An Affordance-Based Approach to Visually Guided Overtaking

When an automobile driver overtakes a lead vehicle while avoiding oncoming traffic, does he or she do so with reference to the limits of his or her car? We investigated overtaking from the perspective of the theory of affordances. We define the overtake-ability affordance as a ratio of the minimum satisfying velocity required for safe overtaking (MSV) to the maximum velocity of the driver's car (V_max). Two groups of participants performed overtaking maneuvers, if deemed possible, by driving either a slow (V_max = 25 m/s) or a fast (V_max = 32.5 m/s) virtual car in overtaking situations constrained by 14 values of MSV. For any given MSV condition, participants in the fast car group were more likely to attempt an overtaking maneuver. However, when MSV was expressed in intrinsic units as a ratio of V_max for both groups, the frequency of overtaking was not significantly different across groups. Furthermore, overtaking frequency dropped to near 0% for both groups when MSV exceeded V_max. In accordance with the affordance-based framework (Fajen, 2007), our results suggest that participants select their overtaking maneuvers by perceiving an overtake-ability affordance.     

Development of Visually Guided Locomotion

This article presents a developmental account of changes in the visual guidance of locomotion. In contrast to the impressive efficiency of adult locomotion, locomotor activity is not under prospective control at the onset of human mobility. Infants require extensive crawling and walking experience before responding adaptively to variations in the terrain. At the same time that they are learning to navigate in increasingly varied environments, their bodies and skills are rapidly changing. Learning generalizes from safe, flat ground to novel surfaces but it does not transfer to new methods of locomotion. We account for these patterns of generality and specificity of learning by focusing on the role of exploratory behavior in detecting threats to balance control.     

Human Eye Movements During Visually Guided Stepping

Visually guided locomotion was studied in an experiment in which human subjects (N = 8) had to accurately negotiate a series of irregularly spaced stepping-stones while infrared reflectometry and electrooculography were used to continuously record their eye movements. On average, 68% of saccades made toward the next target of footfall had been completed (visual target capture had occurred) while the foot to be positioned was still on the ground; the remainder were completed in the first 300 ms of the swing phase. The subjects' gaze remained fixed on a target, on average, until 51 ms after making contact with it, with little variation. A greater amount of variation was seen in the timing of trailing footlift relative to visual target capture. Assuming that subjects sampled the visual cues as and when they were required, visual information appeared most useful when the foot to be positioned was still on the ground.     

Asymmetries in the Regulation of Visually Guided Aiming

An experiment was conducted to examine the contribution of sensory information to asymmetries in manual aiming. Movements were performed in four vision conditions. In the full-vision condition (FV), subjects were afforded vision of both the hand and the target throughout the course of the movement. In the ambient-illumination-off condition (AO), the room lights were extinguished at movement initiation, preventing vision of the moving limb. In the target-off (TO) condition, the target was extinguished upon initiation of the movement. In a no-vision (NV) condition, ambient illumination was removed and the target was extinguished upon initiation of the response movement. Results indicated that accuracy was superior in the full-vision and target-off conditions and when movements were made by the right hand. Movements made by the right hand were also of shorter mean duration. The magnitudes of performance asymmetries were uninfluenced by vision condition. Analyses of movement kinematics revealed that movements made in conditions in which there was vision of the limb exhibited a greater number of discrete modifications of the movement trajectory. On an individual-trial basis, no relationship existed between accuracy and the occurrence of discrete modifications. These data suggest that although vision greatly enhances accuracy, discrete modifications subserved by vision reflect the imposition of nonfunctional zero-order control processes upon continuous higher-order control regimes.     

Visually Based Descriptions of an Everyday Action

A technique is described for analyzing everyday behaviors into units of action. The basis of this technique is both theoretical and empirical. Theoretically, an argument is made that a basic unit level exists in everyday action and that this basic unit is characterized as a minimal transformation of the environment by the agent. In this study, we demonstrate that observers naive to this hypothesis about the nature of action nevertheless segmented actions they observed in the way predicted by the theory.     

