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The effect of manipulability and religion on the multisensory integration of objects in peripersonal space Michiel van Elk

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Department of Social Psychology , University of Amsterdam , Amsterdam , The Netherlands Published online: 01 Jul 2013.

To cite this article: Cognitive Neuroscience (2013): The effect of manipulability and religion on the multisensory integration of objects in peripersonal space, Cognitive Neuroscience, DOI: 10.1080/17588928.2013.808612 To link to this article: http://dx.doi.org/10.1080/17588928.2013.808612

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COGNITIVE NEUROSCIENCE, 2013, http://dx.doi.org/10.1080/17588928.2013.808612

The effect of manipulability and religion on the multisensory integration of objects in peripersonal space Michiel van Elk

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Department of Social Psychology, University of Amsterdam, Amsterdam, The Netherlands

In this study participants were required to respond to vibrotactile stimuli applied to the hand while ignoring visual distractors superimposed on pictures representing Christian, Hindu, or profane objects that were categorized as manipulable or non-manipulable. Overall, participants responded slower when the visual distractor appeared at an incongruent location with respect to the vibrotactile stimulus, which is known as the crossmodal congruency effect (i.e., CCE). The CCE was modulated by the type of object involved (i.e., Christian, Hindu, or Profane), the object manipulability (i.e., manipulable vs. non-manipulable) and the religious background of the participant (i.e., Christian, Hindu, or non-religious). The finding that both object manipulability, the religious significance of the object, and the religious background of the participant have a combined effect on multisensory integration suggests important interactions between low-level body-object integration and the symbolic extension of the self.

Keywords: Religious objects; Extended self; Manipulable objects; Multisensory integration; Crossmodal Congruency Task.

In daily life we use many objects that have become a natural extension of our body and greatly extend our bodily capabilities, such as glasses, shoes, cars, or hammers. Recent discoveries in cognitive neuroscience have provided direct evidence for the notion that the self can be extended towards external objects and that the possibility to manipulate the object is a key feature determining body-object integration (Goldenberg & Iriki, 2007; Holmes, 2012; Maravita & Iriki, 2004). It has been found, for instance, that the visual receptive field size of parietal neurons of the macaque monkey increased after tool use (Iriki, Tanaka, & Iwamura, 1996). In humans, similar effects of tool use on the extension of peripersonal space have been observed (for review, see e.g., Maravita & Iriki, 2004). A task that has often been used to measure the extension of peripersonal space is the crossmodal congruency task in which participants respond to tactile vibrations while ignoring visual distractor stimuli

(Aspell, Lenggenhager, & Blanke, 2009; Spence, Pavani, Maravita, & Holmes, 2004; van Elk & Blanke, 2011). Typically, participants respond slower if the visual distractor stimulus appears at a different vertical location than the tactile stimulus, which is known as the crossmodal congruency effect (CCE). The CCE provides a direct measure of multisensory integration in peripersonal space and it has been shown that the CCE is enhanced for objects that can be easily integrated in our body representation, such as rubber hands (Pavani, Spence, & Driver, 2000; Tsakiris & Haggard, 2005), everyday objects that can be easily manipulated with one’s hands (van Elk & Blanke, 2011), and even virtual bodies (Aspell et al., 2009). Most experimental studies on object use have used relatively simple manipulable tools, devoid of any personal, symbolic, or cultural significance. However, it is specifically the fact that objects have a symbolic meaning which underlies the notion of the extended self (Belk,

Correspondence should be addressed to: Michiel van Elk, University of Amsterdam, Department of Social Psychology, Weesperplein 4, 1018 XA Amsterdam, The Netherlands. E-mail: [email protected]

