The influences of target size and recent experience on the vigour of adjustments to ongoing movements

Reach Corrections Toward Moving Objects are Faster Than Reach Corrections Toward Instantaneously Switching Targets

Visually guided reaching is a common motor behavior that engages subcortical circuits to mediate rapid corrections. Although these neural mechanisms have evolved for interacting with the physical world, they are often studied in the context of reaching toward virtual targets on a screen. These targets often change position by disappearing from one place reappearing in another instantaneously. In this study, we instructed participants to perform rapid reaches to physical objects that changed position in different ways. In one condition, the objects moved very rapidly from one place to another. In the other condition, illuminated targets instantaneously switched position by being extinguished in one position and illuminating in another. Participants were consistently faster in correcting their reach trajectories when the object moved continuously.

Slightly perturbing the arm influences choices between multiple targets

We constantly make choices about how to interact with objects in the environment. Do we immediately consider changes in our posture when making such choices? To find out, we examined whether motion in the background, which is known to influence the trajectory of goal-directed hand movements, influences participants' choices when suddenly faced with two options. The participants' task was to tap on as many sequentially presented targets as possible within 90 seconds. Sometime after a new target appeared, it split into two targets and participants had to choose which of them to hit. Shortly before the split, the background moved in a way that was expected to result in the finger shifting slightly towards one of the two new targets. We examined whether such shifts influenced the choice between the two targets. The moving background influenced the finger movements in the expected manner: participants moved in the direction of the background motion. It also influenced the choice that participants made between the two targets: participants more frequently chose the target in the direction of the background motion. There was a positive correlation across participants between the magnitude of the response to background motion and the bias to choose the target in the direction of such motion. Thus, people consider sudden changes in their posture when choosing between different movement options.

Continuous use of visual information about the position of the moving hand

People generally look at a target when they want to reach for it. Doing so presumably helps them continuously update their judgments about the target's position and motion. But not looking at their hand does not prevent people from updating judgments about its position on the basis of visual information, because people do respond to experimental perturbations of visual information about the position of their hand. Here, we study such responses by adding jitter to the movement of a cursor that follows participants' fingers. We analyse the response to the jitter in a way that reveals how the vigour of the response depends on the moment during the movement at which the change in cursor position occurs. We compare the change in vigour to that for equivalent jitter in the position of the target. We find that participants respond to jitter in the position of a cursor in much the same way as they respond to jitter in the target's position. The responses are more vigorous late in the movement, when adjustments need to be made within less time, but similarly so for the cursor as for the target. The responses are weaker for the cursor, presumably because of the jitter-free kinaesthetic information about the position of the finger.

Tapping on a target: dealing with uncertainty about its position and motion

Reaching movements are guided by estimates of the target object's location. Since the precision of instantaneous estimates is limited, one might accumulate visual information over time. However, if the object is not stationary, accumulating information can bias the estimate. How do people deal with this trade-off between improving precision and reducing the bias? To find out, we asked participants to tap on targets. The targets were stationary or moving, with jitter added to their positions. By analysing the response to the jitter, we show that people continuously use the latest available information about the target's position. When the target is moving, they combine this instantaneous target position with an extrapolation based on the target's average velocity during the last several hundred milliseconds. This strategy leads to a bias if the target's velocity changes systematically. Having people tap on accelerating targets showed that the bias that results from ignoring systematic changes in velocity is removed by compensating for endpoint errors if such errors are consistent across trials. We conclude that combining simple continuous updating of visual information with the low-pass filter characteristics of muscles, and adjusting movements to compensate for errors made in previous trials, leads to the precise and accurate human goal-directed movements.

How similar are responses to background motion and target displacements?

When making a goal-directed movement towards a target, our hand follows abrupt background motion. This response resembles that of a shift in the target’s position. Does background motion simply change the position towards which the movement is guided? If so, the response to background motion should resemble the response to a target displacement. To find out whether this is the case, we ran two exploratory studies where we asked participants to hit a moving target at a specified moment. At various times during the hand’s movement, the background could move briefly at one of several speeds, and for various durations. The response to abrupt background motion was larger when the background moved later in the movement and when the background moved faster, in line with known responses to target displacements. The response to a second epoch of background motion was smaller than it would have been if there had been no first epoch, in contrast to responses to multiple target displacements. If the background was already moving before the target appeared, the hand even moved in the opposite direction. Thus, the response to background motion and that to a target displacement are clearly not identical, but they do share several features.