Running together influences where you look.

To read this article, you have to constantly direct your gaze at the words on the page. If you go for a run instead, your gaze will be less constrained, so many factors could influence where you look. We show that you are likely to spend less time looking at the path just in front of you when running alone than when running with someone else, presumably because the presence of the other runner makes foot placement more critical.

Methods matter: Exploring how expectations influence common actions.

Behavior in controlled laboratory studies is not always representative of what people do in daily life. This has prompted a recent shift toward conducting studies in natural settings. We wondered whether expectations raised by how the task is presented should also be considered. To find out, we studied gaze when walking down and up a staircase. Gaze was often directed at steps before stepping on them, but most participants did not look at every step. Importantly, participants fixated more steps and looked around less when asked to navigate the staircase than when navigating the same staircase but asked to walk outside. Presumably, expecting the staircase to be important made participants direct their gaze at more steps, despite the identical requirements when on the staircase. This illustrates that behavior can be influenced by expectations, such as expectations resulting from task instructions, even when studies are conducted in natural settings.

Spatial contextual cues that help predict how a target will accelerate can be used to guide interception.

Objects in one's environment do not always move at a constant velocity but often accelerate or decelerate. People are very poor at visually judging acceleration and normally make systematic errors when trying to intercept accelerating objects. If the acceleration is perpendicular to the direction of motion, it gives rise to a curved path. Can spatial contextual cues help one predict such accelerations and thereby help interception? To answer this question, we asked participants to hit a target that moved as if it were attached to a rolling disk, like a valve (target) on a bicycle wheel (disk) moves when cycling: constantly accelerating toward the wheel's center. On half the trials, the disk was visible such that participants could use the spatial relations between the target and the rolling disk to guide their interception. On the other half, the disk was not visible, so participants had no help in predicting the target's complicated pattern of accelerations and decelerations. Importantly, the target's path was the same in both cases. Participants hit more targets when the disk was visible than when it was invisible, even when using a strategy that can compensate for neglecting acceleration. We conclude that spatial contextual cues that help predict the target's accelerations can help intercept it.

Effect of the number and diversity of visual stimuli on the reproduction of short time intervals.

Presenting more items within a space makes the space look and feel bigger. Presenting more tones within a time interval makes the interval seem longer. Does presenting more visual items also make a time interval seem longer? Does it matter what these items are? A series of 2-4 images were presented sequentially on a screen. Participants had to press the spacebar to indicate either the interval between the first and the last item or the intervals between all items. The first and last items were red squares with onset asynchronies of 700, 900, or 1,100 ms. We found that the times between key presses were longer when additional items had different shapes and colors than when they were also red squares. With only red squares, the time may even decrease with the number of items. Whether one had to tap for all targets or only the first and the last hardly mattered.

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.

Online updating of obstacle positions when intercepting a virtual target.

People rely upon sensory information in the environment to guide their actions. Ongoing goal-directed arm movements are constantly adjusted to the latest estimate of both the target and hand's positions. Does the continuous guidance of ongoing arm movements also consider the latest visual information of the position of obstacles in the surrounding? To find out, we asked participants to slide their finger across a screen to intercept a laterally moving virtual target while moving through a gap that was created by two virtual circular obstacles. At a fixed time during each trial, the target suddenly jumped slightly laterally while still continuing to move. In half the trials, the size of the gap changed at the same moment as the target jumped. As expected, participants adjusted their movements in response to the target jump. Importantly, the magnitude of this response depended on the new size of the gap. If participants were told that the circles were irrelevant, changing the gap between them had no effect on the responses. This shows that obstacles' instantaneous positions can be considered when visually guiding goal-directed movements.

The cost of aiming for the best answers: Inconsistent perception.

The laws of physics and mathematics describe the world we live in as internally consistent. As these rules provide a very effective description, and our interaction with the world is also very effective, it seems self-evident that our perception follows these laws. As a result, when trying to explain imperfections in perception, we tend to impose consistency and introduce concepts such as deformations of visual space. In this review, we provide numerous examples that show that in many situations we perceive related attributes to have inconsistent values. We discuss how our tendency to assume consistency leads to erroneous conclusions on how we process sensory information. We propose that perception is not about creating a consistent internal representation of the outside world, but about answering specific questions about the outside world. As the information used to answer a question is specific for that question, this naturally leads to inconsistencies in perception and to an apparent dissociation between some perceptual judgments and related actions.

How the timing of visual feedback influences goal-directed arm movements: delays and presentation rates.

