Tuning in to Another Agent s Action Capabilities

Transcription

1 Tuning in to Another Agent s Action Capabilities Tehran J. Davis (davtj@ .uc.edu) Verónica C. Ramenzoni (ramenzvc@ .uc.edu) Kevin Shockley (kevin.shockley@.uc.edu) Michael A. Riley (michael.riley@uc.edu) Abstract Successful social coordination usually requires one person to be able to judge the limits and abilities of another person. Two experiments investigating perception of the maximum height to which an actor could jump and reach were conducted. In both experiments those estimates were compared to estimates of the perceiver s own maximum jump-with-reach height. Results of Experiment 1 showed that perceptions for the self improved over time, in the absence of explicit feedback, but perceptions for the other actor remained constant. Experiment 2 demonstrated that providing perceivers with perceptual information about the other actor s action capabilities enabled perceivers to recalibrate their perceptual reports for the other actor s maximum jump-withreach height. Keywords: Action understanding; embodiment; affordances; visual perception; social perception-action. Most naturally occurring human behavior takes place within a social environment. In these settings, choosing an appropriate action for oneself may depend upon the actions of others. Whether cooperating to lift a couch, or competing in a tennis match, successful social coordination often demands that one be able to predict another's actions and their consequences. One perspective on how agents solve this problem a perspective motivated by the discovery of the mirror neuron system emphasizes the role of embodied simulation processes that enable observers to interpret others actions via their own neural systems for movement (e.g., Gallese, 2005; see also Sebanz, Bekkering, & Knoblich 2006). A second (but perhaps not mutually exclusive) perspective is that action prediction and social coordination may call upon one's ability to perceive affordances (opportunities for action; Gibson, 1986) for another person one must be able to apprehend what another person is capable of doing before being able to predict or understand another person s actions (Ramenzoni, Riley, Shockley, & Davis, 2008). In the present research we addressed two specific questions related primarily to this second perspective: (1) How does perception of affordances for another person change over time and with visual experience, and (2) How does the ability to perceive affordances for another person depend upon one s own action capabilities, as predicted by embodied simulation? A number of studies have established that people are able to make accurate perceptual estimates of what the environment affords others, as well as what the environment affords themselves, by making use of information that is publicly available in the optic array to all observers in a given setting (Mark, 2007; Ramenzoni, Riley, Davis, & Snyder, 2005; Ramenzoni, Riley, Davis, Shockley, & Armstrong, in press; Stoffregen, Gorday, Sheng, & Flynn, 1999). Much of this research has focused on what are called body-scaled affordances. Body-scaled affordances, such as sitting and reaching, can be described by referring only to geometric attributes of the actor (leg length, arm span, etc.) (e.g., Mark, 1987; Warren, 1984). This class of affordances represents only a small spectrum of the possibilities for action that may be available to an individual at any given moment, however. Recently, there has been increased interest in so-called action-scaled affordances that are defined by dynamic properties of the actor (i.e., force generation, acceleration, etc.; see, e.g., Cesari, Formenti, & Olivato, 2003; Fajen, 2007; Konczak, Meeuwsen, & Cress, 1992; Oudejans, Michaels, Bakker, & Dolné, 1996; Pepping & Li, 1997, 2000). For instance, the ability to catch a fly ball depends upon, among other things, the acceleration and velocity of the approaching ball, as well as the individual s ability to accelerate to the point where the ball will land (Oudejans et al.). Action-scaled affordances present a particularly appealing case for the study of the social perception-action. The perception of action-scaled affordances for others is contingent upon the perceiver s sensitivity to properties of other person s action system that may not be obviously 2462

2 available in the optic array (e.g., the ability to exert force to lift an object), in the sense that the actor properties such as these are not simply geometric ones that can be visibly compared to the complementary geometric property of the environment. Previous work exploring the ability to perceive actionscaled affordances for another person (i.e., jump-withreaching) has shown that observers are able to do so with a similar level of accuracy as perceiving the same affordance for themselves (Ramenzoni et al., 2005). Ramenzoni, Riley, Davis et al. (in press) found that perceivers are sensitive to changes in an actor s maximum jumping height (effected by attaching ankle weights to the actor) when the only visual information about this change is available in the kinematic patterns of the actor s gait. In another study Ramenzoni, Riley, Shockley, and Davis (2008) found that perception of affordances for another person was influenced strongly by the perceiver