The flip side of perception-action coupling: locomotor experience and the ontogeny of visual-postural coupling.

The possible role of motor development on psychological function is once again a topic of great theoretical and practical importance. The revival of this issue has stemmed from a different approach to the topic, away from Gesell's interest in the long-term prediction of psychological functions from early motoric assessments, toward an attempt to understand how the acquisition of motor skills orchestrates psychological changes. This paper describes how the acquisition of one motor skill, prone locomotion, has been linked to developmental changes in an infant's ability to regulate posture based on information available in patterns of optic flow. It is argued that the onset of prone locomotion presses the infant to differentiate spatially delimited regions of optic flow to effectively and efficiently control the important subtasks nested within the larger task of locomotion, namely, steering, attending to the surface of support, and maintaining postural control. Following this argument, a research program is described that aims to determine if locomotor experience is causally linked to improvements in the ability to functionalize peripheral optic flow for postural control or whether locomotor experience is merely a maturational forecaster of such improvements. Finally, a hypothesis is put forward that links the emergence of wariness of heights to infants' ability to regulate posture on the basis of peripheral optic flow. The paper's overarching theoretical point is the principle of probabilistic epigenesis, which states that one developmental acquisition produces experiences that bring about a host of new developmental changes in the same and different domains.     

Stability in young infants' discrimination of optic flow

Although considerable progress has been made in understanding how adults perceive their direction of self-motion, or heading, from optic flow, little is known about how these perceptual processes develop in infants. In 3 experiments, the authors explored how well 3- to 6-month-old infants could discriminate between optic flow patterns that simulated changes in heading direction. The results suggest that (a) prior to the onset of locomotion, the majority of infants discriminate between optic flow displays that simulate only large (> 22 deg.) changes in heading, (b) there is minimal development in sensitivity between 3 and 6 months, and (c) optic flow alone is sufficient for infants to discriminate heading. These data suggest that spatial abilities associated with the dorsal visual stream undergo prolonged postnatal development and may depend on locomotor experience.     

Responsiveness to terrestrial optic flow in infancy: does locomotor experience play a role?

Human infants show a peak in postural compensation to optic flow at approximately nine months of age. The current experiment tested whether the magnitude of visual-postural coupling in 9-month-olds increases when terrestrial optic flow is added to a moving room. A secondary objective was to explore whether locomotor experience plays any role in enhancing responsiveness to the additional terrestrial information. Ninety-one infants (experienced creepers, nascent creepers, and prelocomotors) were exposed to two conditions of optic flow: global optic flow (G) and global optic flow minus terrestrial optic flow (G-T). The additional terrestrial optic flow led to significantly higher visual-postural coupling. Consistent with previous findings, locomotor experience had no effect on responsiveness to the G-T condition, though there was weak evidence that the nascent creepers were more strongly influenced by the difference between flow conditions than the other infants. Unexpectedly, the prelocomotor females showed significantly lower visual-postural coupling than the prelocomotor males. These findings support the notion that the ground provides an important source of information for the control of posture and locomotion. The findings also suggest that locomotor experience most likely helps to functionalize smaller (partial), rather than larger (global), optic flow fields for postural control.     

Judgments of path, not heading, guide locomotion

To steer a course through the world, people are almost entirely dependent on visual information, of which a key component is optic flow. In many models of locomotion, heading is described as the fundamental control variable; however, it has also been shown that fixating points along or near one's future path could be the basis of an efficient control solution. Here, the authors aim to establish how well observers can pinpoint instantaneous heading and path, by measuring their accuracy when looking at these features while traveling along straight and curved paths. The results showed that observers could identify both heading and path accurately (approximately 3 degrees ) when traveling along straight paths, but on curved paths they were more accurate at identifying a point on their future path (approximately 5 degrees ) than indicating their instantaneous heading (approximately 13 degrees ). Furthermore, whereas participants could track changes in the tightness of their path, they were unable to accurately track the rate of change of heading. In light of these results, the authors suggest it is unlikely that heading is primarily used by the visual system to support active steering.     

