Welber Marinovic

Corticospinal modulation induced by sounds depends on action preparedness

•  Unexpected loud auditory stimuli can trigger the involuntary release of motor actions during preparation to move. •  Because acoustic stimulation can suppress motor cortex excitability during action, this early release could be independent of the motor cortex, and interpreted as pre‐planned action stored and triggered from subcortical areas. •  In contrast, we show that corticospinal excitability in response to the loud auditory stimuli was increased when people were highly prepared to move, and reduced otherwise. •  Our results also indicate that auditory stimuli can affect intracortical excitability by increasing intracortical facilitation and reducing short‐interval intracortical inhibition. •  Together, our findings demonstrate that the early release of motor responses by auditory stimuli involves the motor cortex.

The Utilisation of Visual Information in the Control of Rapid Interceptive Actions

When intercepting a moving target, accurate timing depends, in part, upon starting to move at the right moment. It is generally believed that this is achieved by triggering motor command generation when a visually perceived quantity such as the targets time-to-arrival reaches a specific criterion value. An experimental method that could be used to determine the moment when this visual event happens was introduced by Whiting and coworkers in the 1970s, and it involves occluding the vision of the target at different times prior to the time of movement onset (MO). This method is limited because the experimenter has no control over MO time. We suggest a method which provides the needed control by having people make interceptive movements of a specific duration. We tested the efficacy of this method in two experiments in which the accuracy of interception was examined under different occlusion conditions. In the first experiment, we examined the effect of changing the timing of an occlusion period (OP) of fixed duration (200 ms). In the second experiment, we varied the duration of the OP (180-430 ms) as well as its timing. The results demonstrated the utility of the proposed method and showed that performance deteriorated only when the participants had their vision occluded from 200 ms prior to MO. The results of Experiment 2 were able to narrow down the critical interval to trigger the interceptive action to within the period from 200 to 150 ms prior to MO, probably closer to 150 ms. In addition, the results showed that the execution of brief interceptive movements (180 ms) was not affected by the range of OPs used in the experiments. This indicates that the whole movement was prepared in advance and triggered by a visual stimulus event that occurred at about 150 ms before onset.

Manual interception of moving targets in two dimensions: performance and space-time accuracy.

We report results from four experiments that examined performance of an interceptive task that restricted movement of the hand and moving target to a horizontal plane. The task required accurate control over both where and when interception takes place. Three experiments studied the effects of four independent variables: target speed, target size, manipulandum size and movement amplitude. For small amplitude movements, small, fast targets were hit harder than larger slower ones and targets were hit harder with smaller manipulanda; movement time (MT) was unaffected by target size, but was shorter when the manipulandum was smaller. For larger amplitude movements, smaller, faster targets were also hit harder, but MTs tended to be greater when targets were smaller. The results support the idea that MT and peak movement speed can be independently controlled to some degree in order to meet the accuracy demands of the task. Analysis of the task showed that spatial and temporal accuracy demands are interdependent, indicating that the spatial and temporal variable errors should covary such that increases in one are accompanied by decreases in the other. This can be tested if there is no variation in interception location; which was not the case in the first three experiments. In a final experiment variation in interception location was restricted by requiring that the target be struck through an aperture. Both spatial and temporal variable errors could be estimated. As predicted, it was found that when spatial errors were small, temporal errors were large.

Neural prediction of complex accelerations for object interception

To intercept or avoid moving objects successfully, we must compensate for the sensorimotor delays associated with visual processing and motor movement. Although straightforward in the case of constant velocity motion, it is unclear how humans compensate for accelerations, as our visual system is relatively poor at detecting changes in velocity. Work on free-falling objects suggests that we are able to predict the effects of gravity, but this represents the most simple, limiting case in which acceleration is constant and motion linear. Here, we show that an internal model also predicts the effects of complex, varying accelerations when they result from lawful interactions with the environment. Participants timed their responses with the arrival of a ball rolling within a tube of various shapes. The pattern of errors indicates that participants were able to compensate for most of the effects of the ball acceleration (∼85%) within a relatively short practice (∼300 trials). Errors on catch trials in which the ball velocity was unexpectedly maintained constant further confirmed that participants were expecting the effect of acceleration induced by the shape of the tube. A similar effect was obtained when the visual scene was projected upside down, indicating that the mechanism of this prediction is flexible and not confined to ecologically valid interactions. These findings demonstrate that the brain is able to predict motion on the basis of prior experience of complex interactions between an object and its environment.

Responses to loud auditory stimuli indicate that movement-related activation builds up in anticipation of action

Previous research using a loud acoustic stimulus (LAS) to investigate motor preparation in reaction time (RT) tasks indicates that responses can be triggered well in advance of the presentation of an imperative stimulus (IS). This is intriguing given that high levels of response preparation cannot be maintained for long periods (≈ 200 ms). In the experiments reported here we sought to assess whether response-related activation increases gradually over time in simple RT tasks. In experiment 1, a LAS was presented at different times just prior to the presentation of the IS to probe the level of activation for the motor response. In experiment 2, the same LAS was presented at different times after the presentation of the IS. The results provide evidence that response-related activation does increase gradually in anticipation of the IS, but it remains stable for a short time after this event. The data display a pattern consistent with the response being triggering by the LAS, rather than a reaction to the IS.