s own action capabilities attaching ankle weights to the perceiver produced reductions in estimates of an unencumbered actor s jumping height. Taken together these findings seem to indicate that the ability to perceive affordances for others is shaped by sensitivity to the other person s as well as one s own action capabilities. A number of questions remain unanswered about the relative influences of the observer s own action capabilities on perception of what another actor is capable of doing. For example, one would expect that perceptual experience or exposure to new information might produce a change in affordance judgments through processes of perceptual attunement or calibration (Fajen, 2005). This raises the question of whether perceptual reports for one s own and another s maximum jumping height show a similar pattern of change over time or when new information about the other person s action capabilities is provided to the perceiver. The present study sought to address these issues. In Experiment 1 we explored whether estimates of the maximum height one could reach by jumping improved in accuracy over time as a result of calibration to information that specifies the affordance. In the absence of new information about the other person s action capabilities, a parallel pattern of change for self and other-actor affordance judgments would suggest that perception of another person s actions is dependent on one s own action capabilities. In Experiment 2 we determined whether watching an actor perform an action that offered perceivers the opportunity to attune to specifying information for the actor s jumping ability could lead to an improvement in estimating the actor s maximum jump-with-reach height. Experiment 1 In Experiment 1, we examined whether estimates of maximum jump-with-reach for the self and the other actor would improve in accuracy over time, in the absence of explicit feedback about the accuracy of the estimates. Mark, Balliet, Craver, Douglas, and Fox (1990) have shown that even in the absence of explicit feedback about affordance judgments visual information obtained from the optic flow produced by postural sway is sufficient for recalibrating an observer s estimations of affordances for the self when the observer s action capabilities have been modified. The present experiment was conducted to determine whether this finding transfers to perceptual judgments for another actor. The experiment also permitted an examination of the relative dependence or independence of affordance estimates for another actor on the observer s own action capabilities. If, as in the Mark et al. study, the observer s estimates for the self improved in accuracy over time but estimates for the other actor did not, it would suggest some degree of independence. Method Twenty participants ranging in maximum jump-with-reach height from to 294 cm (mean = ± cm) reported to the laboratory in pairs. On a given trial, each member of the pair was assigned the role of either observer or model. Observers were asked to make estimates of the limits of their own vertical jump-with-reach or that of the actor when jumping to grasp a small (5 cm 4 cm) cylindrical object suspended from the ceiling (see Figure 1). Jump-with-reach was defined as the maximum height at which the cylinder could be reached while performing a vertical jump (i.e., no steps were allowed) with the preferred arm extended overhead. This definition was provided to participants prior to beginning the experiment, but participants did not perform the actual jumping task until all perceptual judgment trials had been completed. Participants therefore did not see the other member of the pair perform the task (reach with jump) until the experiment was over, but they did have opportunities to observe each other walking around the laboratory during the experiment before data collection occurred (cf. Ramenzoni, Riley, Davis et al., in press). Figure 1: Apparatus used in both experiments and depiction of the perceptual task of estimating the maximum jumpwith-reach height for the self and the other actor. 2463

3 In all conditions observers stood 3 m away from a vertical surface in front of which was suspended the target cylinder. The model was stationed directly beside the apparatus (from the observer s perspective, to the right side) at all times. Both the observer and the model were instructed to remain stationary with the arms at the sides for the duration of the experiment. The model was blindfolded and wore headphones playing music to prevent being influenced by hearing the perceptual estimates given by the observer. Perceptual reports were obtained using the method of adjustments (Guilford, 1954). Participants verbally instructed an experimenter who, while standing out of sight behind the apparatus, alternately raised or lowered the object by means of a rope and pulley until the observer indicated the object was just at the perceived jump-withreach height for the self or for the model, depending on the condition. Observers were allowed to fine-tune their responses until they were satisfied. Between estimates, observers closed their eyes while the experimenter reset the apparatus. Successive ascending-descending estimates were averaged as one trial. Each participant completed six such trials for