© 2013 Taylor & Francis

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1988; James, 1891, vol. 1, chap. 10). According to the idea of the “extended self,” our sense of who we are is determined to a strong extent by what we have and the objects that we own (Sirgy, 1982). The symbolic value of objects is well exemplified by cultural and religious objects (e.g., consecrated host during the Eucharist, the use of prayer beads, etc.). It has been found, for instance, that when people are reminded of their mortality, they are more reluctant to use cultural or religious objects in an inappropriate fashion, likely because these objects represent their worldview (Greenberg, Porteus, Simon, Pyszczynski, & Solomon, 1995). The symbolic significance of religious objects was already noted by Émile Durkheim, who stated that the sacred and the profane are inherently different classes in religious thinking (1976, p. 38). Durkheim argues that the part-whole relationship of the sacred object is what distinguishes it from profane objects. Sacred objects have an intrinsic value, such that through their part-whole relationship they represent something bigger than the object itself. As such, religious objects often exemplify a way of magical thinking (Rozin, Millman, & Nemeroff, 1986), as evidenced, for instance, by the worship of remains and relics of saints that are on display in many religious shrines or the national flag of the USA. In sum, one line of research suggests that objects can functionally extend the representation of our body, while another line of research indicates that objects can symbolically extend the self, as in the case of personal, religious, and cultural objects. The aim of the present study was to investigate to what extent the functional properties of an object are dependent upon its symbolic significance. More specifically, the relative importance of both the cultural familiarity and the symbolic significance for the integration of objects in the body representation was investigated—similar to the process described for relatively simple tools (Maravita & Iriki, 2004). In line with a previous study—showing that object manipulability is a key feature determining body-object integration (van Elk & Blanke, 2011)—in the present study both manipulable and nonmanipulable objects were included1. In addition, object

1

Manipulability is primarily defined in terms of the action associations of an object. Some objects are strongly associated with many different actions and are frequently interacted with (e.g., a cup), whereas other objects have less potential for bodily interaction (e.g., a bookend; for a similar distinction, see Rueschemeyer, Pfeiffer, & Bekkering, 2010). Thus, although some non-manipulable objects used in the present study can be grasped as well, these objects are less strongly associated with specific bodily actions than manipulable objects. The classification according to object manipulability is supported by the object rating data as well (see below), indicating that participants rated manipulable objects as easier to manipulate than non-manipulable objects.

pictures could represent either Christian, Hindu, or Profane objects. Given that a majority of the participants had either a Christian or a Hindu background, in this way it could be investigated whether the effect of object manipulability was further modulated by the cultural familiarity of the object. Body-object integration was measured by the crossmodal congruency task, in which participants responded to tactile vibrations applied to their hand while ignoring visual distractors superimposed on the object (Aspell et al., 2009; Spence, Pavani, Maravita, et al., 2004; van Elk & Blanke, 2011).

METHODS Participants In total 31 people participated in the experiment (eight females, mean age ¼ 22.2 years), who were all students at the Swiss Federal Institute of Technology, Lausanne, Switzerland. Participants declared themselves through informal verbal inquiry to be right-handed. The study was conducted in accordance with the declaration of Helsinki. The experiment was approved by the local ethics committee: La Commission d’ethique de la recherché Clinique de la Faculté de Biologie et de Médecine - at the University of Lausanne, Switzerland. All subjects verbally gave informed consent prior to participation and were fully debriefed after the experiment. Owing to the non-invasive, purely behavioral nature of our study, the ethics committee considered verbal consent was appropriate and approved this consent procedure.

Stimuli As stimuli pictures of manipulable and nonmanipulable objects were selected. Objects could be Catholic, Hindu, or Profane (see Figure 1 for example stimuli). The classification of objects as belonging to the different religions was based on publicly available internet resources, such as Wikipedia, and through informal conversations with religious persons. As profane objects, antique or old objects were selected that would resemble the religious objects in visual appearance. Each object was scaled according to its size and placed on a white background subtending 400 x 400 pixels. In a pre-test involving the ratings by 15 participants who did not participate in the main experiment, objects within each religious category were matched for familiarity and visual complexity, resulting in a total of eight objects per category (see Table 1). The pre-test also established that participants were well able

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EXAMPLE STIMULI AND EXPERIMENTAL SETUP AND PROCEDURE

Figure 1. Upper part: Example stimuli used in the experiment. Objects could be Christian (upper row), Hindu (middle row) or Profane (lower row) and could be manipulable (left column) or non-manipulable (right column). Lower part: Experimental setup and procedure. Subjects were seated behind a table, facing a computer screen. Tactile vibrators were attached to the thumb and index finger of the subject’s right hand and the subject responded with the left hand by pressing one of three buttons on a button box. Each trial started with a fixation cross (1st panel from left), followed by the presentation of an object picture (e.g. a monstrance; 2nd panel from left), visual distractor and tactile stimulation (3rd panel from left) and an object classification task (right panel). Subjects responded by indicating whether the thumb or index finger was stimulated (Button press 1) and by classifying the object according to its religion (Button press 2).

