Learning to visually perceive the relative mass of colliding balls in globally and locally constrained task ecologies.

Novice observers differ from each other in the kinematic variables they use for the perception of kinetic properties, but they converge on more useful variables after practice with feedback. The colliding-balls paradigm was used to investigate how the convergence depends on the relations between the candidate variables and the to-be-perceived property, relative mass. Experiment 1 showed that observers do not change in the variables they use if the variables with which they start allow accurate performance. Experiment 2 showed that, at least for some observers, convergence can be facilitated by reducing the correlations between commonly used nonspecifying variables and relative mass but not by keeping those variables constant. Experiments 3a and 3b further demonstrated that observers learn not to rely on a particular nonspecifying variable if the correlation between that variable and relative mass is reduced.

Individual differences and the use of nonspecifying variables in learning to perceive distance and size: comments on McConnell, Muchisky, and Bingham (1998).

McConnell, Muchisky, and Bingham (1998) showed that observers are able to judge the distance and size of falling, rolling, and swinging balls and that performance improves after practice with feedback. They concluded that observers use information that specifies the spatial scales of the different event types--namely, event duration in combination with event-specific constants. The improvement was interpreted as the calibration of the event-specific constants. We argue that their analyses should have considered the use of optical variables that do not specify the to-be-perceived metrics and individual differences in variable use. Furthermore, we propose convergence on the more useful variables as an alternative explanation for the observed improvement. The viability of these arguments is demonstrated with an experiment in which participants are trained with feedback to judge the distance and size of freely falling balls.

Information and action in punching a falling ball.

Lee, Young, Reddish, Lough, and Clayton (1983) reported that the timing control of jumping and vertically punching a dropping ball exploits the inverse of the rate of change of optical expansion, tau(r). We raise a number of methodological and logical criticisms against their experiment and conclusions and attempt to rectify them by examining elbow joint angles only, in seated punchers, under both monocular and binocular conditions, with two ball sizes, dropped from two heights. Differences between the binocular and monocular cases suggest the exploitation of different information. We present several techniques to help determine the operative variable(s) controlling the action. The optical variable used to initiate and guide flexion appeared to be expansion velocity (looming), rather than tau(r); extension appeared to be under the control of different variables in the monocular and binocular cases. Simulations using single variables and single perceptuo-motor intervals were of mixed success.

Stimulus-response compatibility for absolute and relative spatial correspondence in reaching and in button pressing.

Three experiments tested whether stimulus-response (S-R) compatibility might be a function of absolute (as opposed to relative) spatial correspondence--that is, the distance between a stimulus and the place of response. Experiment 1 studied reaching movements toward one of two targets in response to one of six visual stimuli. Stimulus-response pairs that shared relative position were faster than those that did not, and reaction time was faster when the stimulus and one of the potential targets were in close proximity. In Experiment 2 the same effects were found when the hands started from a different position, implicating stimulus-target distance, rather than stimulus-hand distance as the critical variable. Experiment 3 employed keypress responses instead of reaches, and the distance effect was nearly absent. The implications of these results are discussed in terms of categorical (e.g. left-right) vs. quantitative (e.g. distance) S-R variables in spatial compatibility.

Learning to perceive the relative mass of colliding balls: the effects of ratio scaling and feedback.

The theory of direct perception holds that competent observers are able to detect optical patterns that specify the relative mass of colliding balls. Heuristic theorists, on the other hand, claim that judgments of relative mass are based on variables that do not specify relative mass. We contrasted these views with an experiment in which participants were given feedback on their ratio-scaled estimates of the relative mass of simulated colliding balls. Correlations between judged relative mass and various kinetic and kinematic measures of the collisions revealed that (1) judgments of relative mass become more accurate with feedback, (2) different observers use different variables, (3) during training, many observers change which variables they use, (4) before training, observers tend to use nonspecifying variables or combinations thereof, (5) after a minimal amount of training, at least some observers seem to detect mass-specifying information, and (6) the judgments do not support a generalization of the heuristic model of Gilden and Proffitt (1989, 1994). These findings suggest that direct perception of relative mass is a skill that can be developed through appropriate training.

Shedding some light on catching in the dark: perceptual mechanisms for catching fly balls.

To catch a lofted ball, a catcher must pick up information that guides locomotion to where the ball will land. The acceleration of tangent of the elevation angle of the ball (AT) has received empirical support as a possible source of this information. Little, however, has been said about how the information is detected. Do catchers fixate on a stationary point, or do they track the ball with their gaze? Experiment 1 revealed that catchers use eye and head movements to track the ball. This means that if AT is picked up retinally, it must be done by means of background motion. Alternatively, AT could be picked up by extraretinal mechanisms, such as the vestibular and proprioceptive systems. In Experiment 2, catchers reliably ran to intercept luminous fly balls in the dark, that is, in absence of a visual background, under both binocular and monocular viewing conditions. This indicates that the optical information is not detected by a retinal mechanism alone.

Higher order and lower order variables in the visual perception of relative pulling force.

