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A longitudinal investigation of self–other discrimination and the emergence of mirror self-recognition
Infant Behavior & Development 26 (2003) 213–226
A longitudinal investigation of self–other discrimination and the emergence of mirror self-recognition Mark Nielsen a,∗ , Cheryl Dissanayake b , Yoshi Kashima c a
Early Cognitive Development Unit, School of Psychology, University of Queensland, Brisbane, Qld. 4072, Australia b School of Psychological Science, La Trobe University, Melbourne, Australia c School of Behavioural Science, University of Melbourne, Melbourne, Australia
Received 9 August 2002; received in revised form 9 December 2002; accepted 16 December 2002
Abstract The aim in this study was to investigate the association between infants’ developing interest in their self-image and the onset of mirror self-recognition (MSR). A longitudinal study was conducted with 98 infants who were seen at intervals of 3 months from 9–24 months of age. At each session, the infants were administered a preferential-looking test whereby they were presented with a video image of themselves alongside a video image of a same-aged peer in two conditions, unmarked and marked. From the 12-month session onwards, the infants were also administered a version of the standard “mark test” of MSR. The infants showed a significant preference for looking at images of themselves in both conditions coincident with the onset of MSR. This result indicates that developing an interest in the self-image is an important component in the development of MSR. © 2003 Elsevier Science Inc. All rights reserved. Keywords: Mirror self-recognition; Mark test; Preferential-looking; Self–other discrimination
1. Introduction It is common for infants in industrialized cultures to be exposed to mirrors from an early age. Once infants understand that it is themselves who they see when they are directly facing a mirror, they can begin to appreciate how they look and how others might see them (Lacan, ∗
Corresponding author. Tel.: +61-7-3365-6421; fax: +61-7-3365-4466. E-mail address: [email protected] (M. Nielsen). 0163-6383/03/$ – see front matter © 2003 Elsevier Science Inc. All rights reserved. doi:10.1016/S0163-6383(03)00018-3
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1949, 1977; Merleau-Ponty, 1964). Mirror self-recognition (MSR) has thus been afforded an important place in the development of a conceptual sense of “self” (see Rochat, 2002). Over the course of the last three decades, the principal tool used to assess MSR in infants is what has been referred to as the “mark task” whereby infants are surreptitiously marked on their faces and then exposed to a mirror. The premise of this task is that if infants appreciate that the reflected image corresponds to themselves they will react by touching and exploring the marked region on their own faces. The ability to demonstrate appropriate mark-directed behavior emerges in the second year and is present in most infants by 24 months of age (Amsterdam, 1972; Bertenthal & Fischer, 1978; Johnson, 1983; Lewis & Brooks-Gunn, 1979; Schulman & Kaplowitz, 1977). Having been widely used in studies with humans and non-humans, the mark test has provided a valuable foundation for speculation regarding the phylogenetic and ontogentic development of self-awareness (e.g., Parker, Mitchell, & Boccia, 1994). However, precisely what aspects of “self-awareness” are indexed by the mark test remain unresolved. In this context, distinct psychological mechanisms have been proposed to underpin the exhibition of MSR, such as kinaesthetic-visual matching (Mitchell, 1993, 1997), secondary representation (Suddendorf & Whiten, 2001) and the development of a “self-concept” (Gallup, 1979). Regardless of the veracity of these contrasting theories, implicit in each account is that in order to pass the mark test infants must become interested in their own facial features. If infants do not develop an interest in their own face there is little basis upon which they can begin to appreciate the legitimate connection between themselves and the individual in the mirror. Consistent with this argument, it has been demonstrated that prior to the second year (i.e., before the onset of MSR) infants prefer to look at images depicting the faces of same-aged peers rather than images depicting their own faces. Using a preferential-looking paradigm, a recent series of studies has recorded the visual responses of infants aged between 4 and 9 months to the presentation of non-contingent1 video and photograph images of themselves and same-aged peers (Bahrick, Moss, & Fadil, 1996; Legerstee, Anderson, & Schaffer, 1998; Rochat & Striano, 2002). In each of these studies, the infants spent a greater amount of time looking at the peer- rather than the self-images. That is, infants younger than 12 months of age show an ability to discriminate images of themselves from images of same-aged peers. Bahrick et al. (1996) argue that this discrimination is a function of infants having become familiar with their own facial features through experience with mirrors. Hence, in line with the premise that infants of this age orient to novel rather than familiar stimuli (for a review see Hunter & Ames, 1988), they prefer to look at the unique peer-image. The findings reviewed above demonstrate that prior to their first birthday infants are able to discriminate their own facial features from those of same-aged peers. Importantly, infants express this discrimination by visually orienting to the peer-image. However, as outlined previously, an important component in the exhibition of MSR is the development of an interest in one’s own face. If this argument is valid, coincident with the onset of MSR, infants should prefer to look at images of their own face rather than images of the faces of same-aged peers. To date, there has been no study reported in which gaze orientation on the preferential-looking test has been assessed with reference to the development of MSR. This was the overall objective in the investigation reported here.
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The ability of young infants to discriminate images of themselves from images of others was further investigated in a study by Bahrick (1995). Fourteen 5- to 8-month-old infants were presented with the forced-choice (self- vs. peer-image) preference test introduced above. However, in an analogue of the MSR test, the infants were marked on both cheeks with a large red spot prior to filming. It was considered that marking the face of the infants would “distort” their self-image and in so doing entice them to look at the self rather than at the normally preferred peer-image. Although the infants in this study did not significantly orient to one image over the other, when compared to the results reported for the unmarked condition (Bahrick et al., 1996), infants in this marked version showed a significant change in their orientation of gaze. That is, when the images were unmarked young infants oriented to the novel face but when the images were marked infants found the images equally appealing. This intriguing finding leads to speculation that the interest infants show in their own face may be more pronounced when something is inconsistent with the way their face typically appears—as is the case in the mark test. Therefore, in the current study, looking preferences in the marked condition of the preferential-looking task were also assessed with reference to the emergence of MSR. To achieve the objectives outlined above, the pattern of visual orientation displayed by infants in the unmarked and marked conditions of the preferential-looking test was charted from 9 to 24 months of age with regard to the onset of MSR. It was hypothesized that infants would show a preference for looking at their own image in both the unmarked and marked conditions of the preferential-looking test in the session during which MSR was first exhibited.
2. Method 2.1. Participants Ninety-eight infants (48 males, 50 females), recruited from maternal and child health centers (in suburbs surrounding La Trobe University, Melbourne, Australia) participated in the study. The infants were predominantly Caucasian and middle class.2 Of the 98 infants recruited for this study, 88 commenced participation at 9 months of age with an additional 10 commencing at 12 months of age. With the exception of those described below, all infants were seen six times at intervals of 3 months from 9–24 months of age. All infants were seen within 1 month of the target age. Eight of the infants who commenced participation in the study at 9 months of age were used for pilot testing throughout the study.3 In addition, two infants were diagnosed with developmental delays during the course of the study. The data from these 10 infants were excluded in all subsequent analyses. Nine infants were unable to attend one of the six sessions due to extraneous circumstances, and six infants were withdrawn by their carers altogether at various stages of the study. Of the infants who were withdrawn from the study, one did so following the 9-month session, three following the 12-month session, two following the 15-month session and two following the 18-month session. The reasons given for withdrawal included moving house and an increase in either work or family commitments. Data across all six sessions was thus available from 63 infants. However, where possible, data from as many infants as possible were included in the analyses.
