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.

Active Braking of Whole-Arm Reaching Movements Provides Single-Trial Neuromuscular Measures of Movement Cancellation.

Movement inhibition is an aspect of executive control that can be studied using the countermanding paradigm, wherein subjects try to cancel an impending movement following presentation of a stop signal. This paradigm permits estimation of the stop-signal reaction time or the time needed to respond to the stop signal. Numerous countermanding studies have examined fast, ballistic movements, such as saccades, even though many movements in daily life are not ballistic and can be stopped at any point during their trajectory. A benefit of studying the control of nonballistic movements is that antagonist muscle recruitment, which serves to actively brake a movement, presumably arises in response to the stop signal. Here, nine human participants (2 female) performed a center-out whole-arm reaching task with a countermanding component, while we recorded the activity of upper-limb muscles contributing to movement generation and braking. The data show a clear response on antagonist muscles to a stop signal, even for movements that have barely begun. As predicted, the timing of such antagonist recruitment relative to the stop signal covaried with conventional estimates of the stop-signal reaction time, both within and across subjects. The timing of antagonist muscle recruitment also attested to a rapid reprioritization of movement inhibition, with antagonist latencies decreasing across sequences consisting of repeated stop trials; such reprioritization also scaled with error magnitude. We conclude that antagonist muscle recruitment arises as a manifestation of a stopping process, providing a novel, accessible, and within-trial measure of the stop-signal reaction time.SIGNIFICANCE STATEMENT The countermanding or stop-signal paradigm permits estimation of how quickly subjects cancel an impending movement. Traditionally, this paradigm has been studied using simple movements, such as saccadic eye movements or button presses. Here, by measuring upper limb muscle activity while human subjects countermand whole-arm reaching movements, we show that movement cancellation often involves prominent recruitment of antagonist muscles that serves to actively brake the movement, even on movements that have barely begun. The timing of antagonist muscle recruitment correlates with traditional estimates of movement cancellation. Because they can be detected on a single-trial basis, muscle-based measures may provide a new way of characterizing movement cancellation at an unprecedented within-trial resolution.

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.

Causal Inference for Spatial Constancy across Saccades.

Our ability to interact with the environment hinges on creating a stable visual world despite the continuous changes in retinal input. To achieve visual stability, the brain must distinguish the retinal image shifts caused by eye movements and shifts due to movements of the visual scene. This process appears not to be flawless: during saccades, we often fail to detect whether visual objects remain stable or move, which is called saccadic suppression of displacement (SSD). How does the brain evaluate the memorized information of the presaccadic scene and the actual visual feedback of the postsaccadic visual scene in the computations for visual stability? Using a SSD task, we test how participants localize the presaccadic position of the fixation target, the saccade target or a peripheral non-foveated target that was displaced parallel or orthogonal during a horizontal saccade, and subsequently viewed for three different durations. Results showed different localization errors of the three targets, depending on the viewing time of the postsaccadic stimulus and its spatial separation from the presaccadic location. We modeled the data through a Bayesian causal inference mechanism, in which at the trial level an optimal mixing of two possible strategies, integration vs. separation of the presaccadic memory and the postsaccadic sensory signals, is applied. Fits of this model generally outperformed other plausible decision strategies for producing SSD. Our findings suggest that humans exploit a Bayesian inference process with two causal structures to mediate visual stability.

No perisaccadic mislocalization with abruptly cancelled saccades.

Every saccadic eye movement that we make changes the image of the world on our retina. Yet, despite these retinal shifts, we still perceive our visual world to be stable. Efference copy from the oculomotor system to the visual system has been suggested to contribute to this stable percept, enabling the brain to anticipate the retinal image shifts by remapping the neural image. A psychophysical phenomenon that has been linked to this predictive remapping is the mislocalization of a stimulus flashed around the time of a saccade. If this mislocalization is initiated by saccade preparation, one should also observe localization errors when a saccade is planned, but abruptly aborted just before its execution. We tested this hypothesis in human subjects using a novel paradigm that combines a flash localization task with a countermanding component that occasionally requires saccade cancellation. Surprisingly, we found no trace of mislocalization, even for saccades cancelled close to the point of no return. This strongly suggests that the actual execution of the saccade is a prerequisite for the typical localization errors, which rejects various models and constrains neural substrates. We conclude that perisaccadic mislocalization is not a direct consequence of saccade preparation, but arises after saccade execution when the flash location is constructed from memory.

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.

Luminance contrast in the background makes flashes harder to detect during saccades.