In 7 experiments, undergraduates judged the force exerted by a videotaped standing puller, a computer-generated (stick-figure) puller, or a computer-generated inverted pendulum. Single and stepwise multiple regression analyses determined the kinematic variables exploited by the participants. Results show that (a) judgments correlated highly with force and improved with feedback; (b) judgments correlated more highly with lower order kinematic variables than with force itself; (c) participants differed in the kinematic variables exploited; (d) participants changed over blocks of trials in the variables exploited; (e) some participants used compound kinematic variables; (f) the variables exploited depended on the type of feedback; and (g) judgments to upright pullers, inverted pullers, and simple pendula showed the same qualitative patterns. Implications for theories of direct perception, directed perception, and heuristics are considered.

Stimulus-target compatibility for reaching movements.

Reaction time, movement time, and initial direction of reaching movements toward a target in left or right hemispace were measured. In Experiment 1, the target of movement and hand had to be selected; movements toward the imperative stimulus were initiated faster than movements toward the alternate target, and ipsilateral reaches were initiated faster than contralateral reaches. In Experiment 2, the difference between ipsilateral and contralateral reaches disappeared when no selection of the hand had to occur. In Experiment 3, no target had to be selected, and only a stimulus-hand compatibility effect appeared. The results reveal different compatibility effects (stimulus-target, stimulus-hand, target-hand), implying that participants exploit different correspondences, depending on the degrees of freedom of the action. The notion of compatibility effects relating to movement targets offers a new perspective on the negative Simon effect and it questions the general concept of response codes.

The effects of baseball experience on movement initiation in catching fly balls.

Previous research has shown that skilled athletes are able to respond faster than novices to skill-specific information. The aim of this study was to ascertain whether expert outfielders are faster than non-experts in acting on information about the flight of a fly ball. It was hypothesized that expert outfielders are better attuned to this information; as a result, faster and more accurate responses were expected. This hypothesis was tested by having non-expert and expert outfielders judge, as quickly as possible, where a ball would land in the front-behind dimension (perceptual condition) and, in another condition, to attempt to catch such balls (catching condition). The results of the perceptual condition do not support the hypothesis that expert outfielders are more sensitive to ball flight information than non-experts, but the results of the catching condition reveal that experts are more likely to initiate locomotion in the correct direction.

The relevance of action in perceiving affordances: perception of catchableness of fly balls.

The catchableness of a fly ball depends on whether the catcher can get to the ball in time; accurate judgments of catchableness must reflect both spatial and temporal aspects. Two experiments examined the perception of catchableness under conditions of restricted information pickup. Experiment 1 compared perceptual judgments with actual catching and revealed that stationary observers are poor perceivers of catchableness, as would be expected by the lack of information about running capabilities. In Experiment 2, participants saw the 1st part of ball trajectories before their vision was occluded. In 1 condition, they started to run (as if to catch the ball) before occlusion; in another, they remained stationary. Moving judgments were better than stationary judgments. This supports the idea that perceiving affordances that depend on kinematic, rather than merely geometric, body characteristics may require the relevant action to be performed.

The Organization of Multisegmental Pulls Made by Standing Humans: II. Submaximal Pulls.

Previously, an autonomous oscillator model with three parameters was derived that describes the relationship between anterior-posterior center of mass motions and pulling force for near-maximal, bimanual pulls made by standing subjects (Michaels, Lee, Pai, 1993). The present study evaluated the extent to which a full range of pulling forces could be fit by the model and how the model's three parameters changed with intended pulling force. How much variation in force each parameter could contribute was determined by simulating the model. Qualitative and quantitative analyses of pulls made by 6 well-practiced subjects at 5prcent;, 10prcent;, 20prcent;, 40prcent;, 60prcent;, 80prcent;, and 95prcent; of their maximum pulling force revealed that the model holds well, except for the least forceful pulls of some subjects. Two parameters appeared to be controlled; one, related to the position of the center of pressure, varied most among less forceful pulls; the second, related to the position of the center of mass at the time of handle-force onset, varied most among more forceful pulls. How these parameters might be set is discussed.

Destination compatibility, affordances, and coding rules: a reply to Proctor, Van Zandt, Lu, and Weeks.

Proctor, Van Zandt, Lu, and Weeks (1993) argued that the invocation of affordances to explain stimulus-response (S-R) compatibilities in reaction time is not needed because left-right direction compatibility, from the coding rules approach, explains the apparent "destination compatibility" effect of Michaels (1988). In this reply, an experiment demonstrates that destination compatibility can be shown even when contradicted by relative left-right motion. The second half of the article addresses theoretical issues separating and joining these two approaches. It is argued that the domain of the affordance approach in S-R compatibility is the guidance of action by information, whereas the domain of coding rules is S-R incompatibility and noncompatibility, situations in which required responses are not afforded and rules must be invoked. The manipulation of rules can mimic some of the consequences of more fundamental perception-action couplings, but principles of the latter sort are needed.

The organization of multisegmental pulls made by standing humans: I. Near-maximal pulls.