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2.2. Procedure All testing was conducted in the Child Development Unit at La Trobe University. All children were tested individually with their primary carer present. The current investigation formed part of a larger longitudinal study. Due to constraints of the larger study, it was not possible to counterbalance the order of presentation of the preferential-looking and MSR tests. Thus, the preferential-looking test was conducted first in all sessions. 2.3. The preferential-looking test The preferential-looking test was administered at each session from 9 to 24 months of age, and was adapted from Bahrick (1995; Bahrick et al., 1996). 2.3.1. Preparation phase The infant, dressed in a yellow bib designed to cover his or her clothes, was placed in an age-appropriate chair positioned in front of a plain opaque screen and facing a wall-mounted video camera. The camera was focused on the face (head and neck) in order to provide a color video image. The infant’s carer sat in a chair next to the infant (randomly to the left or right of the infant) but out of camera view. The experimenter stood beside the camera (randomly to the left or right) and attracted the infant’s attention with a brightly colored hand puppet. Once the infant had fixated on the puppet the experimenter moved it randomly around the camera. This provided a film of the infant looking in the direction of the camera and moving naturally from the shoulders up. This video recording comprised the unmarked condition. The infant’s carer then surreptitiously placed a spot of bright red lipstick on each of the infant’s cheeks and the above procedure was repeated using a different puppet. Thus, in the marked condition, the infant’s image had a bright red dot displayed on each cheek. 2.3.2. Video preparation Following completion of the Preparation phase, the carer cleaned the infant’s face, removed the bib, and took the infant out of the seat. The infant and carer were left to play with a set of toys while the experimenter prepared the videotapes for the Presentation phase. The video taken of the infant was used in conjunction with a previously recorded video of a same-aged peer to prepare two videotapes for the Presentation phase—one each to be displayed on the two television monitors. Thus, a yoked control design was implemented whereby the face of each infant served as the peer face for the next infant (no attempt was made to consistently pair any infant with a specific “peer” throughout testing and hence the “peer” video was always of the infant who attended the Child Development Unit in the previous session). The aim in using this approach was to control for any differences between faces (e.g., facial attractiveness, activity level, affect, etc.) with respect to the main variable in question (self vs. other).4 Each videotape comprised a sequence of two 15-s trials.5 In one trial the unmarked self-image and the unmarked peer-image appeared simultaneously on two TV monitors. On the alternate trial both images were marked. There were no trials in which one face was unmarked and the other face marked. In addition, the order and lateral position of the images was counterbalanced across infants.
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2.3.3. Presentation phase At the 9-month session, the Presentation phase was conducted while the infants were seated in an age-appropriate chair with the carer seated behind them. The infants sat on their carers’ laps for all remaining sessions. The infants sat facing two 40-cm color video monitors located 1 m away. The monitors were placed 75 cm apart and facing inwards at a 15◦ angle. The monitors were separated by a piece of black ply board with a small hole centered between the monitors. The hole permitted an unobtrusive color video camera to be positioned directly in line with the upper torso and head of the infants, enabling their direction of gaze to be captured on video. A 15-W red light was positioned directly above the camera hole. The aim in using the light was to center the infant’s gaze prior to the commencement of each trial. The room was dimly lit to increase the saliency of the video events on the monitors. The infants were exposed to two 15-s trials whereby they were presented simultaneously with the self-image on one monitor and the peer-image on a second monitor. Prior to the presentation of the first trial, the screens went blank and the red light was illuminated for 5 s. Similarly, a 5-s interval was imposed between the presentation of Trials 1 and 2 whereby the screen again went blank and the red light was illuminated. The carer was instructed not to point or direct the infant to either image but to calm him or her if, during the course of the presentation, the infant appeared distressed. 2.3.4. Coding The duration of gaze at each monitor was coded from the videotapes recorded during the Presentation phase. These videotaped segments were imprinted during recording with a 0.1-s timer. A Panasonic video player with a jog-shuttle facility was used for coding which enabled each video to be coded millisecond by millisecond. The video recordings were coded for the duration of looking to the right monitor, the left monitor, or “away” (looking at neither left nor right monitor), giving a measure of the total duration the infants spent looking at each image in both unmarked and marked conditions. Following the approach adopted by Bahrick (1995) and Bahrick et al. (1996), the dependent measure was the proportion of total looking time (PTLT) the infants spent oriented to the peer-image (i.e., time spent looking at the peer-image divided by the total time spent looking at both images).
2.4. Mirror self-recognition The MSR test was administered from the 12-month session onwards. The infants sat at a table (120 cm × 60 cm) opposite the experimenter. A wall-mounted camera was positioned behind the experimenter and was used to videotape the infant during administration of the MSR test. Traditionally, marking for the MSR test has been achieved by having the mother or experimenter surreptitiously apply rouge to the infant’s face. Following a procedure first implemented by Povinelli, Landau, and Perilloux (1996), a brightly colored sticker was used, instead of rouge, in the current investigation. The MSR procedure consisted of three stages, administered in the following order:
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2.4.1. Mirror–no sticker stage The experimenter presented a medium sized mirror (46 cm × 89 cm) to the infant, allowing interaction with his or her reflection for 20 s. This established a baseline of the infant’s natural inclination to touch the target area. No infant spontaneously brushed his or her hair during this stage. 2.4.2. Mirror–sticker stage The experimenter placed the mirror out of sight and, while the infant was occupied with a toy, rubbed the forehead, surreptitiously fixing a sticker around the fringe area. To ensure that the infant was unaware of the sticker, the infant and experimenter continued to play with the toy for a further 60 s. If the infant reached for the sticker within this 60-s play period the task was not administered. If the infant did not respond to the sticker within the 60-s play period, the mirror was re-introduced and the infant was again given 20 s of mirror exposure. If the infant responded by bringing the hand within 2 cm of the sticker, while visually observing him or herself in the mirror, he or she was classified as a “self-recognizer” and the test was terminated. If the infant failed to respond, he or she was classified as a “non-recognizer” and the Sticker-saliency stage was implemented. 2.4.3. Sticker-saliency stage In contrast to a number of non-human primate studies (Anderson, 1984; Gallup, Wallnau, & Suarez 1980; Shillito, Gallup, & Beck, 1999; Suarez & Gallup, 1981), child development researchers, to date, have not investigated how children respond to marks on their own bodies that are clearly visible without a mirror. Therefore, if the infant was classified as a “non-recognizer,” the experimenter placed the mirror out of sight and surreptitiously placed a second sticker on the back of the infant’s hand.6 The aim of this stage was to ensure that the failure of an infant to reach for the sticker placed on his or her fringe was not due to a lack of motivation to retrieve the sticker (i.e., a false negative). 2.4.4. Coding MSR was coded from the videotapes of each session. The observer was first required to judge if the infant attempted to reach for the sticker prior to the mirror being introduced in the Mirror–sticker stage, and if not, whether he or she responded after the mirror was introduced by bringing either hand within 2 cm of the sticker while observing him or herself in the mirror. If the infant was judged as failing to respond to the sticker in the Mirror–sticker stage the observer was further required to judge if the infant removed the sticker from his or her hand in the Sticker-saliency condition. The age of emergence (AOE) of MSR7 was determined to be the age at which the infant first reached for the sticker in the Mirror–sticker stage. 2.5. Reliability Two trained coders independently observed and coded the videotapes of 12 randomly selected infants (2 from each session). Intraclass correlation coefficients (Shrout & Fleiss, 1979) were calculated between the scores of the two coders for the PTLT oriented to the peer-image for both the unmarked and marked conditions in the preferential-looking test, and between the