The role of direction information in the perception of geometric optic flow components

Theoretically, optic flow, an important source of information for the perception of locomotion and three-dimensional structure of the environment, is described in terms of divergence, curl, and shear components. We measured how the detection of the type of flow field depends on directional information. We manipulated the local directions by rotating them through an angle x relative to the original direction (i.e., the direction of motion at that locus in an unaltered flow field). The results of the first experiments showed that divergence, curl, and shear can be detected even if the directional range of the individual motion vectors is as broad as 180 degrees. Subsequent experiments revealed that the detection of the geometric components of the optic flow field is merely based on the integration of a few (10% of vectors) local directions correctly (within 10 degrees of original direction) specifying the type of flow field. Other directions are irrelevant to this process. This is actually what one would expect if the optic flow is analyzed by special purpose mechanisms that detect and process the geometric components on the basis of the integration of motion information. The results indicate that as far as they integrate motion information, detectors for divergence, curl, and shear operate in a similar manner. Implications of the results for modeling such mechanisms are discussed.     

How Is Human Gait Controlled by Vision

In his seminal article, Gibson (1958/this issue) laid the foundation for understanding the visuomotor transformations necessary for adaptive locomotor behavior. In this article, I review the work on visual control of locomotion done in our lab in Waterloo and discuss some new experiments. The major findings are put forward as three new postulates. Postulate 1: Visual exteroceptive information about the environment is used in a sampled, feed-forward mode to control locomotion. Postulate 2: Visual exproprioceptive information about the posture and movement of the lower limb is used in a sampled, online mode to control the swing phase trajectory. Postulate 3: Visual exproprioceptive information about self-motion is used in a sampled, online mode to control locomotion. Our results in most cases provide empirical support for the many insightful postulates of Gibson, in some cases amplify his statements, and in a few cases add to our understanding of how we get about by vision.     

Postural and Locomotor Contributions to Affordance Perception

The authors sought to evaluate the relative importance of locomotor control and postural control in the perception of affordances. While seated in a stationary wheelchair, participants made a series of judgments about the minimum lintel height under which they could roll in the wheelchair. Prior to making judgments, participants were given brief (～2 min) experience with wheelchair locomotion. They expected that this practice would influence the accuracy of subsequent affordance judgments. During practice, participants moved under their own power (using their hands on the wheels) or with an experimenter pushing the wheelchair. Also during wheelchair locomotion the participant's head was restrained, or was not. Results revealed that head restraint during the practice session had no effect on the accuracy of subsequent judgments. By contrast, the judgments of participants who controlled locomotion during practice were significantly more accurate than the judgments of participants who had not controlled their locomotion during practice.     

Gauging possibilities for action based on friction underfoot

Standing and walking generate information about friction underfoot. Five experiments examined whether walkers use such perceptual information for prospective control of locomotion. In particular, do walkers integrate information about friction underfoot with visual cues for sloping ground ahead to make adaptive locomotor decisions? Participants stood on low-, medium-, and high-friction surfaces on a flat platform and made perceptual judgments for possibilities for locomotion over upcoming slopes. Perceptual judgments did not match locomotor abilities: Participants tended to overestimate their abilities on low-friction slopes and underestimate on high-friction slopes (Experiments 1-4). Accuracy improved only for judgments made while participants were in direct contact with the slope (Experiment 5), highlighting the difficulty of incorporating information about friction underfoot into a plan for future actions.     

The organization of exploratory behaviors in infant locomotor planning

How do infants plan and guide locomotion under challenging conditions? This experiment investigated the real‐time process of visual and haptic exploration in 14‐month‐old infants as they decided whether and how to walk over challenging terrain – a series of bridges varying in width. Infants’ direction of gaze was recorded with a head‐mounted eye tracker and their haptic exploration and locomotor actions were captured on video. Infants’ exploration was an organized, efficient sequence of visual, haptic, and locomotor behaviors. They used visual exploration from a distance as an initial assessment on nearly every bridge. Visual information subsequently prompted gait modifications while approaching narrow bridges and haptic exploration at the edge of the bridge. Results confirm predictions about the sequential, ramping‐up process of exploration and the distinct roles of vision and touch. Exploration, however, was not a guarantee of adaptive decisions. With walking experience, exploratory behaviors became increasingly efficient and infants were better able to interpret the resulting perceptual information in terms of whether it was safe to walk.     

