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. Author manuscript; available in PMC: 2014 Dec 1.
Published in final edited form as: Psychon Bull Rev. 2014 Jun;21(3):676–681. doi: 10.3758/s13423-013-0545-6

My shadow, myself: Cast-body shadows are embodied

Christopher Kuylen 1,2, Benjamin Balas 1,2, Laura E Thomas 1,2
PMCID: PMC4041201  NIHMSID: NIHMS585040  PMID: 24243137

Abstract

Objects that serve as extensions of the body can produce a sensation of embodiment, feeling as if they are a part of us. We investigated the characteristics that drive an object’s embodiment, examining whether cast-body shadows, a purely visual stimulus, can become embodied. Tools are represented as an extension of the body when they enable observers to interact with distant targets, perceptually distorting space. We examined if perceptual distortion would also result from exposure to cast-body shadows in two separate distance estimation perceptual matching tasks. If observers represent cast-body shadows as extensions of their bodies, then when these shadows extend toward a target, it should appear closer than when no shadow is present (Experiment 1) or when a non cast-body shadow is cast toward a target (Experiment 2). We found perceptual distortions in both cast-body shadow and tool-use conditions, but not in our non cast-body shadow condition. These results suggest that, although cast-body shadows do not enable interaction with objects or provide direct tactile feedback, observers nonetheless represent their shadows as if they were a part of them.

Keywords: cast-body shadows, embodiment, perceptual distortion

“I don’t need a friend who changes when I change and who nods when I nod; my shadow does that much better.”

---Plutarch c. 100 AD

Plutarch, whether he was aware of it or not, may have been the first person to realize that the cast-body shadow is unique from other objects in the environment. As long as there is light, our shadows are always with us, extending from and moving with the bodies they resemble. Shadows are linked to the body, but unlike all other parts of the body, they are purely visual, changing with sources of illumination and lacking tactile or proprioceptive sensors. Although cast-body shadows bear a clear relationship to the physical body, it is unclear whether observers represent their shadows as a part of their own bodies. If body shadows are embodied—that is, if observers process properties of their shadows in the same way as properties of their own bodies under similar spatial, motor, and affective circumstances (de Vignemont, 2011)—then a shadow cast beyond the boundaries of the physical body should bias observers to experience themselves as extending further into the environment. A number of objects that are physically distinct from the body such as allografts (Dubernard et al., 2003; Farnè, Roy, Giraux, Dubernard, & Sirigu, 2002), prostheses (Lotze et al., 1999; Murray, 2004), rubber hands (Botvinick & Cohen, 1998; Tsakiris & Haggard, 2005), and tools (Cardinali et al., 2009a; Farnè & Làdavas, 2000; Maravita & Iriki, 2004) can become embodied in this sense, stretching the impression of occupying space beyond the physical body’s boundaries via incorporation into or extension of the body schema (see Botvinick, 2004; De Preester & Tsakiris, 2009; Legrand, 2009; Thompson and Stapleton, 2009 for differences between incorporation and extension.) In these cases, embodiment typically results from objects providing both visual and tactile feedback to the observer that potentially enables action (e.g., Murray, 2004; Ramachandran & Rogers-Ramachandran, 1996; Yamamoto, Moizumi, & Kitazawa, 2005). Like tools and other objects that can be embodied, shadows visually extend beyond the body, but unlike these objects, shadows are physically tenuous. Movement creates visual-motor synchronies between the cast-body shadow and the physical body, but shadows themselves exist only as a two-dimensional projection. While people can act with tools or prostheses, feeling when these objects come into contact with other objects, they cannot experience proprioceptive or tactile feedback via cast-body shadows. Can objects that never enable action become embodied?

We leverage a well-documented consequence of the embodiment of tools (e.g., Cardinali et al., 2009a; de Vignemont; 2011)—altered spatial perception—to determine if the cast-body shadow is also embodied. When people use tools to reach toward and interact with a distant object, the tool acts as an extension of the body, leading observers to perceive the object as significantly closer to them (Witt, Proffitt, & Epstein, 2005; Witt & Proffitt, 2008). This occurs when using a tool to interact with objects just beyond reach, presumably because tool use expands peripersonal space (Cardinali, Brozzoli, & Farnè, 2009b; Farnè & Ladavas, 2000). Neural evidence supports this notion; bimodal neurons of the macaque monkey that code for somatosensation and vision increase the size of their visual receptive fields to include the area now within reach of the tool immediately following tool use (Maravita & Iriki, 2004). Behavioral results also support these findings; attention normally observed for space around the hand (Kennet, Spence, & Driver, 2002) shifts to the functional end of a tool following its use (Farnè, Iriki, & L davas, 2005). Tools can also alter spatial perception (Davoli, Brockmole, & Witt, 2012) when they are used to interact with targets well beyond the boundaries of peripersonal space (Rizzolatti, Fadiga, Fogassi, & Gallese, 1997), suggesting that the ability to interact with an object at any distance shrinks the perceived distance between object and observer. To determine if cast-body shadows are also embodied despite their inability to provide tactile feedback or enable action, we investigated whether cast-body shadows are perceived as an extension of the body that perceptually distorts space.