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TABLE 1 Objects used in the experiment (upper panel) and ratings for the different stimulus categories (lower panel)

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NonManipulable manipulable Christian

Chalice Monstrance Dikirion Censer Blessing cross Altar bell Pyx Chaplet

Crucifix Ciborium Icon Analogion Holy water font Prié dieu Retable Botafumeiro

Hindu

Dhunachi Diya Flywhisk Garland Ghanta Karatalas Rudraksha Shankha

Kalasha Lingam Murti Rangoli Sarpa Kavu Diva lamp Lingam Hamsa hand statue

Profane

Cup Hairpin Jewellery pot Pocketknife Necklace Teapot Pen gift box

Candle Fire basket Clock

Hammer

World globe Match cup Wooden box Tower of Pisa souvenir Eiffel Tower souvenir

Christian Manipulability Familiarity Visual Complexity Usage

2.1 (.51) 2.7 (.68) 3.0 (.72) 4.2 (.50)

2.9 (.41) 2.6 (.72) 3.1 (1.1) 4.3 (.37)

Hindu Manipulability Familiarity Visual Complexity Usage

1.9 (.44) 3.9 (.48) 2.5 (.76) 4.2 (.48)

2.7 (.93) 3.8 (.74) 2.9 (.71) 4.2 (.42)

Profane Manipulability Familiarity Visual Complexity Usage

1.5 (.23) 2.6 (.88) 2.2 (.81) 3.1 (.86)

2.1 (.26) 2.1 (.65) 2.3 (.71) 3.6 (.60)

Notes: Manipulability ratings (1st row; 1 ¼ easy to manipulate, 5 ¼ difficult to manipulate), familiarity ratings (2nd row; 1 ¼ familiar, 5 ¼ unfamiliar), visual complexity ratings (3rd row; 1 ¼ visually simple, 5 ¼ visually complex), and frequency of usage ratings (4th row; 1 ¼ used in a daily fashion, 5 ¼ rarely used) are represented for the different object categories. Standard errors are between brackets.

to classify objects according to the category and that manipulable objects were rated as more easy to manipulate than non-manipulable objects (see Table 1).

Experimental setup and procedure An overview of the experimental setup and procedure is represented in Figure 1 (for a similar setup, see: van Elk & Blanke, 2011). During the experiment a white fixation cross and two asterisks, presented 70 pixels above and below the fixation cross, were continuously visible on the screen (refresh rate: 70Hz). At the beginning of each trial a fixation cross was presented for 2000–3000ms, followed by the presentation of an object picture. Five hundred milliseconds after the onset of the picture a visual distractor was presented for 100ms (i.e., one of the asterisks turned red) followed by a vibrotactile stimulation of the participant’s index finger or thumb for 100ms (i.e., there was a temporal delay of 100ms between the visual distractor and the tactile stimulation). Each object picture was presented twice with a congruent visuotactile stimulation and twice with an incongruent visuotactile stimulation. Participants were required to indicate whether the tactile stimulation was applied to their thumb or index finger by pressing the left or the right button of the response box respectively with their left hand. After the participant responded the picture disappeared from the screen and three words appeared on the screen referring to the three object categories used in the experiment (i.e., Catholic, Hindu, or Profane). The participant was required to indicate to which religion the object belonged by pressing one of three buttons with their left hand. The position of the words referring to the different categories was randomized from trial to trial, in order to prevent the participant from preparing the classification response already during the CCE interval. In total, the experiment consisted of 192 trials plus an additional 16 practice trials at the beginning of the experiment. The experiment was implemented using Presentation software (NeuroBehavioral Systems, California). At the end of the experiment participants completed a questionnaire. Participants were required to indicate their country of origin, their native language and their religious background. In addition, they were required to indicate their religiosity, their frequency of church / temple / mosque visit, and their use of religious objects.