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two coders on each criterion of the MSR task. There was 100% agreement (and hence Intraclass correlation coefficients were equal to 1.00) between the two coders on each measure. 3. Results 3.1. The preferential-looking test Following Bahrick et al. (1996), in order to ensure that the infants had noticed both images, infants who did not gaze at each image for more than 1 s or who devoted less than 3% of the total fixation time to one of the displays in any condition were excluded from analysis involving that condition. Thus, in the marked condition, four infants were excluded at 9 months, two infants at 12 months, three infants at 15 months, one infant at 18 months and one infant at 24 months. In the unmarked condition, seven infants were excluded at 9 months, five infants at 12 months, three infants at 15 months and four infants at 18 months. In addition, not all infants were able to attend each session and some infants were unable to be tested on the preferential-looking test as they were either distressed or uncooperative during the Preparation or Presentation phases. The number of infants included in the subsequent analysis for each respective session is presented in Table 1 along with the mean proportion of available time (15 s) that they spent looking at both images (self and peer combined) in the unmarked and marked conditions. Given that the infants spent approximately 80% of the available time looking at either the self- or peer-image across all sessions, it can be concluded that the images were attractive stimuli. The PTLT spent fixating the image of the peer was calculated for the unmarked and marked conditions at each session.8 A MANOVA was performed for each session including these two dependent variables with order (unmarked condition first vs. marked condition first) as an independent variable. No significant effect for order was revealed for any session from 9 to 24 months of age. Therefore, the order of presentation was not considered further. Following Bahrick (1995) and Bahrick et al. (1996), to determine whether or not the infants were discriminating between the self- and peer-images, single-sample t tests were conducted against the chance value of .50. The results of these analyses are presented in Table 2. The infants discriminated between the two marked images in the 12-month session where they Table 1 Mean proportion of the available time (15 s) the infants spent looking at both monitors in the “unmarked” and “marked” conditions of the preferential-looking test Months
Condition Unmarked
9 12 15 18 21 24
Marked
N
Mean proportion
SD
N
Mean proportion
SD
68 79 74 70 66 66
0.82 0.77 0.79 0.80 0.84 0.88
0.18 0.19 0.19 0.16 0.17 0.11
70 80 75 73 66 64
0.78 0.80 0.85 0.86 0.85 0.89
0.20 0.16 0.16 0.16 0.14 0.12
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Table 2 Means, standard deviations and single-sample t tests (against a chance value of .50) for the PTLT oriented towards the peer-imagea for the “unmarked” and “marked” conditions of the preferential-looking test at each session Months
Condition Unmarked
9 12 15 18 21 24
Marked
N
Mean
SD
t value
N
Mean
SD
t value
68 79 74 70 66 66
0.50 0.53 0.47 0.46 0.46 0.44
0.18 0.19 0.17 0.17 0.19 0.16
0.14 0.34 1.79 2.06∗ 1.56 2.61∗∗
70 80 75 73 66 64
0.51 0.45 0.48 0.50 0.46 0.48
0.18 0.16 0.15 0.18 0.18 0.17
0.35 2.60∗∗ 1.09 0.17 1.60 0.83
a
A mean greater than 0.50 indicates a preference for the peer-image. p < .05 (two tailed). ∗∗ p < .01 (two tailed). ∗
showed a significant preference for the self-image. The infants also discriminated between the two unmarked images in the 18- and 24-month sessions where they again showed a significant preference for the self-image. To evaluate whether or not the means in the unmarked conditions were equal across sessions a one-way repeated-measures ANOVA was conducted. The repeated-measures ANOVA indicated that the means for the six sessions were not different, F (5, 160) = 1.19, p > .05. However, a significant linear trend was revealed, F (1, 32) = 5.44, p < .05, reflecting a change in fixation from the 12- to the 15-month session, t (66) = 2.11, p < .05. Similarly, to evaluate whether or not the means in the marked conditions were equal, a one-way repeated-measures ANOVA was conducted. The repeated-measures ANOVA indicated that the means for the six sessions were not significantly different, F (5, 185) = 0.71, p > .05. No significant trends were revealed. 3.2. Mirror self-recognition Three infants reached for the sticker prior to the mirror being introduced in the Mirror–sticker stage. However, as each of these infants had exhibited MSR in a previous session their AOE of MSR was not affected. Similarly, one infant failed to show MSR in one session (24 months) having previously exhibited it in previous sessions (18 and 21 months). The AOE data for these four infants were therefore retained. Six infants were withdrawn from the study prior to exhibiting MSR. With the exception of one infant, all infants who failed to respond to the sticker in Stage 2 (Mirror–sticker stage) of the mirror task removed the sticker from their hand in Stage 3 (Sticker-saliency). As the infant who failed to respond to the sticker being placed on his hand in Stage 3 had not previously exhibited mark-directed behavior, his failure to show interest in the sticker meant that his AOE of MSR could not be reliably determined. Therefore, these seven infants were excluded in subsequent analyses involving the AOE of MSR. In addition, seven infants failed to show mark-directed behavior in any session. These seven infants were also excluded in subsequent analyses involving the AOE of MSR.9
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Fig. 1. The cumulative percentage of infants showing evidence of mirror self-recognition (MSR) at each session. Note: The percentages for MSR were calculated including those seven infants who failed to exhibit MSR by the 24-month session.
Consistent with past research, the mean, median and mode AOE for the remaining 76 infants was 18 months. The cumulative percentage of infants showing evidence of MSR at each session (including those seven infants who failed to exhibit MSR by the 24 months of age) is presented in Fig. 1. 3.3. Mirror self-recognition and the preferential-looking test In order to test the hypothesis that infants would show a preference for their own image coincident with the onset of MSR, gaze orientation in each condition of the preferential-looking test was calculated for each infant for the sessions before, during and after the session at which he or she first exhibited MSR (see Fig. 2).
Fig. 2. The PTLT oriented to the peer-image in the “unmarked” and “marked” conditions of the preferential-looking test according to the AOE of MSR. Note: Means greater than 0.50 indicate a preference for the peer-image and means less than 0.50 indicate a preference for the self-image.
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Table 3 Means, standard deviations and one-sample t tests (vs. 0.50) for the PTLT oriented to the peer-image for the “unmarked” and “marked” conditions of the preferential-looking test according to the session in which MSR was first exhibited Months
Condition Unmarked
Less 9 Less 6 Less 3 At MSR Plus 3 Plus 6 ∗
Marked
N
Mean
SD
t value
N
Mean
SD
t value
38 58 67 63 58 38
0.52 0.50 0.51 0.45 0.48 0.45
0.19 0.18 0.20 0.17 0.20 0.18
0.49 0.03 0.27 2.28∗ 0.79 1.62
39 61 69 64 57 39
0.51 0.54 0.47 0.45 0.49 0.49
0.18 0.16 0.17 0.17 0.16 0.19
0.27 1.95 1.51 2.33∗ 0.68 0.19
p < .05 (two tailed).