To explore a visual scene we make many fast eye movements (saccades) every second. During those saccades the image of the world shifts rapidly across our retina. These shifts are normally not detected, because perception is suppressed during saccades. In this paper we study the origin of this saccadic suppression by examining the influence of luminance borders in the background on the perception of flashes presented near the time of saccades in a normally illuminated room. We used different types of backgrounds: either with isoluminant red and green areas or with black and white areas. We found that the ability to perceive flashes that were presented during saccades was suppressed when there were luminance borders in the background, but not when there were isoluminant color borders in the background. Thus, masking by moving luminance borders plays an important role in saccadic suppression. The perceived positions of detected flashes were only influenced by the borders between the areas in the background when the flashes were presented before or after the saccades. Moreover, the influence did not depend on the kind of contrast forming the border. Thus, the masking effect of moving luminance borders does not appear to play an important role in the mislocalization of flashes that are presented near the time of saccades.

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.

Temporal uncertainty separates flashes from their background during saccades.

It is known that spatial localization of flashed objects fails around the time of rapid eye movements (saccades). This mislocalization is often interpreted in terms of a combination of shifts and deformations of the brain's representation of space to account for the eye movement. Such temporary remapping of positions in space should affect all elements in a scene, leaving ordinal relationships between positions intact. We performed an experiment in which we presented flashes on a background with red and green regions to human subjects. We found that flashes that were presented on the green part of the background around the time of a saccade were readily reported to have been presented on the red part of the background and vice versa. This is inconsistent with the notion of a temporary shift and deformation of perceived space. To explain our results, we present a model that illustrates how temporal uncertainty could give rise to the observed spatial mislocalization. The model combines uncertainty about the time of the flash with a bias to localize targets where one is looking. It reproduced the pattern of mislocalization very accurately, showing that perisaccadic mislocalization can best be explained in terms of temporal uncertainty about the moment of the flash.

Peri-saccadic mislocalization is not influenced by the predictability of the saccade target location.

Flashes presented around the time of a saccade are often mislocalized. The precise pattern of mislocalization is influenced by many factors. Here we study one such factor: the predictability of the saccade target's location. The experiment examines two conditions. In the first the subject makes the same horizontal rightward saccade to the same target location over and over again. In the second the subject makes saccades to a target that is jumping in unpredictable radial directions. A dot is flashed in the vicinity of the saccade target near the time of saccade onset. Subjects are asked to localize the flash by touching its location on the screen. Although various saccade parameters differed, the errors that subjects made were very similar in both conditions. We conclude that the pattern of mislocalization does not depend on the predictability of the location of the saccade target.

The use of the saccade target as a visual reference when localizing flashes during saccades.

Flashes presented around the time of a saccade are often mislocalized. Such mislocalization is influenced by various factors. Here, we evaluate the role of the saccade target as a landmark when localizing flashes. The experiment was performed in a normally illuminated room to provide ample other visual references. Subjects were instructed to follow a randomly jumping target with their eyes. We flashed a black dot on the screen around the time of saccade onset. The subjects were asked to localize the black dot by touching the appropriate location on the screen. In a first experiment, the saccade target was displaced during the saccade. In a second experiment, it disappeared at different moments. Both manipulations affected the mislocalization. We conclude that our subjects' judgments are partly based on the flashed dot's position relative to the saccade target.

Corticospinal beta-band synchronization entails rhythmic gain modulation.

Rhythmic synchronization of neurons in the beta or gamma band occurs almost ubiquitously, and this synchronization has been linked to numerous nervous system functions. Many respective studies make the implicit assumption that neuronal synchronization affects neuronal interactions. Indeed, when neurons synchronize, their output spikes reach postsynaptic neurons together, trigger coincidence detection mechanisms, and therefore have an enhanced impact. There is ample experimental evidence demonstrating this consequence of neuronal synchronization, but beyond this, beta/gamma-band synchronization within a group of neurons might also modulate the impact of synaptic input to that synchronized group. This would constitute a separate mechanism through which synchronization affects neuronal interactions, but direct in vivo evidence for this putative mechanism is lacking. Here, we demonstrate that synchronized beta-band activity of a neuronal group modulates the efficacy of synaptic input to that group in-phase with the beta rhythm. This response modulation was not an addition of rhythmic activity onto the average response but a rhythmic modulation of multiplicative input gain. Our results demonstrate that beta-rhythmic activity of a neuronal target group multiplexes input gain along the rhythm cycle. The actual gain of an input then depends on the precision and the phase of its rhythmic synchronization to this target, providing one mechanistic explanation for why synchronization modulates interactions.

Temporal information can influence spatial localization.

To localize objects relative to ourselves, we need to combine various sensory and motor signals. When these signals change abruptly, as information about eye orientation does during saccades, small differences in latency between the signals could introduce localization errors. We examine whether independent temporal information can influence such errors. We asked participants to follow a randomly jumping dot with their eyes and to point at flashes that occurred near the time they made saccades. Such flashes are mislocalized. We presented a tone at different times relative to the flash. We found that the flash was mislocalized as if it had occurred closer in time to the tone. This demonstrates that temporal information is taken into consideration when combining sensory information streams for localization.