Analysis For the analysis, trials with incorrect responses and trials that exceeded the subject’s mean by more than two standard deviations were excluded from analysis. To control for speed accuracy trade-offs reaction times and error rates were combined in one measure, the inverse efficiency (IE), by dividing the reaction times

EXAMPLE STIMULI AND EXPERIMENTAL SETUP AND PROCEDURE

by the proportion of correct trials per condition (Schicke, Bauer, & Roder, 2009; Spence, Kingstone, Shore, & Gazzaniga, 2001). Data were analyzed using a repeated measures ANOVA with the factors Category (Christian, Hindu, Profane), Manipulability (Manipulable vs. Non-manipulable) and Congruency (Congruent vs. Incongruent visuotactile stimulation). Analysis focused on differences in the cross-modal congruency effect (CCE; i.e., the difference between incongruent and congruent trials; cf. Spence, 2011) between the different stimulus categories.

TABLE 2 Behavioural data in response to the crossmodal congruency task Congruent Incongruent Reaction Times

Error Rates

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Background of participants Ten participants reported having a Christian background (Catholic or Orthodox), nine participants had a Hindu background, four participants came from a Muslim background and eight participants reported having no specific religious background. On average, participants considered themselves not to be very religious (2.6, SD ¼ 1.6; 1 ¼ not religious at all, 7 ¼ very religious), they did not very often visit a church, temple or mosque (4.9, SD ¼ 1.9; 1 ¼ very often, 7 ¼ never), and did not very often use one of the objects that were presented in the experiment (5.5, SD ¼ 1.9; 1 ¼ very often, 7 ¼ never). Most Hindu participants reported using some of the Hindu objects both during their temple services and at home. In addition, two catholic participants reported using icons, candles, and a prié dieu (i.e., prayer desk) in the church.

CCE results The reaction times, error rates, and the inverse efficiency data to the crossmodal congruency task are represented in Table 2. All statistical analyses were conducted using the inverse efficiency (IE), as this measure directly controls for eventual speed-accuracy trade-offs (Schicke et al., 2009; Spence et al., 2001). On average, 6.0% of all trials was excluded due to incorrect responses and 4.8% of all trials was excluded due to outliers. A main effect of Religion, F(2, 60) ¼ 18.3, p < .001, η2 ¼ .38, reflected that reaction times were modulated according to Category: Hindu objects were responded to slower (1117ms) than Christian objects (1094ms) and Profane objects (1038ms). A main effect of Manipulability, F(1, 30) ¼ 44.5, p < .001, η2 ¼ .60, reflected faster reaction times to non-manipulable

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Inverse Efficiency

Inverse Efficiency

Christian Manipulable Non-manipulable Hindu Manipulable Non-manipulable Profane Manipulable Non-manipulable Christian Manipulable Non-manipulable Hindu Manipulable Non-manipulable Profane Manipulable Non-manipulable Christian Manipulable Non-manipulable Hindu Manipulable Non-manipulable Profane Manipulable Non-manipulable

Christian Manipulable Non-manipulable Hindu Manipulable Non-manipulable Profane Manipulable Non-manipulable

1036 (53) 981 (40)

1166 (41) 1170 (44)

1070 (42) 991 (43)

1203 (43) 1178 (40)

1025 (45) 959 (43)

1193 (43) 960 (41)

.08 (.03) .08 (.04)

.85 (.18) .67 (.16)

.03 (.02) .04 (.02)

1.1 (.21) .78 (.16)

.08 (.04) .08 (.06)

.99 (.25) .03 (.02)

1038 (53) 983 (41)

1177 (42) 1178 (45)

1070 (42) 992 (43)

1217 (45) 1188 (41)

1026 (45) 960 (43)

1206 (45) 961 (41)

Up

Down

1040 (53) 1042 (42)

1097 (57) 1054 (48)

1050 (50) 1053 (42)

1127 (54) 1072 (45)

1041 (46) 958 (43)

1044 (48) 914 (38)