self-estimates and six trials for other-model estimates, with self-and other trials presented in alternating blocks. Block order was counterbalanced across participants. When all trials were completed for the first participant, members of the pair switched roles. At the conclusion of the experimental trials, each participant performed three jumpwith-reach actions in order to empirically determine each participant s action capabilities. Results and Discussion Overall, participants were fairly accurate but exhibited a tendency to underestimate jump-with-reach ability for the self (mean estimate = cm; mean error = mean estimate - actual jump-with-reach height = cm) and for the other actor (mean estimate = cm; mean error = cm). Ratios of the raw estimates to the participants empirically determined actual jump-with-reach heights were calculated with self-estimates scaled by the observers own action capabilities and other-estimates scaled by the model s capabilities. The ratios were near unity for both self- and other-estimates (self: 0.96; other: 0.95). To determine the stability of these estimates over time, an analysis of variance (ANOVA) was conducted on the estimates with judgment type (self vs. model) and trial (trials 1 through 6) as within-subjects factors. The analysis revealed a significant main effect for trial, F(5,95) = 4.62, p <.05, η p 2 = The judgment type trial interaction was also significant, F(5,19) = 4.06, p <.05, η p 2 = 0.17 (see Figure 2). Simple-effects tests revealed a significant effect of trial for self-estimates, F(5,95) = 7.94, p <.05, η p 2 = 0.18, but not for other-estimates. Estimates increased and became more accurate over time for the self, but estimates for the other actor remained constant over time. Figure 2. Results of Experiment 1. Mean perceptual estimates of maximum jump-with-reach height for the self and for the other participant as a function of trials. Bar at right represents the mean actual maximum jump-with-reach height across all participants. Error bars correspond to one standard error. The difference in the self- and other-estimates over time suggests that the ability to perceive possibilities for action afforded by the environment for oneself might be independent of the ability to perceive behaviors afforded for others. Had the percepts been strongly dependent, they would have exhibited similar patterns of change over time. The relative independence of the percepts could be explained by differences in the nature of the information that specifies what is afforded for the self versus what is afforded for the other. Differences in the relative availability of such information might be responsible for the observed results; that is, the lack of improvement in the estimates for others may reflect a lack of information specific to the other agent's ability to jump-with-reach. After all, the other actor simply stood beside the apparatus, without jumping or even extending the arms overhead. While body sway might have provided participants with enough information about their own relation to the environment to allow them to tune to the information that specified their own maximum jump-withreach (cf. Mark et al., 1990), a shift to a different source of information about the other model s relation to the environment might be necessary to allow for an improvement in estimating maximum jump-with-reach for the other person. In Experiment 2 we explored this possibility. We investigated whether participants estimates of maximum jump-with-reach height for the other person changed if access to additional information about the action capabilities of the other person was provided to the observer. Rather than simply allowing the observers to watch the other person jump, however, we allowed observers to watch the model perform a different task that was functionally related (but not identical) to jumping squatting to lift a weight. 2464

4 Experiment 2 Previous findings have demonstrated that information contained in kinematics of a model s movements is sufficient to inform observers about the model s action capabilities (Ramenzoni, Riley, Davis et al., in press). According to the kinematic specification of dynamics (KSD) principle (Runeson, 1983; Runeson & Frykholm, 1983; Runeson, Juslin, & Olsson, 2000) the kinematics and dynamics of an event are specific to one another (i.e., they relate in a 1:1 fashion because the forces and masses lawfully determine the resulting motion according to Newton s laws), from which it follows that kinematic information is a reliable source of information for perceiving facts about the underlying dynamics of an event. The kinematics of a person s movement patterns the patterns of displacement, velocity, and acceleration of the limbs and body segments thus may be informative about the person s action capabilities the person s dynamic capacity to produce force for jumping. How high a person can jump is determined by an actor s ability to produce vertical force impulses relative to the actor s mass. Based on these considerations, we hypothesized that providing perceivers with information about a person s dynamic capabilities even in the context of a behavior that differed from the behavior about which perceivers made judgments, namely a lifting (squatting) task would enable perceivers to