It is evident in Fig. 2 that the infants oriented their gaze towards the self-image in both conditions coincident with the onset of MSR. To determine whether or not the infants were discriminating between the two images, single-sample t tests against the chance value of .50 were conducted on the PTLT oriented to the peer-image (see Table 3). In line with Fig. 2, the only session in which the infants significantly discriminated between the self- and other-images was in the same session during which they first exhibited MSR. In this session, infants showed a significant preference for the self-image in both the unmarked, t (62) = 2.28, p < .05, and marked, t (63) = 2.33, p < .05, conditions. 4. Discussion The primary aim in this investigation was to evaluate the hypothesis that the onset of success on the mark test would coincide with infants showing a preference for looking at the self-image in the preferential-looking test. This hypothesis was supported. The infants in this study showed a significant preference for their self-image in both marked and unmarked conditions in the session that they first exhibited MSR. In contrast, they did not show a significant preference for either image in the sessions prior to or following the session in which MSR was first exhibited. As noted previously, each of the accounts of mark-directed behavior (Mitchell, 1993, 1997; Suddendorf & Whiten, 2001; Gallup, 1979) maintain that in order to pass the mark test infants must become interested in their own facial features. By developing an interest in their own face infants can begin to appreciate the connection between themselves and their mirror image. The preference shown by the infants for looking at their own image in the session that they first passed the mark test is congruent with this suggestion. The finding that infants fixated their gaze to their own image coincident with the onset of MSR also provides evidence that the mark test is a reliable index of the developing ability infants have for recognizing their own physical features. Alternative interpretations of the test posit that mark-directed behavior need not evoke self-recognition in any form. For example, it has been argued that success on the mark test can be reduced to the ability of individuals for detecting the contingency inherent in mirrored feedback (Heyes, 1994, 1998; Menzel, Savage-Rumbaugh,
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& Lawson, 1985). Heyes (1994) has similarly suggested that individuals can pass the mark test via a process of distinguishing “across a fairly broad range, sensory inputs resulting from the physical state and operations of [their] own body, from sensory inputs originating elsewhere” (p. 915). It now becomes necessary for proponents of these alternative interpretations to explain the findings reported here. Curiously, the infants in the current study did not maintain a strong preference for their own faces following the onset of MSR. That is, the infants did not continue to significantly discriminate between the images in either condition of the preferential-looking task following the session in which they first succeeded on the mark test. This failure to discriminate between the images may be due to individual differences that are likely to exist in the preferences infants have for looking at novel over familiar stimuli (see Kagan, Resnick, & Snidman, 1987) and for looking at images of “me” over images of “not me.” Further research is therefore needed to evaluate the preference infants have for looking at images of themselves over other familiar faces and for looking at images of familiar over unfamiliar faces. Such comparisons will not only help to delineate the kinds of individual differences that may have impacted on the results reported here, but will also help to identify the bases upon which infants differentiate images of themselves from images of other people. Past research has reported that infants aged between 4 and 9 months orient to the peer-image in the unmarked condition of the preferential-looking test (Bahrick et al., 1996; Legerstee et al., 1998; Rochat & Striano, 2002). The present investigation did not find this preference in infants aged from 9 to 24 months. Rather, at 18 and 24 months the infants oriented to the self-image. As the majority of infants had become self-recognizers by 18 months, this orientation is consistent with the data showing a preference for the self-image coincident with the onset of MSR. However, the failure to reveal a similar preference at 21 months of age is incongruent with this interpretation. As such, these findings may be significant by chance and should be treated with caution. Bahrick (1995) reported that 5- to 8-month-old infants showed a significant change in their orientation of gaze in the marked compared to the unmarked version of the preferential-looking test. When the images were unmarked infants oriented to the novel face, but when the images were marked infants made no discrimination. This difference in looking patterns was not evident in the current study. The only session in which the PTLTs differed significantly between conditions was at 12 months where the infants showed a significant preference for the marked self-image (PTLT = 0.45) but did not discriminate between the unmarked images (PTLT = 0.53).10 The preference for the self-image at 12 months was also the only session in which infants showed such a preference in the marked condition. As with the results reported above for the unmarked condition, in the absence of any reliable pattern of looking preference across sessions, the lone significant orientation to the marked self-image remains challenging to interpret and should be treated with caution. The looking preferences observed in the current study suggest that the processes driving gaze orientation in the studies with younger infants (Bahrick, 1995; Bahrick et al., 1996; Legerstee et al., 1998; Rochat & Striano, 2002) are distinct from those driving the preferences in the older infants tested here. However, these apparent differences may be a function of assessing older infants with a test designed to evaluate development in younger infants.11 Moreover, in order to maintain the interest of the older infants tested here, it was necessary to use a
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shorter presentation time than has been previously used. The current investigation was also the first in which the same infants were assessed on both unmarked and marked conditions of the preferential-looking test, and it remains possible that assessing the gaze orientation of the same infants in both conditions altered their regular patterns of looking preference. Together, these methodological differences may have contributed to the lack of correspondence between the findings reported here and those reported in the studies cited above. Further research is needed to disentangle which of these methodological differences account for the present results. Upon cursory observation of the behavior of the infants studies here, it was notable that no infant (at any age) responded to seeing his or her marked image in the preferential-looking test by exploring the marked region on his or her face. This observation is in line with the finding that young 2 year olds fail video or photograph analogues of the mark test (Povinelli et al., 1996; Suddendorf, 1999; Zelazo, Somerville, & Nichols, 1999). That is, in the absence of contingency cues, young children do not appear to connect images of the self to their current reality, and this is something they need to transcend if they are to bind time into one conceptual present (Suddendorf & Corballis, 1997). In stark contrast to the vast corpus of research conducted using the traditional mirror test, the use of the preferential-looking paradigm to investigate self-recognition has been minimal. Therefore, continued refinement of the preferential-looking test is warranted. In so doing, preferential-looking may prove to be a valuable adjunct to the standard mirror test in the investigation of self-recognition. There is a continued importance for assessing the development of how human and non-human individuals deal with external representations of themselves. Such research will enrich our understanding of what may prove to be a crucial process in both ontogenetic and phylogenetic development. Notes 1. Young infants use contingency information to determine the relatedness of external representations to the self (e.g., Bahrick & Watson, 1985). 2. More detail on the cohort can be obtained from the first author. 3. The infants were also assessed on a range of tasks that are not discussed here. 4. It is relevant to note that the video used as the 15-s ‘self’ stimuli was same video used as the 15-s ‘peer’ stimuli for the following infant. 5. Previous research with the preferential looking task has used presentation times ranging from 30 s (Bahrick, 1995; Bahrick et al., 1996) to 1 min (Legerstee et al., 1998; Rochat & Striano, 2002). However, pilot testing indicated that a 15-s presentation time was sufficient to assess discrimination while maintaining the interest of the older infants studied here. 6. Note that this provided tactile cues. 7. The AOE measure indicates the approximate age at which the infants first developed MSR. It is acknowledged that MSR may have emerged in the 3 months preceding the AOE session. 8. The alternative to calculating PTLTs is to compare actual looking times for each stimulus. Thus, all analyses reported here were also conducted using actual looking times. As this
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alternative analysis did not effect the substantive interpretations made (and were identical with regard to the onset of MSR), in order to maintain consistency with Bahrick (1995) and Bahrick et al. (1996) and for the sake of parsimony all analyses are reported using PTLTs. 9. An alternative approach would be to credit these seven infants with an arbitrarily determined AOE that is greater than 24 months. However, this approach is rendered problematic by reports that, for example, some children as old as 4 years of age still fail to pass the mark test (Rochat, MacGillivray, Callaghan, Genest, & Lewis, 2002). Thus, the relevant measure, individual differences in the AOE of MSR, would potentially be confounded by such an approach. 10. t (75) = 2.98, p < .01. 11. However, the preferential looking task has been successfully used with older infants in studies of language acquisition (e.g., Bavin & Growcott, 2000; Naigles & Gelman, 1995).
Acknowledgments The work described in this article was funded by a La Trobe University Postgraduate Research Scholarship to the first author and formed part of his doctoral thesis under the supervision of the second author. The authors would like to thank the carers and children who gave their time to participate in this study. The authors also thank Virginia Slaughter and Thomas Suddendorf for their comments and suggestions on this article.