Notes: The 1st panel of the table represents Reaction Times, the 2nd panel Error Rates, and the 3rd panel the Inverse Efficiency data for the different experimental conditions. The left column represents responses to congruent visuo-tactile stimulation, the right column represents responses to incongruent visuo-tactile stimulation. The 4th panel of the table represents the Inverse Efficiency Data according to whether the visual distractor stimulus appeared at the upper (left column) or the lower part of the screen. Standard errors are between brackets.

objects (1044ms) compared to manipulable objects (1122ms). A main effect of Congruency, F(1, 30) ¼ 76.2, p < .001, η2 ¼ .72, reflected faster reaction times to congruent (1011ms) compared to incongruent visuotactile stimulation (1155ms). An interaction between Category and Manipulability, F(2, 60) ¼ 17.2, p < .001, η2 ¼ .37, reflected that the

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VAN ELK TABLE 3 Classification of objects according to category (upper panel) and according to the religious background of the participant (middle and lower panel)

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Christian Objects Hindu Objects Profane Objects

Figure 2. Crossmodal congruency effect. Bars represent the crossmodal congruency effect (i.e. the difference in inverse efficiency between incongruent and congruent visuotactile stimulation) for the different stimulus categories: Christian objects (left bars), Hindu objects (middle bars) and Profane objects (right bars). Dark bars represent manipulable objects and light bars represent non-manipulable objects. Error bars represent standard errors.

difference between manipulable and non-manipulable objects was strongest for Profane (difference ¼ 155ms) compared to Christian (difference ¼ 27ms) and Hindu objects (difference ¼ 53ms). An interaction between Category and Congruency, F(2, 60) ¼ 7.6, p < .001, η2 ¼ .20, reflected stronger congruency effects for Christian objects (congruency effect ¼ 167ms) and Hindu objects (congruency effect ¼ 171ms) compared to Profane objects (congruency effect ¼ 101ms). Finally a significant three-way interaction was found between Category, Manipulability, and Congruency, F (2, 60) ¼ 15.6, p < .001, η2 ¼ .34. This interaction reflected that the CCE (i.e., the difference between incongruent and congruent trials) was modulated by both Category and Manipulability (see Figure 2). To explore this three-way interaction, three separate ANOVAs were conducted with the factors Manipulability and Congruency for Christian, Hindu, and Profane objects. For Christian objects, a marginally significant interaction was found between Manipulability and Congruency, F(1, 30) ¼ 3.6, p ¼ .07, η2 ¼ .11, indicating a stronger CCE for Christian non-manipulable objects (CCE ¼ 196ms) compared to Christian manipulable objects (CCE ¼ 140ms). For Hindu objects, the interaction between Manipulability and Congruency did not reach statistical significance, F (1, 30) ¼ 2.2, p ¼ 2.2, η2 ¼ .15. For Profane objects, a significant interaction between Manipulability and Congruency, F(1, 30) ¼ 32.0, p < .001, η2 ¼ .52, reflected a stronger CCE for profane manipulable objects (CCE ¼ 181ms) compared to non-manipulable objects (CCE ¼ 1ms).

Participant: Christian Hindu Other Participant: Christian Hindu Other

Classification Christian

Hindu

Profane

65% (2) 16% (1) 11% (1)

14% (1) 69% (2) 25% (2)

21% (1) 15% (2) 64% (2)

1046 (73) 1016 (77) 1192 (67)

1117 (65) 986 (69) 1215 (59)

1010 (70) 951 (74) 1127 (64)

26% (1.4) 23.5% (1.5) 25.3% (1.3)

16.8% (1.1) 18.5% (1.3) 23.3% (1.2) 22.8% (1.4) 17.4% (1.0) 21.7% (1.2)

Notes: The upper panel reflects the percentage of objects that were classified according to each category (i.e., Christian, Hindu, Profane). The middle panel represents the reaction times (ms) according to the religious background of the participant and the object involved. The lower panel reflects the percentage of correctly classified objects according to the religious background of the participant and the object involved.