Visual feedback normally helps guide movements to their goal. When moving one's hand, such guidance has to deal with a sensorimotor delay of about 100 ms. When moving a cursor, it also has to deal with a delay of tens of milliseconds that arises between the hand moving the mouse and the cursor moving on the screen. Moreover, the cursor is presented at a certain rate, so only positions corresponding with the position of the mouse at certain moments are presented. How does the additional delay and the rate at which cursor positions are updated influence how well the cursor can be guided to the goal? We asked participants to move a cursor to consecutive targets as quickly as they could. They did so for various additional delays and presentation rates. It took longer for the mouse to reach the target when the additional delay was longer. It also took longer when a lower presentation rate was achieved by not presenting the cursor all the time. The fraction of the time during which the cursor was present was more important than the rate at which the cursor's position was updated. We conclude that the way human arm movements are guided benefits from continuous access to recent visual feedback.

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.

Where do people look when walking up and down familiar staircases?

Many activities in daily life do not impose strict requirements on gaze. We investigated gaze when walking up and down staircases within one's own house. We anticipated that using a variety of staircases in different environments and not informing participants that stair climbing was the focus of investigation might provide a description of gaze behavior that is closer to that used in our daily life than doing so under circumstances in which the focus is explicitly and exclusively directed at the stairs. We analyzed several measures, including the order in which participants fixated the steps. We confirmed that people often look at the steps sequentially, but found that they often made fixations back to steps they had already fixated. They also regularly skipped looking at several steps to fixate further ahead. On average, they directed their gaze at about half the steps. They looked further ahead when ascending than when descending staircases. Overall, the results are similar to those found under highly constrained laboratory conditions, although we do report some differences. One such difference is a tendency to fixate fewer steps. Another is that participants fixated steps that were less far ahead when descending staircases. We also introduced some new analyses that may help understand gaze behavior during stair climbing.

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.

Eye Tracking to Assess the Functional Consequences of Vision Impairment: A Systematic Review.

BACKGROUND: Eye tracking is a promising method for objectively assessing functional visual capabilities, but its suitability remains unclear when assessing the vision of people with vision impairment. In particular, accurate eye tracking typically relies on a stable and reliable image of the pupil and cornea, which may be compromised by abnormalities associated with vision impairment (e.g., nystagmus, aniridia). OBJECTIVES: This study aimed to establish the degree to which video-based eye tracking can be used to assess visual function in the presence of vision impairment. DATA SOURCES: A systematic review was conducted using PubMed, EMBASE, and Web of Science databases, encompassing literature from inception to July 2022. STUDY ELIGIBILITY CRITERIA, PARTICIPANTS, AND INTERVENTIONS: Studies included in the review used video-based eye tracking, included individuals with vision impairment, and used screen-based tasks unrelated to practiced skills such as reading or driving. STUDY APPRAISAL AND SYNTHESIS METHODS: The included studies were assessed for quality using the Strengthening the Reporting of Observational Studies in Epidemiology assessment tool. Data extraction and synthesis were performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. RESULTS: Our analysis revealed that five common tests of visual function were used: (i) fixation stability, (ii) smooth pursuit, (iii) saccades, (iv) free viewing, and (v) visual search. The studies reported considerable success when testing individuals with vision impairment, yielding usable data from 96.5% of participants. LIMITATIONS: There was an overrepresentation of conditions affecting the optic nerve or macula and an underrepresentation of conditions affecting the anterior segment or peripheral retina. CONCLUSIONS AND IMPLICATIONS OF KEY FINDINGS: The results offer promise for the use of eye tracking to assess the visual function of a considerable proportion of those with vision impairment. Based on the findings, we outline a framework for how eye tracking can be used to test visual function in the presence of vision impairment.

Pursuing a target with one's eyes helps judge its velocity.

When intercepting moving targets, people perform slightly better if they follow their natural tendency to pursue the target with their eyes. Is this because the velocity is judged more precisely when pursuing the target? To find out, we compared how well people could determine which of two sequentially presented moving bars was moving faster. There was always also a static bar on the screen. People judged the moving bar's velocity about 10% more precisely when pursuing it than when fixating the static bar.

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.

Having several options does not increase the time it takes to make a movement to an adequate end point.

Throughout the day, people constantly make choices such as where to direct their gaze or place their foot. When making such movement choices, there are usually multiple acceptable options, although some are more advantageous than others. How much time does it take to make such choices and to what extent is the most advantageous option chosen from the available alternatives? To find out, we asked participants to collect points by tapping on any of several targets with their index finger. It did not take participants more time to direct their movements to an advantageous target when there were more options. Participants chose targets that were advantageous because they were easier to reach. Targets could be easier to reach because the finger was already moving in their direction when Amsterdam they appeared, or because they were larger or oriented along the movement direction so that the finger could move faster towards them without missing them. When the target's colour indicated that it was worth more points they chose it slightly less fast, presumably because it generally takes longer to respond to colour than to respond to attributes such as size. They also chose it less often than they probably should have, presumably because the advantage of choosing it was established arbitrarily. We conclude that having many options does not increase the time it takes to move to an adequate target.