tune in to the model s action capabilities and improve their estimates of what is afforded the other actor. Perceivers initially provided two sets of estimates before watching the model perform the lifting task. Then they watched the model repeatedly lift (squat) 10% of his body mass. We expected that observing the actor perform the lifting task could provide information about the actor s ability to produce force with the legs in order to raise a mass (Dowling & Vamos, 1993), and therefore provide information about the actor s ability to lift his own body from the ground during a jump. Estimates provided for the model before and after watching the model lift the mass were compared to estimates provided by a second group of control participants who observed the actor perform a task that did not share underlying dynamics with jumping standing while rotating the torso (twisting about the waist). Based on the results of Experiment 1, we expected to observe a significant improvement in perceptual estimates over time for self-estimates for both the experimental and control groups. We furthermore expected the other-model estimates to become more accurate for the experimental group only, and not simply with the passage of time but only after watching the model perform the lifting task. Method Thirty participants, ranging in height from cm to 196 cm (mean = ± cm) and in maximum jump-withreach height from cm to 298 cm (mean = ± cm), were asked to make jump-with-reach estimates for themselves and a model. The model was 180 cm tall and weighed kg, with a maximum standing reach of cm and a maximum jump-with-reach height of cm. The apparatus from Experiment 1 was used along with a 134 cm long, 11 cm diameter tube containing weights equaling 10% of the model s body mass (8 kg), used by the model when performing the squatting task. The procedures for the second experiment closely followed those in Experiment 1. Participants made estimates about the limits of their own vertical jump-with-reach, as well as for the model. Estimates were again obtained using the method of limits. Participants were given a block of 8 practice trials for each judgment type to familiarize them with the task. Trials were presented in blocks of 4 successive estimates for the self and 4 successive estimates for the model. The order of judgment type conditions (self vs. model) was counterbalanced across blocks and across participants. Two blocks of each estimate condition were administered, for a total of 16 trials. After the first block of experimental trials both the participant and model were led into an adjoining room. The participant and model faced one another, standing 2 m apart. The participant was instructed to watch the model perform either the squatting or torso rotation task, depending on the group assignment for the participant. In the control condition, the participant observed the model who stood rotating his torso from one side to another a total of 15 times. The rotation task did not involve any flexionextension about the ankle, knee, or hip effectively the model stood upright, with the legs straight, and twisted about the waist while holding the tube at chest level with both hands. In the experimental condition, the participant observed the model lifting the tube using a squatting technique. In this condition, the model held the tube at chest level with both hands. While holding the tube, the model lowered his torso by bending at the knees and keeping his back straight until his knees were flexed 90 and his upper legs were parallel to the floor; at this point the actor stopped and raised himself back to a standing position. The model lifted the tube in this manner a total of fifteen times. Just prior to observing the model, every participant lifted the tube once. At the completion of the model s task, both the participant and model returned to the other room and completed the remaining block of estimates. Results and Discussion Estimates from the four repeated trials in each condition for each subject were averaged to yield one mean estimate per condition per subject for both the experimental and control groups (see Figure 3). For the control group there was a tendency to underestimate maximum jump-with-reach height for the self (mean estimate = cm; mean error = mean estimate - actual jump-with-reach height = cm) and the model (mean estimate = cm; mean error = cm). A similar tendency was found for the experimental group in the estimates made for the self (mean estimate = cm; mean error = cm) and the model (mean estimate = cm; mean error =