Classification of objects As can be seen in Table 3, overall participants were well able to determine to which religion objects belonged. On average, participants correctly classified about two-third of all objects. Misclassifications occurred for all three categories: Religious objects were assigned to the incorrect religion or were categorized as profane. Vice versa: Some profane objects were classified as religious. As expected, analysis of the classification data using a repeated measures ANOVA revealed an interaction between Category and Classification, F(4, 120) ¼ 227.6, p < .001, η2 ¼ .94, indicating that overall participants assigned objects to the correct category.

Post-hoc analysis: Religiosity & religious background When using participant’s score on the religiosity and the object use scale as a covariate, self-reported religiosity interacted with the Category * Manipulability * Congruency interaction, F(2, 58) ¼ 3.6, p < .05, η2 ¼ .11. Similarly, self-reported object use interacted with the Category * Manipulability * Congruency interaction, F(2, 58) ¼ 3.3, p < .05, η2 ¼ .10. These interactions reflected that religious participants showed a stronger CCE for manipulable religious objects compared to non-manipulable religious objects, whereas no such

EXAMPLE STIMULI AND EXPERIMENTAL SETUP AND PROCEDURE

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relation was observed for profane objects. Self-reported religiosity and object use did not interact with the classification of objects (F < 1). When using religious background (i.e., Christian, Hindu, or Other) as an additional between-subjects variable, for the reaction times in the CCE task an interaction was found between Religious Background and Category, F(4, 56) ¼ 2.7, p < .05, η2 ¼ .16, indicating that religious participants responded fastest to objects from their own religion (see Table 3). For the classification of objects an interaction between Religious Background, Category, and Classification was found, F(8, 112) ¼ 5.1, p < .001, η2 ¼ .27. This three-way interaction reflected that religious participants classified objects from their own religion more correctly than objects from a different religion (see Table 3).

Control for location of visual distractors An important question is to what extent the current findings truly reflect the integration of visuotactile information in the body representation or the simple interference of visual distractors. To address this issue, an additional analysis was conducted in which the data was averaged according to whether the visual distractor appeared at the upper or lower side of the screen, while collapsing across congruent and incongruent visuotactile stimulation. A marginally significant interaction was found between UpDown and Category, F(2,54) ¼ 2.9, p ¼ .07, η2 ¼ .10, reflecting that for religious objects participants responded faster to visual stimuli appearing at the upper compared to the lower side of the screen, while for non-religious objects they responded faster to stimuli appearing at the lower side of the screen (see lower panel of Table 2)2. In addition, a marginally significant interaction was found between UpDown and Manipulability, F(1,27) ¼ 3.3, p ¼ .08, η2 ¼ .11, indicating that for manipulable objects participants responded faster to visual stimuli appearing at the upper compared to the lower side, whereas for non-manipulable objects no such difference was observed. In contrast to the main analysis no significant three-way interaction was observed (F < 1). Thereby this analysis indicates that the effects of visual distractors on responding were clearly different from the visuotactile effects reported in the main analysis.

2 This finding is reminiscent of studies showing faster processing of religious words when presented in the upper compared to the lower visual field (Meier, Hauser, Robinson, Friesen, & Schjeldahl, 2007)

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DISCUSSION The present study shows that the integration of objects in the body representation depends on (1) the manipulability of the object, (2) the object category (religious vs. non-religious), and (3) the religious background of the participant. Thereby this study links the notion that objects can extend our body functionally (Maravita & Iriki, 2004) to the idea that objects extend our self symbolically as well (Belk, 1988). In addition, the present study adds to the growing body of studies showing that individual differences regarding one’s cultural or religious background can have a profound influence on basic neurocognitive processes (see also: Hommel, Colzato, Scorolli, Borghi, & van den Wildenberg, 2011). As such, the study of religion and religious objects provides a first stepping-stone towards assessing the impact of culture on cognition more broadly. A stronger CCE was found for manipulable compared to non-manipulable profane objects. This finding replicates a previous study using a similar experimental design with everyday objects (van Elk & Blanke, 2011) and indicates that manipulable objects facilitate multisensory integration in peripersonal space. That is, based on our action experience, the mere observation of manipulable objects results in the automatic retrieval of the sensorimotor programs associated with grasping these objects (Martin, 2007), which in turn may facilitate multisensory integration. The present study extends these findings, by including non-canonical or non-prototypical pictures of everyday objects, suggesting that it is the object type rather than its physical appearance that underlies this effect. It was expected that because of their symbolic significance, non-manipulable religious objects would evoke a feeling of reverence or bodily distance compared to manipulable religious objects, which should be reflected in a stronger reduction in the CCE. However, it was found that non-manipulable religious objects resulted in a comparable or even stronger crossmodal integration compared to manipulable religious objects. It could be that this unexpected effect is partly related to the stronger visual saliency for religious compared to non-religious objects, resulting in the automatic capture of visual attention and an enhanced interference of the visual distractor stimuli superimposed on the object picture. However, visual attention cannot fully account for the effects observed. Firstly, previous studies have suggested that the crossmodal congruency effect is not modulated by spatial attention (Spence, 2001; Spence, Pavani, & Driver, 2004; Spence, Pavani, Maravita, et al., 2004).