Familiarity with an Object's Size Influences the Perceived Size of Its Image.

It is known that judgments about objects' distances are influenced by familiar size: a soccer ball looks farther away than a tennis ball if their images are equally large on the retina. We here investigate whether familiar size also influences judgments about the size of images of objects that are presented side-by-side on a computer screen. Sixty-three participants indicated which of two images appeared larger on the screen in a 2-alternative forced-choice discrimination task. The objects were either two different types of balls, two different types of coins, or a ball and a grey disk. We found that the type of ball biased the comparison between their image sizes: the size of the image of the soccer ball was over-estimated by about 5% (assimilation). The bias in the comparison between the two balls was equal to the sum of the biases in the comparisons with the grey disk. The bias for the coins was smaller and in the opposite direction (contrast). The average precision of the size comparison was 3.5%, irrespective of the type of object. We conclude that knowing a depicted object's real size can influence the perceived size of its image, but the perceived size is not always attracted towards the familiar size.

Hand movements respond to any motion near the endpoint.

Hand movements are pulled in the direction of motion near their planned endpoints. Is this an automatic response to motion signals near those positions, or do we consider what is moving? To find out, we asked participants to hit a target that moved rightward across a patterned surface when it reached an interception zone that was indicated by a circle. The circle was initially at the center of a square. The square was either filled, occluding the patterned surface (tile), or open, such that the patterned surface was not occluded (frame). The square briefly moved leftward or rightward shortly after the target appeared. Thus, participants were either aiming to hit the target on the surface that moved (the tile) or to hit the target on the patterned surface that did not move. Moving the two types of squares produced very similar local motion signals, but for the tile this could be interpreted as motion of an extended surface, while for the frame it could not. Motion onset of the two types of squares yielded very similar responses. Increasing the size of the square, and thus the eccentricity of the local motion signal, reduced the magnitude of the response. Since this reduction was seen for both types of squares, the surface on which the interception zone was presented was clearly not considered. We conclude that the response is driven by local motion signals near the endpoint of the action without considering whether the local surface is moving.

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

People adjust their on-going movements to changes in the environment. It takes about 100 ms to respond to an abrupt change in a target's position. Does the vigour of such responses depend on the extent to which responding is beneficial? We asked participants to tap on targets that jumped laterally once their finger started to move. In separate blocks of trials the target either remained at the new position so that it was beneficial to respond to the jump, or jumped back almost immediately so that it was disadvantageous to do so. We also varied the target's size, because a smaller, less vigorous adjustment is enough to place the finger within a larger target. There was a systematic relationship between the vigour of the response and the remaining time until the tap: the shorter the remaining time the more vigorous the response. This relationship did not depend on the target's size or whether or not the target jumped back. It was already known that the vigour of responses to target jumps depends on the magnitude of the jump and on the time available for adjusting the movement to that jump. We show that the vigour of the response is precisely tuned to the time available for making the required adjustment irrespective of whether responding in this manner is beneficial.

Gravity Influences How We Expect a Cursor to Move.

We expect a cursor to move upwards when we push our computer mouse away. Do we expect it to move upwards on the screen, upwards with respect to our body, or upwards with respect to gravity? To find out, we asked participants to perform a simple task that involved guiding a cursor with a mouse. It took participants that were sitting upright longer to reach targets with the cursor if the screen was tilted, so not only directions on the screen are relevant. Tilted participants' performance was indistinguishable from that of upright participants when the screen was tilted slightly in the same direction. Thus, the screen's orientation with respect to both the body and gravity are relevant. Considering published estimates of the ocular counter-roll induced by head tilt, it is possible that participants actually expect the cursor to move in a certain direction on their retina.

Further Evidence That People Rely on Egocentric Information to Guide a Cursor to a Visible Target.

Everyday movements are guided by objects' positions relative to other items in the scene (allocentric information) as well as by objects' positions relative to oneself (egocentric information). Allocentric information can guide movements to the remembered positions of hidden objects, but is it also used when the object remains visible? To stimulate the use of allocentric information, the position of the participant's finger controlled the velocity of a cursor that they used to intercept moving targets, so there was no one-to-one mapping between egocentric positions of the hand and cursor. We evaluated whether participants relied on allocentric information by shifting all task-relevant items simultaneously leaving their allocentric relationships unchanged. If participants rely on allocentric information they should not respond to this perturbation. However, they did. They responded in accordance with their responses to each item shifting independently, supporting the idea that fast guidance of ongoing movements primarily relies on egocentric information.