If shadows are represented as if they are a part or extension of the body, then the presence of a cast-body shadow extending toward a target should reduce the distance observers perceive between themselves and that target. However, if objects must provide a combination of visual and tactile feedback that enables goal directed action to become embodied, then extending a tool toward a distant target should lead observers to underestimate their distance to the target, whereas casting a body shadow toward the same target should not distort perception.

Experiment One

To determine if cast-body shadows are capable of becoming embodied, we implemented a design with three experimental conditions: (1) cast-body shadow present, (2) laser pointer present, and (3) baseline. Participants made distance estimations using a perceptual matching paradigm in which they indicated when an experimenter was the same distance away from them as a previously viewed stationary target1. We included a laser pointer condition in our design because previous work has shown that remote tool use can alter spatial perception (Davoli et al., 2012) and we wished to ensure our matching paradigm could document an established case of perceptual distortion resulting from embodied tool use as we examined the influence of cast-body shadows on perception of space. In our baseline condition, we asked participants to make distance estimations without the presence of a cast-body shadow or laser pointer. We anticipated replicating previous findings, hypothesizing participants would provide smaller distance estimations in the laser pointer condition than in the baseline condition. If cast-body shadows can also become embodied, then distance estimates in this condition should also be smaller than in the baseline case. Our results indicated that, compared to baseline, the cast-body shadow and laser pointer conditions resulted in an underestimation of distance, providing the first evidence that the cast-body shadow—a purely visual stimulus—is embodied.

Method

Twelve naïve undergraduate participants were brought into a large gym with strong natural lighting on NDSU’s campus and performed our task in three experimental conditions (cast-body shadow present, laser pointer, baseline). Task order was balanced across participants. In all three conditions, participants stood on a piece of carpet (3′9″× 60′6″) that extended away from them and turned away while the experimenter placed a target (an athletic cone) on the carpet at one of eight distances (20–55 feet away in 5 foot increments). The experimenter then walked back to the participant, and asked them to turn toward the target, stand still, and direct their view down the length of the carpet to look at the target. After observing the target’s distance, participants were instructed to direct their view orthogonally from the length of the carpet, observe the experimenter walk away from them without looking back at the target, and tell the experimenter to stop when they thought he was the same distance away from them as the previously viewed target. The experimenter recorded distance estimations with a rolling measuring wheel (Keson RR182). Participants viewed the target at each of the eight distances twice for each condition.

In the baseline condition, participants performed the distance estimation task under natural lighting. To determine if cast-body shadows perceptually distort space, a 500 watt photoflood lamp encompassed in a 20″ collapsible uLite Softbox sitting atop a 6.5′ light stand was placed directly behind the participant and turned on, casting a body shadow that extended down the length of the carpet toward the target but did not visually overlap with the target. We included a third condition in which participants used a laser pointer to interact with the target to examine the sensitivity of the perceptual matching paradigm in showing tool-driven alterations in spatial perception. Our laser pointer condition was identical to the baseline condition, except that when participants faced the target, they shined a laser pointer on the target and held the pose while facing the experimenter as he walked away from them.

Upon finishing the study, participants completed a posttest questionnaire designed to probe for the presence of demand characteristics. This questionnaire asked participants what they thought the experimenter was investigating, what the purpose of the study was, and what the results of the study would be. Responses on the posttest questionnaire indicated that all participants were unaware of our hypotheses and did not anticipate experiencing changes in perception across conditions.

Results

Figure 1 displays the mean estimated distances across conditions. We analyzed these distances in a repeated measures analysis of variance (ANOVA) with condition (shadow present, laser pointer, baseline) and distance in feet as within-subjects factors. Both condition (F(2, 22)=17.71, p<.001, η2=.617) and physical distance (F(7, 77)=500.70, p<.001, η2=.979) influenced reported distances and these factors interacted with each other (F(14, 154)=1.87, p=.032, η2=.146). Bonferroni post-hoc comparisons revealed that participants’ estimates were significantly different across shadow present and baseline conditions (M=−2.083, SE=.432, CI [−3.302, −.864], p=.002) and laser pointer and baseline conditions (M=−2.438, SE=.550, CI [−3.989, −.886], p=.003), but not the shadow present and laser pointer conditions (M=.354, SE=.314, CI [−.531, 1.239]. These results suggest that both shining a laser pointer at a distant target and casting a body shadow toward a target alter space perception.