Complex multisegmental movements occur when standing subjects exert forceful, impulse-like pulls on a bimanually held handle. The degrees of freedom of this task were analyzed to provide a principled basis for understanding the act's coordination. Body posture was found to be describable by only two degrees of freedom, expressible as the anterior-posterior and vertical coordinates of the center of mass (CM(AP), CM(V)). Kinetic analysis revealed that the two major contributors to pulling force depended only upon CM(AP) motion and the location of the center of the pressure. Kinematic and kinetic data from six well-practiced subjects pulling near their maxima were used to test the prediction of less intersubject variability in CM(AP) than in CM(V) variation led to different movement patterns among subjects. A dynamic model of CM(AP) motion was developed, and manipulation of its three degrees of freedom yielded CM(AP) trajectories that matched the empirical trajectories. It is suggested that the pull might be controlled with reference to these three parameters.

Stimulus-response compatibilities between vertically oriented stimuli and horizontally oriented responses: the effects of hand position and posture.

Stimulus-response (S-R) compatibility effects between vertically oriented stimuli (above or below fixation) and horizontally oriented responses (left or right switch deflections by a single hand) have been shown to depend both on which hand responds (Bauer & Miller, 1982) and on the location at which the response is made (eccentricity on a frontoparallel line; Michaels, 1989). In the latter study, hand position and hand posture were confounded, so it is unclear which variable determined the compatibility effect. In Experiment 1, the importance of effector position was tested. Vertically oriented stimuli were paired with a horizontal response solicited at different locations but always involving the same hand posture. Compatibility effects emerged, and their direction depended on position. In Experiment 2, the compatibilities were not evident in a simple reaction time paradigm, so the effect was not due to differential ease of responses. In Experiment 3, a change in hand posture (palm up or palm down) at the same location (the body midline) also affected the compatibilities. It was concluded that the S-R compatibility of orthogonally oriented stimuli and responses is influenced by (1) which hand responds, (2) the location of that hand, and (3) its posture. The results imply that both postural and positional states of the action system affect S-R compatibility.

The organization of torque and EMG activity during bilateral handle pulls by standing humans.

This study examined whether the torques and EMG activity that precede and accompany bilateral arm pulls made by standing humans demonstrate a pulse height form of organization. Nine adults made abrupt bilateral pulls in the sagittal plane against a handle, to force targets equal to 5, 10, 20, 40, 60, 80 and 95% of their maximal pulling force (%MPF). The force applied at the handle, ground reaction forces, the center of pressure (CP), and EMG activity in gastrocnemius (GS), biceps femoris (BF), tibialis anterior (TA) and quadriceps (QD) muscles were recorded. Our analysis divided the action into a pre-pull phase (events prior to the increase of handle force) and a pulling phase (while handle force was greater than zero). We evaluated the effects of %MPF on the durations and peak amplitudes of the pre-pull and pulling angular impulses about the ankle joint and on pre-pull EMG patterns. The results showed that the angular impulse associated with the pulling torque (due to the reactive force on the body during the pull) had a pulse height organization: peak torque increased linearly with %MPF, and the durations of the pulling torque were relatively constant. In contrast, a pulse height organization did not characterize the pre-pull period for either the angular impulse associated with ankle torque (due to net ground reaction force) or EMG activity in the leg muscles. Rather, peak ankle torque typically increased up to some submaximal %MPF and then plateaued, perhaps due to a constraining effect on foot length on CP. The durations of pre-pull ankle torques increased over the whole range of %MPF, thereby compensating for the limit on ankle torque. Depending on the subject, the muscles were recruited in two different orders: GS-BF-TA-QD, or GS-TA-BF-QD. As the %MPF increased, the EMG onset times of all four muscles occurred earlier, and there was a greater likelihood that the BF, TA and QD muscles would be recruited on a given trial. The changes in the ankle torque and EMG patterns were gradual, suggesting that the pre-pull phase could have one underlying form of organization, with parameters that are tuned to task goals and anatomical constraints.

S-R compatibility between response position and destination of apparent motion: evidence of the detection of affordances.

In choice reaction time, stimuli and responses in some combinations (e.g., based on spatial arrangement) are faster than in other combinations. To test whether motion toward a position yields faster responses at that position, a computer-generated square in front of one hand appeared to move either toward that hand or toward the other hand. Compatible responses (e.g., motion toward left hand/left response) were faster than incompatible responses, even when that opposed traditional positional compatibility. In Experiment 2, subjects responded to the same stimuli but with both hands left, right, or on the body midline. Medial responses were the fastest, showing that destination, rather than mere relative position, was a critical variable. It was suggested that spatial compatibility effects are not unique to position but apply to a variety of task situations, describable by J.J. Gibson's theory of affordances, in which he claims that one perceives the actions (e.g., catching) permitted in a situation.

A note on the natural basis of action categories: the catching distance of mantids.

What is the basis for the categorical distinctions evident in action? For some visually-guided activities it can be shown that categories of action are specified by criterial values of optical properties. In the more general case, the criterial optical properties should be scaled relative to dimensions of the acting animal. The analysis of the prey-catching behavior of praying mantids is used to exemplify the strategy for determining naturally defined boundaries on actions.