Perception of heading without retinal optic flow

How do we determine where we are heading during visually controlled locomotion? Psychophysical research has shown that humans are quite good at judging their travel direction, or heading, from retinal optic flow. Here we show that retinal optic flow is sufficient, but not necessary, for determining heading. By using a purely cyclopean stimulus (random dot cinematogram), we demonstrate heading perception without retinal optic flow. We also show that heading judgments are equally accurate for the cyclopean stimulus and a conventional optic flow stimulus, when the two are matched for motion visibility. The human visual system thus demonstrates flexible, robust use of available visual cues for perceiving heading direction.     

The contributions of static visual cues, nonvisual cues, and optic flow in distance estimation

By systematically varying cue availability in the stimulus and response phases of a series of same-modality and cross-modality distance matching tasks, we examined the contributions of static visual information, idiothetic information, and optic flow information. The experiment was conducted in a large-scale, open, outdoor environment. Subjects were presented with information about a distance and were then required to turn 180 before producing a distance estimate. Distance encoding and responding occurred via: (i) visually perceived target distance, or (ii) traversed distance through either blindfolded locomotion or during sighted locomotion. The results demonstrated that subjects performed with similar accuracy across all conditions. In conditions in which the stimulus and the response were delivered in the same mode, when visual information was absent, constant error was minimal; whereas, when visual information was present, overestimation was observed. In conditions in which the stimulus and response modes differed, a consistent error pattern was observed. By systematically comparing complementary conditions, we found that the availability of visual information during locomotion (particularly optic flow) led to an 'under-perception' of movement relative to conditions in which visual information was absent during locomotion.     

Multimodal Perception of Reachability Expressed Through Locomotion

We investigated the information that supports perception of whether an object is within reach using a locomotor task. Participants adjusted their own position relative to a fixed target by stepping or by propelling a wheelchair until they judged it to be within reach. The to-be-reached object was presented in virtual reality. The display of the target was driven in real time as a function of the observer's movement, thus depicting a stationary virtual object at a definite distance only through the relation across optical and nonoptical patterns of stimulation. We asked participants to judge the distance they could reach with their unaided hand or when holding a rod that extended their effective reach. They could see neither their body nor the rod thereby limiting available visual information about “reachability.” As expected, our results showed that despite the limited information that was available, participants' locomotor adjustments were influenced by (a) their simulated distance from the target, (b) their arm length, and (c) the presence or absence of the rod. The type of motion (stepping or wheelchair) had little influence. However, judgment accuracy was influenced by participants' initial simulated distance from the target. We compare the performance obtained in our locomotor judgment task with previous studies that have used different methods for measuring perceived reaching-ability. We discuss perceptual information that could have supported performance within the framework of the global array.     

Visually Controlled Locomotion: Its Dependence on Optic Flow,Three-Dimensional Space Perception,and Cognition

Gibson (1958/this issue) and his followers have emphasized the role of optic flow in the control of locomotion. In recent years much research has been devoted to the visual control of aiming and braking, mainly in connection with terrestrial locomotion. The goal of this article is to broaden the topic empirically and theoretically. At the empirical level, we argue that there are a number of visually controlled maneuvers that need to be addressed for their own sake, for they involve more than can be learned from research on aiming and braking. At the theoretical level, we argue that optic flow needs to be supplemented by other explanatory primitives, including the actor's perception of three-dimensional spatial layout and the actor's cognitive representations of the spatial envelope and plant dynamics of his or her body or vehicle.     

Retinal flow is sufficient for steering during observer rotation

How do people control locomotion while their eyes are simultaneously rotating? A previous study found that during simulated rotation, they can perceive a straight path of self-motion from the retinal flow pattern, despite conflicting extraretinal information, on the basis of dense motion parallax and reference objects. Here we report that the same information is sufficient for active control ofjoystick steering. Participants steered toward a target in displays that simulated a pursuit eye movement. Steering was highly inaccurate with a textured ground plane (motion parallax alone), but quite accurate when an array of posts was added (motion parallax plus reference objects). This result is consistent with the theory that instantaneous heading is determined from motion parallax, and the path of self-motion is determined by updating heading relative to environmental objects. Retinal flow is thus sufficient for both perceiving self-motion and controlling self-motion with a joystick; extraretinal and positional information can also contribute, but are not necessary.     

Analysis of recent empirical challenges to an account of interceptive timing.