5 cm). The control group showed greater accuracy for both self and model estimates compared to the experimental group. Ratios of the raw estimates to the participants actual jump-with-reach heights were calculated with self-estimates scaled by the observers own action capabilities and otherestimates scaled by the actor s capabilities. The ratios were at or near unity for both groups. As suggested by the error data, the ratios for the control group (self: 0.95; other: 0.91) indicated they showed overall slightly higher accuracy than the experimental group (self: 0.93; other: 0.89). Figure 3. Results of Experiment 2. Mean perceptual estimates of maximum jump-with-reach height for the self and for the other model as a function of block for the control and experimental groups. Bars at right represent the mean actual maximum jump-with-reach height for the participants (self) and the model (other). Error bars correspond to one standard error. ANOVA was performed with judgment (self vs. other) and block (first block before watching the model and second block after watching the model) as within-subject factors, and group (experimental vs. control) as a betweensubjects factor. The analysis revealed significant main effects for judgment, F(1,28) = 21.93, p <.05, η p 2 = 0.44, and for block, F(1,28) = 27.92, p <.05, η p 2 = The group block [F(1,28) = 6.80, p <.05, η p 2 = 0.20] and the judgment block group [F(1,28) = 5.19, p <.05, η p 2 = 0.16] interactions were also significant. Follow-up ANOVAs were performed to further investigate the significant three-way interaction. ANOVA was performed on the data separately for each judgment type. For the self-estimates there were significantly larger (i.e., more accurate) estimates for the second block compared to the first block, F(1,28) = 11.28, p <.05, η p 2 = 0.29, regardless of group. Analysis of the estimates provided for the model also revealed a main effect for block [F(1,28) = 15.16, p <.05, η p 2 = 0.35] and also a significant block group interaction, F(1,28) = 6.31, p <.05, η p 2 = The interaction was driven by experimental group participants providing significantly larger estimates for the model in the second block compared to the first block, t(14) = -5.95, p <.05. No differences between blocks were found for the estimates provided by the control group. The results showed that participants estimates of maximum jump-with-reach height for the self became more accurate in the second block compared to the first block for both the control and experimental groups. Such an increase in accuracy was predicted based on the results of Experiment 1. The control group s estimates for the model s maximum jump-with-reach height did not change across blocks, also similar to Experiment 1. However, the experimental group participants estimates were more accurate in the second block than in the first. Watching the model perform a task related to the dynamics of jumping (experimental group) helped perceivers tune in to the model s action capabilities. Watching the model perform an unrelated action (control group) did not help participants tune in to the model s action capabilities. These results support the idea that individuals may be able to acquire knowledge about another person from the kinematic patterns produced when the other person moves. This ability seems to be limited to the case of observing movements that are functionally related to the task being judged. That is, kinematic information relating to one class of body movements appears to be informative about another non-identical, but functionally similar, class of body movements. Though the specific force-generation requirements of squatting and jumping are not identical, the underlying dynamics of the two tasks are related insomuch as they both involve the capacity to generate vertical force by contracting the muscles of the legs. Our findings suggest that this similarity was sufficient to allow participants to gain information about the model that is important to performing a vertical jump without actually observing the model jump. General Discussion The results of Experiment 1 suggest that perceivers rely on distinct sources of information when perceiving maximum jump-with-reach height for the self and for another actor. Self-estimates improved over time, even without feedback about the accuracy of the estimates, but other-actor estimates did not. In Experiment 2 it was found that greater accuracy of the estimates for the other actor could be achieved by allowing the perceiver to observe the actor perform a functionally related, but non-identical, action. Being able to perceive the actions another person is capable of performing may be a crucial component of the perception-action processes involved in coordinating social behavior or in predicting what another person is about to do. Different theoretical approaches have been proposed to account for social perception-action processes. Embodied simulation (e.g., Gallese, 2005) casts the problem as one of using one s own action capabilities as a model for apprehending another person s actions. The present study did not provide support for that position, and instead 2466

6 provided evidence for independence of perception of another person s potential actions from perception of possible actions for oneself (Experiment 1). Experiment 2 provided evidence that making available optical information about another person s action capabilities can enhance perception of what is afforded the other person. That result is consistent with information-based approaches to social perception-action (e.g., Ramenzoni et al., 2008). In the present experiments, however, the perceivers and models never engaged in any direct form of social interaction. In future studies it may be interesting to manipulate the degree of interaction between the perceiver and model, as the opportunity for genuine interaction may promote a greater sensitivity of the perceiver to the other person s action capabilities. 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