Coordination of Eye and Leg Movements During Visually Guided Stepping

In the present study, 2 related hypotheses were tested: first, that vision is used in a feedforward control mode during precision stepping onto visual targets and, second, that the oculomotor and locomotor control centers interact to produce coordinated eye and leg movements during that task. Participants' (N = 4) eye movements and step cycle transition events were monitored while they performed a task requiring precise foot placement at every step onto irregularly placed stepping stones under conditions in which the availability of visual information was either restricted or intermittently removed altogether. Accurate saccades, followed by accurate steps, to the next footfall target were almost always made even when the information had been invisible for as long as 500 ms. Despite delays in footlift caused by the temporary removal (and subsequent reinstatement) of visual information, the mean interval between the start of the eye movement and the start of the swing toward a target did not vary significantly (p > .05). In contrast, the mean interval between saccade onset away from a target and a foot landing on that target (stance onset) did vary significantly (p < .05) under the different experimental conditions. Those results support the stated hypotheses.     

Anxiety Influences the Perceptual-Motor Calibration of Visually Guided Braking to Avoid Collisions

We investigated whether anxiety influences perceptual-motor calibration in a braking to avoid a collision task. Participants performed either a discrete braking task (Experiment 1) or a continuous braking task (Experiment 2), with the goal of stopping before colliding with a stop sign. Half of participants performed the braking task after an anxiety induction. We investigated whether anxiety reduced the frequency of crashing and if it influenced the calibration of perception (visual information) and action (brake pressure) dynamically between-trials in Experiment 1 and within-trials in Experiment 2. In the discrete braking task, anxious participants crashed less often and made larger corrective adjustments trial-to-trial after crashing, suggesting that the influence of anxiety on behavior did not occur uniformly, but rather dynamically with anxiety amplifying the reaction to previous crashes. However, when performing continuous braking, anxious participants crashed more often, and their within-trial adjustments of deceleration were less related to visual information compared to controls. Taken together, these findings suggest that the timescale and nature of the task mediates the influence of anxiety on the performance of goal-directed actions.     

Unpredictable yet Guided: Arendt on Principled Action

Political action is unpredictable because it unfolds among a plurality of independent actors. This unpredictability generates a fundamental puzzle: If an actor cannot know where her initiative will lead, what motivates and guides her in her doings? The aim of this paper is to develop and defend the solution to the puzzle that we can find in the thought of Hannah Arendt, namely the idea that political action is – or should be – motivated and guided by principles, principles like justice, equality, or honour. Such principles must be distinguished from goals because they do not designate a specific state of affairs, but are general ideas that allow for diverse implementations. I show that this distinction is of great practical importance for the actors involved, since their outlook and their attitude towards others will differ depending on whether they see their activity as aimed at a goal or as guided by a principle.     

Action,Control and Sensations of Acting

Sensations of acting and control have been neglected in theory of action. I argue that they form the core of action and are integral and indispensible parts of our actions, participating as they do in feedback loops consisting of our intentions in acting, the bodily movements required for acting and the sensations of acting. These feedback loops underlie all activities in which we engage when we act and generate our control over our movements.The events required for action according to the causal theory, or Searle’s theory, do not add up to agency. I find the agent at work, her engagement in her action, in her sensation of acting.It is often thought that bodily movements are either mere events or actions. I suggest that there is a third possibility: that they are activities which are not actions. Activities, whether they are actions or not, differ from mere events in that they are controlled from within the brain. They are actions only if this control forms agent control, that is, control in which the conscious mind, the agents’ intentions in acting and her sensations of acting, participates.Internally wayward actions are characterized by the fact that the person involved does not have a sensation of action and therefore does not control her movements. In the absence of such a sensation she does not act.     

Visually Perceiving What is Reachable

Four experiments designed to assess observers' ability to perceive whether or not something is within reach, under a variety of circumstances, are reported. Actual reaching ability is influenced by the posture in which reaching is to be done (e.g., with the arm alone, extending the arm while bending from the hip, and main- taining balance while bending from an upright standing position) and by the surface layout that confronts the reacher (e.g., the height of a surface to be reached and the presence of barriers to bending). Our investigation reveals that perceivers are sensitive to these influences on their reaching ability, although some are quite complex biomechanically. The results are discussed in the context of the demands that they place on computational and ecological accounts of perceiving what is within reach.     

Visually induced reorientation illusions.