Figure 1.

Figure 1

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Mean estimated distance from target for the baseline, shadow present, and laser pointer conditions. Error bars represent one standard error of the mean.

Discussion

The results of Experiment 1 suggest that both the cast-body shadow and laser pointer act as extensions of the physical body that shape perception of space. The perceptual distortion we found in the laser pointer condition aligns with previous findings of distortion in verbal estimate experiments (Davoli et al., 2012), pointing to the effectiveness of our perceptual matching paradigm in studying these effects. Our results demonstrate that a visual object that affords no interaction—the body shadow—is capable of perceptually distorting space. However, we cannot conclude that the cast-body shadow is unique in its influence on space perception based on the results of Experiment 1. We posit that participants perceived the target as closer to them in the shadow condition because they represented their shadows as a part of themselves that extended toward the target, but this leaves open the possibility that any visual trace extending from an observer to a target may likewise influence perception. In Experiment 2, we examine the influence of shadows that do not originate from the caster’s body on distance perception.

Experiment Two

To investigate how the presence of a visual signal extending toward a target influences distance perception, we ran a second design with three experimental conditions: (1) cast-body shadow present, (2) non cast-body shadow present, and (3) baseline. Participants performed the same task as in Experiment 1, but instead of using a laser pointer, they made distance estimates in a condition where a large file cabinet was placed directly behind them that cast a blocky shadow down the length of the carpet. If any visual trace present in the space between participants and the target can influence distance perception, then participants should estimate shorter distances in both the cast-body and non cast-body shadow conditions than in the baseline condition. However, if the cast-body shadow altered perception in Experiment 1 specifically because it was embodied, we expect distance estimates in the cast-body shadow condition to be shorter than in either the non cast-body shadow or baseline conditions. Our results supported this prediction, providing additional evidence that the cast-body shadow—and only the cast-body shadow—is embodied.

Method

Fourteen naïve undergraduate participants were brought into a hallway of the psychology department at NDSU and performed our task in three experimental conditions: (cast-body shadow, non cast-body shadow, baseline). The design was identical to experiment one, with the exception that a smaller space led us to test distances in 4-foot increments (16–44 feet).

In the non cast-body shadow condition, participants performed the distance estimation task in the presence of a shadow that was not their own. A file cabinet behind participants cast a shadow down the length of the carpet that encompassed the participant’s own shadow. By comparing performance in this condition against the cast-body shadow condition, we are able to determine if any condition that creates a visual signal extending toward a target alters perception. After finishing the study, participants completed the same posttest questionnaire as in Experiment 1. Responses on these questionnaires suggested participants were unaware of our hypotheses.

Results

Figure 2 displays the mean estimated distances across conditions. We analyzed these distances in a repeated measures analysis of variance (ANOVA) with condition (cast-body shadow present, non cast-body shadow preset, baseline) and distance in feet as within subject factors. Both condition (F(2,26)=4.191, p<.05, =.244, η2=.244) and physical distance (F(7,91)=306.314, p<.001, η2=.959) influenced reported distances, but these factors did not interact with each other. Bonferroni post-hoc comparisons revealed that participants estimated significantly shorter distances in the cast-body shadow condition than either the non cast-body shadow condition (M=−1.763, SE=.523, CI[−3.199, −.327], p=.015) or the baseline condition (M=−1.790, SE=.574, CI[−3.367,−.213], p=.024), but did not differ in their estimates for the latter conditions (M=−.027, SE=.951, CI[−2.637, 2.583]. These results show that the cast-body shadow perceptually distorts space, but a similar shadow not originating from the body does not.

Figure 2.

Figure 2

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Mean estimated distance from target for the baseline, cast-body shadow present, and non cast-body shadow present conditions. Error bars represent one standard error of the mean.

Discussion

The target in Experiment 2 appeared closer to participants only when their own shadows extended toward the target, suggesting that visual signals only induce perceptual distortions when there is a link between these signals and observers’ bodies. The shadow of the cabinet extended from a participant’s body, but did not resemble the body’s shape or mirror its movements. The cabinet’s shadow did not alter space perception because participants presumably did not represent the cabinet’s shadow as if it were a part or extension of their own body. However, as in Experiment 1, we found that when observers saw their own shadow extending to a target, this personalized visual signal made the target appear closer

General Discussion

Our results provide the first evidence that cast-body shadows are represented as an extension of the physical body that alters space perception. Although cast-body shadows share characteristics with other objects that are capable of becoming embodied, bearing anatomical resemblance to the body like allografts, prostheses, and rubber hands, in these cases of embodiment, a combination of visual and tactile perception drives the sense of personal ownership observers feel over them (e.g., Durgin, Evans, Dunphy, Klosterman, & Simmons, 2007; Murray, 2004; Ramachandran & Rogers-Ramachandran, 1996). And while tools need not resemble the body in order to feel as if they are a natural extension of it, like allografts, prostheses, and rubber hands, combined visual-tactile feedback is key to the process of their embodiment (Yamamoto et al., 2005). The cast-body shadow, however, only provides visual feedback, serving as a unique example of embodiment in the absence of tactile sensation.