Corticospinal excitability during preparation for an anticipatory action is modulated by the availability of visual information

To intercept rapidly moving objects, people must predict the right time to initiate their actions. The timing of movement initiation in interceptions is thought to be determined when a perceptual variable specifying time to contact reaches a criterion value. If a response needs to be aborted, the performer must make a decision before this moment. It has been recently shown that the minimal time to suppress an anticipatory action takes longer during motion extrapolation than during continuous visual information. In experiment 1, we sought to determine whether or not the availability of visual information would 1) affect the latency to inhibit an anticipatory action, and 2) modulate the level of excitability in the motor cortex (M1). The behavioral results showed that the absence of visual information prolonged the latency to stop the movement as previously reported. The neurophysiological data indicated that corticospinal excitability levels were affected by the availability of visual information. In experiment 2, we sought to verify whether corticospinal excitability levels would also differ between the two visual conditions when the task did not involve response suppression. The results of experiment 2 indicated that excitability levels did not differ between visual conditions. Overall, our findings indicated that the buildup of motor activation can also play a role in determining different latencies to inhibit an anticipatory action. They also suggest that the buildup of motor activation in the corticospinal pathways can be strategically modulated to the requirements of the task during continuous visual information.

The time course of direction specification in brief interceptive actions

In fastball sports such as baseball and tennis people are required to produce accurate responses following brief observations of the ball. This limits the time available to prepare the movement. To cope with constrained viewing periods which precede the interception of fast approaching balls, performers are likely to prepare their responses in advance. Although motor preparation may begin before the moving object is seen, accuracy requires that certain program parameters are determined from observations of the target. The aim of the experiment reported here was to determine the last moment at which information about the direction of the target can be incorporated into a motor program. The empirical protocol used in this study allowed us to examine whether new direction information is incorporated discretely or continuously into the program during short intervals prior to movement onset (MO) - the preparation interval. Participants were trained to hit moving targets at two directions with movements of a specific duration (180 ms). This method permitted an estimate of MO. Preparation intervals were controlled by issuing a stimulus cue for movement direction at various times prior to the estimated MO. Results showed that direction information could be fully incorporated into the program with a preparation interval as brief as 250 ms. In addition, the results indicated that direction was specified predominantly in a discrete fashion even at short preparation intervals.

Control of striking velocity by table tennis players.

This study investigated how 7 skilled table tennis players controlled velocity of a forehand drive stroke when the balls trajectory, velocity, and spin were modified. They hit a target in response to balls launched under four different conditions. The relative and absolute times used in the backswing phase showed no significant differences among conditions. When subjects hit fastballs, there was a significant change in the time required for them to reach the peak of velocity in the forward swing phase. In addition, players decreased the velocity of their strokes to hit fast-approaching balls. These results indicate that highly skilled table tennis players need to adjust the striking velocity and striking time (relative and absolute) required to reach the peak of velocity in the forward swing phase for these task modifications. Since they used slower movement velocities to hit faster-approaching balls, skilled table tennis players may override this speed-coupling process.

Triggering prepared actions by sudden sounds: reassessing the evidence for a single mechanism

Loud acoustic stimuli can unintentionally elicit volitional acts when a person is in a state of readiness to execute them (the StartReact effect). It has been assumed that the same subcortical pathways and brain regions underlie all instances of the StartReact effect. They are proposed to involve the startle reflex pathways, and the eliciting mechanism is distinct from other ways in which sound can affect the motor system. We present an integrative review which shows that there is no evidence to support these assumptions. We argue that motor command generation for learned, volitional orofacial, laryngeal and distal limb movements is cortical and the StartReact effect for such movements involves transcortical pathways. In contrast, command generation for saccades, locomotor corrections and postural adjustments is subcortical and subcortical pathways are implicated in the StartReact effect for these cases. We conclude that the StartReact effect is not a special phenomenon mediated by startle reflex pathways, but rather is a particular manifestation of the excitatory effects of intense stimulation on the central nervous system.

Separable temporal metrics for time perception and anticipatory actions

Reliable estimates of time are essential for initiating interceptive actions at the right moment. However, our sense of time is surprisingly fallible. For instance, time perception can be distorted by prolonged exposure (adaptation) to movement. Here, we make use of this to determine if time perception and anticipatory actions rely on the same or on different temporal metrics. Consistent with previous reports, we find that the apparent duration of movement is mitigated by adaptation to more rapid motion, but is unchanged by adaptation to slower movement. By contrast, we find symmetrical effects of motion-adaptation on the timing of anticipatory interceptive actions, which are paralleled by changes in perceived speed for the adapted direction of motion. Our data thus reveal that anticipatory actions and perceived duration rely on different temporal metrics.