual arm crossing movement with a temporal order judgment (TOJ) task. Human participants judged which of two successive vibrotactile stimuli delivered to each index finger arrived first as they were preparing to either cross or uncross their hands. TOJ error rate was found to systematically vary in a time- and direction-dependent manner. When planning to cross the hands, error rate systematically increased as the vibrotactile stimuli were delivered closer in time to the onset of the movement. By contrast, planning to uncross the hands led to a gradual reduction in error rate as movement planning progressed. In both cases, these changes occurred before the actual alteration in hand configuration. We suggest that these systematic changes in error represent an interaction between the evolving state estimation processes and decisions regarding the timing of successive events.

Misjudging where you felt a light switch in a dark room

Previous research has shown that subjects systematically misperceive the location of visual and haptic stimuli presented briefly around the time of a movement of the sensory organ (eye or hand movements) due to errors in the combination of visual or tactile information with proprioception. These briefly presented stimuli (a flash or a tap on the finger) are quite different from what one encounters in daily life. In this study, we tested whether subjects also mislocalize real (static) objects that are felt briefly while moving ones hand across them, like when searching for a light switch in the dark. We found that subjects systematically mislocalized a real bar in a similar manner as has been shown with artificial haptic stimuli. This demonstrates that movement-related mislocalization is a real world property of human perception.