References Amsterdam, B. (1972). Mirror self-image reactions before age two. Developmental Psychobiology, 5, 297–305. Anderson, J. R. (1984). The development of self-recognition: A review. Developmental Psychobiology, 17, 35–49. Bahrick, L. (1995). Intermodal origins of self-perception. In P. Rochat (Ed.), The self in infancy: Theory and research (pp. 349–373). Amsterdam: North-Holland, Elsevier. Bahrick, L., & Watson, J. S. (1985). Detection of intermodal proprioceptive-visual contingency as a potential basis of self-perception in infancy. Developmental Psychology, 21, 963–973. Bahrick, L., Moss, L., & Fadil, C. (1996). Development of visual self-recognition in infancy. Ecological Psychology, 8, 189–208. Bavin, E. L., & Growcott, C. (2000). Infants of 24–30 months understand verb frames. In M. Perkins & S. Howard (Eds.), New directions in language development and disorders (pp. 169–177). New York: Kluwer & Plenum Publishers. Bertenthal, B. I., & Fischer, K. W. (1978). Development of self-recognition in infants. Developmental Psychology, 4, 44–50. Gallup, G. G., Jr. (1979). Self-recognition in chimpanzees and man: A developmental and comparative perspective. In M. Lewis & L. A. Rosenblum (Eds.), The child and its family (pp. 107–126). New York: Plenum Press. Gallup, G. G., Jr., Wallnau, L. B., & Suarez, S. D. (1980). Failure to find self-recognition in mother–infant and infant–infant rhesus monkey pairs. Folia Primatologica, 33, 210–219. Heyes, C. M. (1994). Reflections on self-recognition in primates. Animal Behaviour, 47, 909–919. Heyes, C. M. (1998). Theory of mind in nonhuman primates. Behavioural and Brain Sciences, 21, 101–114 (target article), 134–144 (author’s response).
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Hunter, M. A., & Ames, E. W. (1988). A multifactor model of infant preferences for novel and familiar stimuli. Advances in Infancy Research, 5, 69–95. Johnson, D. B. (1983). Self-recognition in infants. Infant Behaviour and Development, 6, 211–222. Kagan, J., Resnick, J. S., & Snidman, N. (1987). The physiology and psychology of behavioural inhibition in children. Child Development, 58, 1459–1473. Lacan, J. (1949). Le Stad du Miroir comme formateur de la fonction de Je, telle qu’elle nous est revelee dans l’experience psychoanalytique. Revue Francaise de Psychanalyse, 13, 449–455. Lacan, J. (1977). Ecrits: A selection (A. Sheridan, Trans.). New York: International Universities Press. Legerstee, M., Anderson, D., & Schaffer, A. (1998). Five- and eight-month-old infants recognise their faces and voices as familiar and social stimuli. Child Development, 69, 37–50. Lewis, M., & Brooks-Gunn, J. (1979). Social cognition and the acquisition of self. New York: Plenum Press. Menzel, E. W., Jr., Savage-Rumbaugh, E. S., & Lawson, J. (1985). Chimpanzee (Pan troglodytes) problem solving with the use of mirrors and televised equivalents of mirrors. Journal of Comparative Psychology, 99, 211–217. Merleau-Ponty, M. (1964). The child’s relations with others (William Cobb, Trans.). In M. Merleau-Ponty, The primacy of perception. Evanston: Northwestern. Mitchell, R. W. (1993). Mental models of mirror self-recognition: Two theories. New Ideas in Psychology, 11, 295–325. Mitchell, R. W. (1997). A comparison of the self-awareness and kinesthetic-visual matching theories of self-recognition: Autistic children and others. In J. G. Snodgrass & R. L. Thompson (Eds.), The self across psychology: Self-recognition, self-awareness, and the self-concept. Annals of the New York Academy of Sciences, Vol. 818 (pp. 39–62). New York: New York Academy of Sciences. Naigles, L. G., & Gelman, S. A. (1995). Overextensions in comprehension and production revisited: Preferential-looking in a study of dog, cat, and cow. Journal of Child Language, 22, 19–46. Parker, S. T., Mitchell, R. W., & Boccia, M. L. (Eds.) (1994). Self-awareness in animals and humans: Developmental perspectives. New York: Cambridge University Press. Povinelli, D. J., Landau, K. R., & Perilloux, H. K. (1996). Self-recognition in young children using delayed versus live feedback: Evidence of a developmental asynchrony. Child Development, 67, 1540–1554. Rochat, P. (2002). Origins of self concept. In J. G. Bremmer & A. Fogel (Eds.), Blackwell handbook of infant development. Oxford: Basil Blackwell. Rochat, P., & Striano, T. (2002). Who’s in the mirror? Self–other discrimination in specular images by 4- and 9-month-old infants. Child Development, 73, 35–46. Rochat, P., MacGillivray, T., Callaghan, T., Genest, D., & Lewis, M. (2002, March). Social awareness and mirror self-recognition in 12–65 month olds from Canada and Kenya. Poster Presented at the International Conference on Infant Studies, Toronto, Canada. Schulman, A. H., & Kaplowitz, C. (1977). Mirror-image response during the first two years of life. Developmental Psychology, 10, 133–142. Shillito, D. J., Gallup, G. G., Jr., & Beck, B. B. (1999). Factors affecting mirror behaviour in western lowland gorillas, Gorilla gorilla. Animal Behaviour, 57, 999–1004. Shrout, P. E., & Fleiss, J. L. (1979). Intraclass correlations: Uses in assessing rater reliability. Psychological Bulletin, 86, 420–428. Suarez, S. D., & Gallup, G. G., Jr. (1981). Self-recognition in chimpanzees and orang-utans, but not gorillas. Journal of Human Evolution, 10, 175–188. Suddendorf, T. (1999). Children’s understanding of the relation between delayed video representation and current reality: A test for self-awareness? Journal of Experimental Child Psychology, 72, 157–176. Suddendorf, T., & Corballis, M. C. (1997). Mental time travel and the evolution of the human mind. Genetic Social and General Psychology Monographs, 123, 133–167. Suddendorf, T., & Whiten, A. (2001). Mental evolution and development: Evidence for secondary representation in children, great apes and other animals. Psychological Bulletin, 127, 629–650. Zelazo, P. D., Somerville, J. A., & Nichols, S. (1999). Age-related changes in children’s use of external representations. Developmental Psychology, 35, 1059–1071.