It is known that rotation of a furnished room around the roll axis of erect subjects produces an illusion of 360 degrees self-rotation in many subjects. Exposure of erect subjects to stationary tilted visual frames or rooms produces only up to 20 degrees of illusory tilt. But, in studies using static tilted rooms, subjects remained erect and the body axis was not aligned with the room. We have revealed a new class of disorientation illusions that occur in many subjects when placed in a 90 degrees or 180 degrees tilted room containing polarised objects (familiar objects with tops and bottoms). For example, supine subjects looking up at a wall of the room feel upright in an upright room and their arms feel weightless when held out from the body. We call this the levitation illusion. We measured the incidence of 90 degrees or 180 degrees reorientation illusions in erect, supine, recumbent, and inverted subjects in a room tilted 90 degrees or 180 degrees. We report that reorientation illusions depend on the displacement of the visual scene rather than of the body. However, illusions are most likely to occur when the visual and body axes are congruent. When the axes are congruent, illusions are least likely to occur when subjects are prone rather than supine, recumbent, or inverted.     

Visually controlled matching of pattern movement

Utrecht Biophysics Research Institute, Rijksuniversiteit Utrecht, Utrecht, The Netherlands Subjects were asked to match the speeds of two moving random-dot patterns seen through circular apertures. The speed of one pattern that moved horizontally toward the right of a computer screen changed continuously. The speed of this pattern represented the target. It was to be matched with the speed of the second pattern, which moved in the opposite direction. The subject controlled the speed of the second pattern by means of an isometric joystick. The distance between the apertures on the screen as well as the subject’s distance from the screen served as experimental parameters. In this way, the effects of both spatialand temporal transients of pattern speed on human tracking performance were studied. To avoid anticipation by the subject, the amplitude and the frequency of the target pattern speed changed pseudorandomly. The accuracy with which the subject performed the matching task was influenced by the mean pattern speed and the parameters of the visual field. Within lower velocity ranges, the subject’s sensitivity to the instantaneous speed differences varied according to Weber’s law. The cross-correlation of the velocity time courses decreased when the mean speed of the target pattern was increased. Two stimulus parameters had a strong influence on the modulation of the correlation value: (1) the angular size of the stimulus on the retina and (2) the retinal eccentricity of the stimulus.     

Visually controlled matching of pattern movement.

Subjects were asked to match the speeds of two moving random-dot patterns seen through circular apertures. The speed of one pattern that moved horizontally toward the right of a computer screen changed continuously. The speed of this pattern represented the target. It was to be matched with the speed of the second pattern, which moved in the opposite direction. The subject controlled the speed of the second pattern by means of an isometric joystick. The distance between the apertures on the screen as well as the subject's distance from the screen served as experimental parameters. In this way, the effects of both spatial and temporal transients of pattern speed on human tracking performance were studied. To avoid anticipation by the subject, the amplitude and the frequency of the target pattern speed changed pseudorandomly. The accuracy with which the subject performed the matching task was influenced by the mean pattern speed and the parameters of the visual field. Within lower velocity ranges, the subject's sensitivity to the instantaneous speed differences varied according to Weber's law. The cross-correlation of the velocity time courses decreased when the mean speed of the target pattern was increased. Two stimulus parameters had a strong influence on the modulation of the correlation value: (1) the angular size of the stimulus on the retina and (2) the retinal eccentricity of the stimulus.     

Deciding as Intentional Action: Control over Decisions

Common-sense folk psychology and mainstream philosophy of action agree about decisions: these are under an agent's direct control, and are thus intentional actions for which agents can be held responsible. I begin this paper by presenting a problem for this view. In short, since the content of the motivational attitudes that drive deliberation and decision remains open-ended until the moment of decision, it is unclear how agents can be thought to exercise control over what they decide at the moment of deciding. I note that this problem might motivate a non-actional view of deciding—a view that decisions are not actions, but are instead passive events of intention acquisition. For without an understanding of how an agent might exercise control over what is decided at the moment of deciding, we lack a good reason for maintaining commitment to an actional view of deciding. However, I then offer the required account of how agents exercise control over decisions at the moment of deciding. Crucial to this account is an understanding of the relation of practical deliberation to deciding, an understanding of skilled deliberative activity, and the role of attention in the mental action of deciding.