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Second, within each religious category, manipulable and non-manipulable objects were matched for visual complexity and visual saliency, but still an enhanced CCE was observed for non-manipulable compared to manipulable religious objects. Previous studies have also indicated that the CCE is strongly modulated by the familiarity of the object on which the visual distractors are superimposed (i.e., a stronger CCE for familiar objects; Igarashi, Kimura, Spence, & Ichihara, 2008; Igarashi, Kitagawa, Spence, & Ichihara, 2007). It could be that object familiarity makes it easier to distinguish up from down, thereby resulting in a stronger interference of the location of visual distractors on up-down discrimination. However, object familiarity cannot account for the CCE effects observed in the present study, as religious objects were less familiar than profane objects. Rather than reflecting effects of visual saliency or object familiarity, previous studies have shown that the CCE measures multisensory integration in peripersonal space (Heed, Habets, Sebanz, & Knoblich, 2010; Schicke et al., 2009; Spence, Pavani, Maravita, et al., 2004) and the extension of the global aspects of the self (Aspell et al., 2009). Accordingly, this study suggests that religious objects facilitate bodily interaction—irrespective of their manipulability. It could be that the stronger CCE for non-manipulable religious objects reflects the automatic activation of object affordances (Gibson, 1977). Although most non-manipulable religious objects are not associated to specific bodily actions, they may still afford a specific way of grasping (e.g., one can grasp a statue, open the doors of an altar shrine, etc.). Several studies have shown that grasping affordances are automatically activated upon the mere observation of objects (Martin, 2007) and that this effect is even stronger for unfamiliar compared to familiar objects (Handy, Tipper, Borg, Grafton, & Gazzaniga, 2006). As religious objects were less familiar than profane objects, it could be that both manipulable and non-manipulable religious objects were primarily perceived in terms of their graspability. This interpretation is supported by the finding that religious participants showed a stronger CCE for manipulable compared to non-manipulable objects. This may indicate that due to their increased experience with using religious objects, they correctly perceived the affordances only for manipulable religious objects (see also: van Elk & Blanke, 2011). Religious participants responded faster to, and made fewer errors in, the classification of objects from their own religion compared to a different religion. Previous studies have shown that familiarity has a strong influence on how effectively semantic knowledge is used in language and other cognitive processes (Connine,

Mullennix, Shernoff, & Yelen, 1990; Montaldi, Spencer, Roberts, & Mayes, 2006). In addition, object familiarity may influence the ease whereby inferences are made about the conceptual use of an object (Tessari & Rumiati, 2004; van Elk, van Schie, & Bekkering, 2009; van Elk, van Schie, van den Heuvel, & Bekkering, 2010). The present study extends these findings and suggests that one’s cultural background has a strong effect on the ease whereby cultural and religious objects are recognized and classified.

CONCLUSIONS Many objects can extend the self—both functionally and symbolically. The present study shows that religious objects, which have a symbolic or metaphoric significance, can also extend the representation of our body. The finding that both object manipulability, the religious significance of the object, and the religious background of the participant have a combined effect on multisensory integration suggests important interactions between low-level body-object integration and the high-level symbolic extension of the self.

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