A longitudinal investigation of self–other discrimination and the emergence of mirror self-recognition Mark Nielsen a,∗ , Cheryl Dissanayake b , Yoshi Kashima c a
Early Cognitive Development Unit, School of Psychology, University of Queensland, Brisbane, Qld. 4072, Australia b School of Psychological Science, La Trobe University, Melbourne, Australia c School of Behavioural Science, University of Melbourne, Melbourne, Australia
Received 9 August 2002; received in revised form 9 December 2002; accepted 16 December 2002
Abstract The aim in this study was to investigate the association between infants’ developing interest in their self-image and the onset of mirror self-recognition (MSR). A longitudinal study was conducted with 98 infants who were seen at intervals of 3 months from 9–24 months of age. At each session, the infants were administered a preferential-looking test whereby they were presented with a video image of themselves alongside a video image of a same-aged peer in two conditions, unmarked and marked. From the 12-month session onwards, the infants were also administered a version of the standard “mark test” of MSR. The infants showed a significant preference for looking at images of themselves in both conditions coincident with the onset of MSR. This result indicates that developing an interest in the self-image is an important component in the development of MSR. © 2003 Elsevier Science Inc. All rights reserved. Keywords: Mirror self-recognition; Mark test; Preferential-looking; Self–other discrimination
1. Introduction It is common for infants in industrialized cultures to be exposed to mirrors from an early age. Once infants understand that it is themselves who they see when they are directly facing a mirror, they can begin to appreciate how they look and how others might see them (Lacan, ∗
Corresponding author. Tel.: +61-7-3365-6421; fax: +61-7-3365-4466. E-mail address: [email protected] (M. Nielsen). 0163-6383/03/$ – see front matter © 2003 Elsevier Science Inc. All rights reserved. doi:10.1016/S0163-6383(03)00018-3
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1949, 1977; Merleau-Ponty, 1964). Mirror self-recognition (MSR) has thus been afforded an important place in the development of a conceptual sense of “self” (see Rochat, 2002). Over the course of the last three decades, the principal tool used to assess MSR in infants is what has been referred to as the “mark task” whereby infants are surreptitiously marked on their faces and then exposed to a mirror. The premise of this task is that if infants appreciate that the reflected image corresponds to themselves they will react by touching and exploring the marked region on their own faces. The ability to demonstrate appropriate mark-directed behavior emerges in the second year and is present in most infants by 24 months of age (Amsterdam, 1972; Bertenthal & Fischer, 1978; Johnson, 1983; Lewis & Brooks-Gunn, 1979; Schulman & Kaplowitz, 1977). Having been widely used in studies with humans and non-humans, the mark test has provided a valuable foundation for speculation regarding the phylogenetic and ontogentic development of self-awareness (e.g., Parker, Mitchell, & Boccia, 1994). However, precisely what aspects of “self-awareness” are indexed by the mark test remain unresolved. In this context, distinct psychological mechanisms have been proposed to underpin the exhibition of MSR, such as kinaesthetic-visual matching (Mitchell, 1993, 1997), secondary representation (Suddendorf & Whiten, 2001) and the development of a “self-concept” (Gallup, 1979). Regardless of the veracity of these contrasting theories, implicit in each account is that in order to pass the mark test infants must become interested in their own facial features. If infants do not develop an interest in their own face there is little basis upon which they can begin to appreciate the legitimate connection between themselves and the individual in the mirror. Consistent with this argument, it has been demonstrated that prior to the second year (i.e., before the onset of MSR) infants prefer to look at images depicting the faces of same-aged peers rather than images depicting their own faces. Using a preferential-looking paradigm, a recent series of studies has recorded the visual responses of infants aged between 4 and 9 months to the presentation of non-contingent1 video and photograph images of themselves and same-aged peers (Bahrick, Moss, & Fadil, 1996; Legerstee, Anderson, & Schaffer, 1998; Rochat & Striano, 2002). In each of these studies, the infants spent a greater amount of time looking at the peer- rather than the self-images. That is, infants younger than 12 months of age show an ability to discriminate images of themselves from images of same-aged peers. Bahrick et al. (1996) argue that this discrimination is a function of infants having become familiar with their own facial features through experience with mirrors. Hence, in line with the premise that infants of this age orient to novel rather than familiar stimuli (for a review see Hunter & Ames, 1988), they prefer to look at the unique peer-image. The findings reviewed above demonstrate that prior to their first birthday infants are able to discriminate their own facial features from those of same-aged peers. Importantly, infants express this discrimination by visually orienting to the peer-image. However, as outlined previously, an important component in the exhibition of MSR is the development of an interest in one’s own face. If this argument is valid, coincident with the onset of MSR, infants should prefer to look at images of their own face rather than images of the faces of same-aged peers. To date, there has been no study reported in which gaze orientation on the preferential-looking test has been assessed with reference to the development of MSR. This was the overall objective in the investigation reported here.
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The ability of young infants to discriminate images of themselves from images of others was further investigated in a study by Bahrick (1995). Fourteen 5- to 8-month-old infants were presented with the forced-choice (self- vs. peer-image) preference test introduced above. However, in an analogue of the MSR test, the infants were marked on both cheeks with a large red spot prior to filming. It was considered that marking the face of the infants would “distort” their self-image and in so doing entice them to look at the self rather than at the normally preferred peer-image. Although the infants in this study did not significantly orient to one image over the other, when compared to the results reported for the unmarked condition (Bahrick et al., 1996), infants in this marked version showed a significant change in their orientation of gaze. That is, when the images were unmarked young infants oriented to the novel face but when the images were marked infants found the images equally appealing. This intriguing finding leads to speculation that the interest infants show in their own face may be more pronounced when something is inconsistent with the way their face typically appears—as is the case in the mark test. Therefore, in the current study, looking preferences in the marked condition of the preferential-looking task were also assessed with reference to the emergence of MSR. To achieve the objectives outlined above, the pattern of visual orientation displayed by infants in the unmarked and marked conditions of the preferential-looking test was charted from 9 to 24 months of age with regard to the onset of MSR. It was hypothesized that infants would show a preference for looking at their own image in both the unmarked and marked conditions of the preferential-looking test in the session during which MSR was first exhibited.
2. Method 2.1. Participants Ninety-eight infants (48 males, 50 females), recruited from maternal and child health centers (in suburbs surrounding La Trobe University, Melbourne, Australia) participated in the study. The infants were predominantly Caucasian and middle class.2 Of the 98 infants recruited for this study, 88 commenced participation at 9 months of age with an additional 10 commencing at 12 months of age. With the exception of those described below, all infants were seen six times at intervals of 3 months from 9–24 months of age. All infants were seen within 1 month of the target age. Eight of the infants who commenced participation in the study at 9 months of age were used for pilot testing throughout the study.3 In addition, two infants were diagnosed with developmental delays during the course of the study. The data from these 10 infants were excluded in all subsequent analyses. Nine infants were unable to attend one of the six sessions due to extraneous circumstances, and six infants were withdrawn by their carers altogether at various stages of the study. Of the infants who were withdrawn from the study, one did so following the 9-month session, three following the 12-month session, two following the 15-month session and two following the 18-month session. The reasons given for withdrawal included moving house and an increase in either work or family commitments. Data across all six sessions was thus available from 63 infants. However, where possible, data from as many infants as possible were included in the analyses.
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2.2. Procedure All testing was conducted in the Child Development Unit at La Trobe University. All children were tested individually with their primary carer present. The current investigation formed part of a larger longitudinal study. Due to constraints of the larger study, it was not possible to counterbalance the order of presentation of the preferential-looking and MSR tests. Thus, the preferential-looking test was conducted first in all sessions. 2.3. The preferential-looking test The preferential-looking test was administered at each session from 9 to 24 months of age, and was adapted from Bahrick (1995; Bahrick et al., 1996). 2.3.1. Preparation phase The infant, dressed in a yellow bib designed to cover his or her clothes, was placed in an age-appropriate chair positioned in front of a plain opaque screen and facing a wall-mounted video camera. The camera was focused on the face (head and neck) in order to provide a color video image. The infant’s carer sat in a chair next to the infant (randomly to the left or right of the infant) but out of camera view. The experimenter stood beside the camera (randomly to the left or right) and attracted the infant’s attention with a brightly colored hand puppet. Once the infant had fixated on the puppet the experimenter moved it randomly around the camera. This provided a film of the infant looking in the direction of the camera and moving naturally from the shoulders up. This video recording comprised the unmarked condition. The infant’s carer then surreptitiously placed a spot of bright red lipstick on each of the infant’s cheeks and the above procedure was repeated using a different puppet. Thus, in the marked condition, the infant’s image had a bright red dot displayed on each cheek. 2.3.2. Video preparation Following completion of the Preparation phase, the carer cleaned the infant’s face, removed the bib, and took the infant out of the seat. The infant and carer were left to play with a set of toys while the experimenter prepared the videotapes for the Presentation phase. The video taken of the infant was used in conjunction with a previously recorded video of a same-aged peer to prepare two videotapes for the Presentation phase—one each to be displayed on the two television monitors. Thus, a yoked control design was implemented whereby the face of each infant served as the peer face for the next infant (no attempt was made to consistently pair any infant with a specific “peer” throughout testing and hence the “peer” video was always of the infant who attended the Child Development Unit in the previous session). The aim in using this approach was to control for any differences between faces (e.g., facial attractiveness, activity level, affect, etc.) with respect to the main variable in question (self vs. other).4 Each videotape comprised a sequence of two 15-s trials.5 In one trial the unmarked self-image and the unmarked peer-image appeared simultaneously on two TV monitors. On the alternate trial both images were marked. There were no trials in which one face was unmarked and the other face marked. In addition, the order and lateral position of the images was counterbalanced across infants.