While cast-body shadows are similar to tools in that they extend from our body and can modify the representation of perceived space, unlike tools, cast-body shadows cannot enable actions on the environment. Recent theories on the relationship between action and perception suggest that action capabilities and intentions have direct and immediate effects on perception (Proffitt, 2006; Witt, 2011). A variety of empirical evidence supports this perception for action account (e.g., Davoli et al., 2012; Linkenauger et al., 2009; Thomas, Davoli, & Brockmole, 2013; Witt & Proffitt, 2008; Witt et al., 2005) and in our laser pointer condition we replicated the basic effect of an interaction-driven change in perception. We do not wish to dispute the idea of perception for action, but rather to expand the notion of embodiment-based perceptual distortion beyond situations involving target-focused action. Researchers have recently argued that potential expansions of the embodied cognition literature should focus on the role played by the body independent of its role in goal-directed actions (Borghi & Cimatti, 2010). Our work provides a first step in this direction, showing that an extension of the physical body that inherently lacks the capability to interact with objects or complete a goal-directed action can still become embodied.

What characteristics must an object possess to be embodied? Imagined tool use in the absence of visual feedback creates perceptual distortion (Davoli et al., 2012; Witt & Proffitt, 2008), suggesting that motor simulations of interactions can make users feel as if the tool is an extension of their body, presumably via the same patterns of neural activations that occur during real interactions (e.g., Creem-Regehr & Lee, 2005; Higuchi, Imamizu, & Kawato, 2007). Visual feedback that induces illusory tactile sensations can lead observers to incorporate a rubber hand into the body schema (Durgin et al., 2007), suggesting that imagined or illusory synchronous visuo-tactile feedback is also sufficient to create a sense of embodiment. However, it is impossible to act with or receive tactile feedback from one’s shadow, restricting the ability to engage in shadow motor simulations. Yet we have shown that cast-body shadows distort perception as if they are embodied, demonstrating that observers can process an object as if it were a part of them even when that object can never provide direct tactile feedback or enable interaction.

Our results suggest the cast-body shadow was represented as a part of the self that stretched beyond the physical body, shrinking the apparent distance between a target and an observer. Our work shows that observers’ own shadows alter perception in a manner consistent with other extensions of the body, but other non-cast body shadows do not. Shadows may alter observers’ perception of the world because of their strong anatomical resemblance to the physical body, causing observers to process the location of the cast-body shadow in the same way they would process the location of a part of their own body (de Vignemont, 2011). Pavani and Castiello (2004) have shown that visual distractors presented at the location of a cast-hand shadow produce cross modal interference on a tactile judgment task that is similar to conditions in which the visual distractors are presented on the hands themselves, also suggesting that the brain automatically codes cast-body shadows as related to the body itself (Maravita, 2006).

While participants in our study remained stationary during the shadow present condition, it is also possible that subtle synchronies between visual movement of their shadows and small proprioceptive signals—or previous experience with these synchronies—could have been the driving influence behind the embodiment of cast-body shadows. Indeed, Holmes and Spence (2006) propose that one effect of cast-body shadows may be to bind extrapersonal visual events to simultaneously occurring proprioceptive events. Yet previous work on embodiment-driven perceptual distortion (e.g., Davoli et al., 2012; Witt et al., 2005) relies on action as a key theoretical and methodological component. Participants in these designs are explicitly instructed to act upon some object(s) in their environment, drawing attention not only to that object, but also to the implement that is used to interact with that object (Farnè et al., 2005). Our cast-body shadow conditions strip this distance perception research of its action component, and given the responses on our posttest questionnaires, it is unlikely that participants in our study were paying attention to their shadow. Regardless of which characteristics drive the embodiment of cast-body shadows, our work points to the usefulness of these objects in advancing understanding of when, and how, observers come to feel as if an object is a part of them.

Acknowledgments

ND EPSCoR and NSF grant #EPS-0814442 provided financial support for this research. This research was also supported by NIGMS #P20 GM103505. We thank Jeney Anderson for providing us with the gym space.

Footnotes

1

We chose to use a matching paradigm to avoid the heteroscedasticity that commonly accompanies verbal estimates of distance.

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