How do we perceive how long it will be before we reach a certain place when running, driving, or skiing? How do we perceive how long it will be before a moving object reaches us or will arrive at a place where it can be hit or caught? These are questions of how we temporally coordinate our actions with a dynamic environment so as to control collision events. Much of the theoretical work on the control of these interceptive actions has been united in supposing that (1) timing is functionally separable from positioning and the two are controlled using different types of information; (2) timing is controlled using special-purpose time-to-arrival information; (3) the time-to-arrival information used for the timing of fast interceptive actions is a first-order approximation to the actual time-to-arrival, which does not take accelerations into account. Challenges to each of these suppositions have recently emerged, suggesting that a complete rethinking of how interceptions are controlled may be necessary. These challenges are analyzed in detail and it is shown that they are readily accommodated by a recent theory of interceptive timing based on the points just noted.     

Analysis of recent empirical challenges to an account of interceptive timing

How do we perceive how long it will be before we reach a certain place when running, driving, or skiing? How do we perceive how long it will be before a moving object reaches us or will arrive at a place where it can be hit or caught? These are questions of how we temporally coordinate our actions with a dynamic environment so as to control collision events. Much of the theoretical work on the control of these interceptive actions has been united in supposing that (1) timing is functionally separable from positioning and the two are controlled using different types of information; (2) timing is controlled using special-purpose time-to-arrival information; (3) the time-to-arrival information used for the timing of fast interceptive actions is a first-order approximation to the actual time-to-arrival, which does not take accelerations into account. Challenges to each of these suppositions have recently emerged, suggesting that a complete rethinking of how interceptions are controlled may be necessary. These challenges are analyzed in detail and it is shown that they are readily accommodated by a recent theory of interceptive timing based on the points just noted.     

Path integration from optic flow and body senses in a homing task

We examined the roles of information from optic flow and body senses (eg vestibular and proprioceptive information) for path integration, using a triangle completion task in a virtual environment. In two experiments, the contribution of optic flow was isolated by using a joystick control. Five circular arenas were used for testing: (B) both floor and wall texture; (F) floor texture only, reducing information for rotation; (W) wall texture only, reducing information for translation; (N) a no texture control condition, and (P) an array of posts. The results indicate that humans can use optic flow for path integration and are differentially influenced by rotational and translational flow. In a third experiment, participants actively walked in arenas B, F, and N, so body senses were also available. Performance shifted from a pattern of underturning to overturning and exhibited decreased variability, similar responses with and without optic flow, and no attrition. The results indicate that path integration can be performed by integrating optic flow, but when information from body senses is available it appears to dominate.     

The study of locomotor pointing in virtual reality: The validation of a test set-up

The goal of this experiment was to validate an experimental set-up for studying locomotor pointing. The specific and also original element of this set-up was the interactive nature of virtual reality and movement production. This interaction was achieved through the coupling of a treadmill and a Silicon Graphics system. This latter system generated on a screen (3 × 2.3 m) an environmental array that moved according to the action produced by subjects on a treadmill. The task was to place either foot on a spatial target that appeared on the floor in front of the subject’s displacement trajectory. We analyzed the step length patterns of subjects approaching these targets, along with the current target-subject relationship. The results are in agreement with aperception-action coupling type of control mechanism that operates continuously as the subject approaches the desired target. Apparently, these findings mirror observations of real-life locomotion, indicating that the present set-up provides a valid and useful tool for examining human locomotion.     

It's the journey,not the destination: Locomotor exploration in infants

What incites infant locomotion? Recent research suggests that locomotor exploration is not primarily directed toward distant people, places, or things. However, this question has not been addressed experimentally. In the current study, we asked whether a room filled with toys designed to encourage locomotion (stroller, ball, etc.) elicits different quantities or patterns of exploration than a room with no toys. Caregivers were present but did not interact with infants. Although most walking bouts in the toy‐filled room involved toys, to our surprise, 15‐month‐olds in both rooms produced the same quantity of locomotion. This finding suggests that mere space to move is sufficient to elicit locomotion. However, infants' patterns of locomotor exploration differed: Infants in the toy‐filled room spent a smaller percent of the session within arms' reach of their caregiver and explored more locations in the room. Real‐time analyses show that infants in the toy‐filled room took an increasing number of steps per bout and covered more area as the session continued, whereas infants in the no‐toy room took fewer and fewer steps per bout and traveled repeatedly over the same ground. Although not required to elicit locomotion, moving with toys encouraged infants to travel farther from their caregivers and to explore new areas.