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2.3.3. Presentation phase At the 9-month session, the Presentation phase was conducted while the infants were seated in an age-appropriate chair with the carer seated behind them. The infants sat on their carers’ laps for all remaining sessions. The infants sat facing two 40-cm color video monitors located 1 m away. The monitors were placed 75 cm apart and facing inwards at a 15◦ angle. The monitors were separated by a piece of black ply board with a small hole centered between the monitors. The hole permitted an unobtrusive color video camera to be positioned directly in line with the upper torso and head of the infants, enabling their direction of gaze to be captured on video. A 15-W red light was positioned directly above the camera hole. The aim in using the light was to center the infant’s gaze prior to the commencement of each trial. The room was dimly lit to increase the saliency of the video events on the monitors. The infants were exposed to two 15-s trials whereby they were presented simultaneously with the self-image on one monitor and the peer-image on a second monitor. Prior to the presentation of the first trial, the screens went blank and the red light was illuminated for 5 s. Similarly, a 5-s interval was imposed between the presentation of Trials 1 and 2 whereby the screen again went blank and the red light was illuminated. The carer was instructed not to point or direct the infant to either image but to calm him or her if, during the course of the presentation, the infant appeared distressed. 2.3.4. Coding The duration of gaze at each monitor was coded from the videotapes recorded during the Presentation phase. These videotaped segments were imprinted during recording with a 0.1-s timer. A Panasonic video player with a jog-shuttle facility was used for coding which enabled each video to be coded millisecond by millisecond. The video recordings were coded for the duration of looking to the right monitor, the left monitor, or “away” (looking at neither left nor right monitor), giving a measure of the total duration the infants spent looking at each image in both unmarked and marked conditions. Following the approach adopted by Bahrick (1995) and Bahrick et al. (1996), the dependent measure was the proportion of total looking time (PTLT) the infants spent oriented to the peer-image (i.e., time spent looking at the peer-image divided by the total time spent looking at both images).
2.4. Mirror self-recognition The MSR test was administered from the 12-month session onwards. The infants sat at a table (120 cm × 60 cm) opposite the experimenter. A wall-mounted camera was positioned behind the experimenter and was used to videotape the infant during administration of the MSR test. Traditionally, marking for the MSR test has been achieved by having the mother or experimenter surreptitiously apply rouge to the infant’s face. Following a procedure first implemented by Povinelli, Landau, and Perilloux (1996), a brightly colored sticker was used, instead of rouge, in the current investigation. The MSR procedure consisted of three stages, administered in the following order:
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2.4.1. Mirror–no sticker stage The experimenter presented a medium sized mirror (46 cm × 89 cm) to the infant, allowing interaction with his or her reflection for 20 s. This established a baseline of the infant’s natural inclination to touch the target area. No infant spontaneously brushed his or her hair during this stage. 2.4.2. Mirror–sticker stage The experimenter placed the mirror out of sight and, while the infant was occupied with a toy, rubbed the forehead, surreptitiously fixing a sticker around the fringe area. To ensure that the infant was unaware of the sticker, the infant and experimenter continued to play with the toy for a further 60 s. If the infant reached for the sticker within this 60-s play period the task was not administered. If the infant did not respond to the sticker within the 60-s play period, the mirror was re-introduced and the infant was again given 20 s of mirror exposure. If the infant responded by bringing the hand within 2 cm of the sticker, while visually observing him or herself in the mirror, he or she was classified as a “self-recognizer” and the test was terminated. If the infant failed to respond, he or she was classified as a “non-recognizer” and the Sticker-saliency stage was implemented. 2.4.3. Sticker-saliency stage In contrast to a number of non-human primate studies (Anderson, 1984; Gallup, Wallnau, & Suarez 1980; Shillito, Gallup, & Beck, 1999; Suarez & Gallup, 1981), child development researchers, to date, have not investigated how children respond to marks on their own bodies that are clearly visible without a mirror. Therefore, if the infant was classified as a “non-recognizer,” the experimenter placed the mirror out of sight and surreptitiously placed a second sticker on the back of the infant’s hand.6 The aim of this stage was to ensure that the failure of an infant to reach for the sticker placed on his or her fringe was not due to a lack of motivation to retrieve the sticker (i.e., a false negative). 2.4.4. Coding MSR was coded from the videotapes of each session. The observer was first required to judge if the infant attempted to reach for the sticker prior to the mirror being introduced in the Mirror–sticker stage, and if not, whether he or she responded after the mirror was introduced by bringing either hand within 2 cm of the sticker while observing him or herself in the mirror. If the infant was judged as failing to respond to the sticker in the Mirror–sticker stage the observer was further required to judge if the infant removed the sticker from his or her hand in the Sticker-saliency condition. The age of emergence (AOE) of MSR7 was determined to be the age at which the infant first reached for the sticker in the Mirror–sticker stage. 2.5. Reliability Two trained coders independently observed and coded the videotapes of 12 randomly selected infants (2 from each session). Intraclass correlation coefficients (Shrout & Fleiss, 1979) were calculated between the scores of the two coders for the PTLT oriented to the peer-image for both the unmarked and marked conditions in the preferential-looking test, and between the
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two coders on each criterion of the MSR task. There was 100% agreement (and hence Intraclass correlation coefficients were equal to 1.00) between the two coders on each measure. 3. Results 3.1. The preferential-looking test Following Bahrick et al. (1996), in order to ensure that the infants had noticed both images, infants who did not gaze at each image for more than 1 s or who devoted less than 3% of the total fixation time to one of the displays in any condition were excluded from analysis involving that condition. Thus, in the marked condition, four infants were excluded at 9 months, two infants at 12 months, three infants at 15 months, one infant at 18 months and one infant at 24 months. In the unmarked condition, seven infants were excluded at 9 months, five infants at 12 months, three infants at 15 months and four infants at 18 months. In addition, not all infants were able to attend each session and some infants were unable to be tested on the preferential-looking test as they were either distressed or uncooperative during the Preparation or Presentation phases. The number of infants included in the subsequent analysis for each respective session is presented in Table 1 along with the mean proportion of available time (15 s) that they spent looking at both images (self and peer combined) in the unmarked and marked conditions. Given that the infants spent approximately 80% of the available time looking at either the self- or peer-image across all sessions, it can be concluded that the images were attractive stimuli. The PTLT spent fixating the image of the peer was calculated for the unmarked and marked conditions at each session.8 A MANOVA was performed for each session including these two dependent variables with order (unmarked condition first vs. marked condition first) as an independent variable. No significant effect for order was revealed for any session from 9 to 24 months of age. Therefore, the order of presentation was not considered further. Following Bahrick (1995) and Bahrick et al. (1996), to determine whether or not the infants were discriminating between the self- and peer-images, single-sample t tests were conducted against the chance value of .50. The results of these analyses are presented in Table 2. The infants discriminated between the two marked images in the 12-month session where they Table 1 Mean proportion of the available time (15 s) the infants spent looking at both monitors in the “unmarked” and “marked” conditions of the preferential-looking test Months
Condition Unmarked
9 12 15 18 21 24
Marked
N
Mean proportion
SD
N
Mean proportion
SD
68 79 74 70 66 66
0.82 0.77 0.79 0.80 0.84 0.88
0.18 0.19 0.19 0.16 0.17 0.11
70 80 75 73 66 64
0.78 0.80 0.85 0.86 0.85 0.89
0.20 0.16 0.16 0.16 0.14 0.12
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Table 2 Means, standard deviations and single-sample t tests (against a chance value of .50) for the PTLT oriented towards the peer-imagea for the “unmarked” and “marked” conditions of the preferential-looking test at each session Months
Condition Unmarked
9 12 15 18 21 24
Marked
N
Mean
SD
t value
N
Mean
SD
t value
68 79 74 70 66 66
0.50 0.53 0.47 0.46 0.46 0.44
0.18 0.19 0.17 0.17 0.19 0.16
0.14 0.34 1.79 2.06∗ 1.56 2.61∗∗
70 80 75 73 66 64
0.51 0.45 0.48 0.50 0.46 0.48
0.18 0.16 0.15 0.18 0.18 0.17
0.35 2.60∗∗ 1.09 0.17 1.60 0.83
a
A mean greater than 0.50 indicates a preference for the peer-image. p < .05 (two tailed). ∗∗ p < .01 (two tailed). ∗
showed a significant preference for the self-image. The infants also discriminated between the two unmarked images in the 18- and 24-month sessions where they again showed a significant preference for the self-image. To evaluate whether or not the means in the unmarked conditions were equal across sessions a one-way repeated-measures ANOVA was conducted. The repeated-measures ANOVA indicated that the means for the six sessions were not different, F (5, 160) = 1.19, p > .05. However, a significant linear trend was revealed, F (1, 32) = 5.44, p < .05, reflecting a change in fixation from the 12- to the 15-month session, t (66) = 2.11, p < .05. Similarly, to evaluate whether or not the means in the marked conditions were equal, a one-way repeated-measures ANOVA was conducted. The repeated-measures ANOVA indicated that the means for the six sessions were not significantly different, F (5, 185) = 0.71, p > .05. No significant trends were revealed. 3.2. Mirror self-recognition Three infants reached for the sticker prior to the mirror being introduced in the Mirror–sticker stage. However, as each of these infants had exhibited MSR in a previous session their AOE of MSR was not affected. Similarly, one infant failed to show MSR in one session (24 months) having previously exhibited it in previous sessions (18 and 21 months). The AOE data for these four infants were therefore retained. Six infants were withdrawn from the study prior to exhibiting MSR. With the exception of one infant, all infants who failed to respond to the sticker in Stage 2 (Mirror–sticker stage) of the mirror task removed the sticker from their hand in Stage 3 (Sticker-saliency). As the infant who failed to respond to the sticker being placed on his hand in Stage 3 had not previously exhibited mark-directed behavior, his failure to show interest in the sticker meant that his AOE of MSR could not be reliably determined. Therefore, these seven infants were excluded in subsequent analyses involving the AOE of MSR. In addition, seven infants failed to show mark-directed behavior in any session. These seven infants were also excluded in subsequent analyses involving the AOE of MSR.9
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Fig. 1. The cumulative percentage of infants showing evidence of mirror self-recognition (MSR) at each session. Note: The percentages for MSR were calculated including those seven infants who failed to exhibit MSR by the 24-month session.
Consistent with past research, the mean, median and mode AOE for the remaining 76 infants was 18 months. The cumulative percentage of infants showing evidence of MSR at each session (including those seven infants who failed to exhibit MSR by the 24 months of age) is presented in Fig. 1. 3.3. Mirror self-recognition and the preferential-looking test In order to test the hypothesis that infants would show a preference for their own image coincident with the onset of MSR, gaze orientation in each condition of the preferential-looking test was calculated for each infant for the sessions before, during and after the session at which he or she first exhibited MSR (see Fig. 2).
Fig. 2. The PTLT oriented to the peer-image in the “unmarked” and “marked” conditions of the preferential-looking test according to the AOE of MSR. Note: Means greater than 0.50 indicate a preference for the peer-image and means less than 0.50 indicate a preference for the self-image.
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Table 3 Means, standard deviations and one-sample t tests (vs. 0.50) for the PTLT oriented to the peer-image for the “unmarked” and “marked” conditions of the preferential-looking test according to the session in which MSR was first exhibited Months
Condition Unmarked
Less 9 Less 6 Less 3 At MSR Plus 3 Plus 6 ∗
Marked
N
Mean
SD
t value
N
Mean
SD
t value
38 58 67 63 58 38
0.52 0.50 0.51 0.45 0.48 0.45
0.19 0.18 0.20 0.17 0.20 0.18
0.49 0.03 0.27 2.28∗ 0.79 1.62
39 61 69 64 57 39
0.51 0.54 0.47 0.45 0.49 0.49
0.18 0.16 0.17 0.17 0.16 0.19
0.27 1.95 1.51 2.33∗ 0.68 0.19
p < .05 (two tailed).
It is evident in Fig. 2 that the infants oriented their gaze towards the self-image in both conditions coincident with the onset of MSR. To determine whether or not the infants were discriminating between the two images, single-sample t tests against the chance value of .50 were conducted on the PTLT oriented to the peer-image (see Table 3). In line with Fig. 2, the only session in which the infants significantly discriminated between the self- and other-images was in the same session during which they first exhibited MSR. In this session, infants showed a significant preference for the self-image in both the unmarked, t (62) = 2.28, p < .05, and marked, t (63) = 2.33, p < .05, conditions. 4. Discussion The primary aim in this investigation was to evaluate the hypothesis that the onset of success on the mark test would coincide with infants showing a preference for looking at the self-image in the preferential-looking test. This hypothesis was supported. The infants in this study showed a significant preference for their self-image in both marked and unmarked conditions in the session that they first exhibited MSR. In contrast, they did not show a significant preference for either image in the sessions prior to or following the session in which MSR was first exhibited. As noted previously, each of the accounts of mark-directed behavior (Mitchell, 1993, 1997; Suddendorf & Whiten, 2001; Gallup, 1979) maintain that in order to pass the mark test infants must become interested in their own facial features. By developing an interest in their own face infants can begin to appreciate the connection between themselves and their mirror image. The preference shown by the infants for looking at their own image in the session that they first passed the mark test is congruent with this suggestion. The finding that infants fixated their gaze to their own image coincident with the onset of MSR also provides evidence that the mark test is a reliable index of the developing ability infants have for recognizing their own physical features. Alternative interpretations of the test posit that mark-directed behavior need not evoke self-recognition in any form. For example, it has been argued that success on the mark test can be reduced to the ability of individuals for detecting the contingency inherent in mirrored feedback (Heyes, 1994, 1998; Menzel, Savage-Rumbaugh,
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& Lawson, 1985). Heyes (1994) has similarly suggested that individuals can pass the mark test via a process of distinguishing “across a fairly broad range, sensory inputs resulting from the physical state and operations of [their] own body, from sensory inputs originating elsewhere” (p. 915). It now becomes necessary for proponents of these alternative interpretations to explain the findings reported here. Curiously, the infants in the current study did not maintain a strong preference for their own faces following the onset of MSR. That is, the infants did not continue to significantly discriminate between the images in either condition of the preferential-looking task following the session in which they first succeeded on the mark test. This failure to discriminate between the images may be due to individual differences that are likely to exist in the preferences infants have for looking at novel over familiar stimuli (see Kagan, Resnick, & Snidman, 1987) and for looking at images of “me” over images of “not me.” Further research is therefore needed to evaluate the preference infants have for looking at images of themselves over other familiar faces and for looking at images of familiar over unfamiliar faces. Such comparisons will not only help to delineate the kinds of individual differences that may have impacted on the results reported here, but will also help to identify the bases upon which infants differentiate images of themselves from images of other people. Past research has reported that infants aged between 4 and 9 months orient to the peer-image in the unmarked condition of the preferential-looking test (Bahrick et al., 1996; Legerstee et al., 1998; Rochat & Striano, 2002). The present investigation did not find this preference in infants aged from 9 to 24 months. Rather, at 18 and 24 months the infants oriented to the self-image. As the majority of infants had become self-recognizers by 18 months, this orientation is consistent with the data showing a preference for the self-image coincident with the onset of MSR. However, the failure to reveal a similar preference at 21 months of age is incongruent with this interpretation. As such, these findings may be significant by chance and should be treated with caution. Bahrick (1995) reported that 5- to 8-month-old infants showed a significant change in their orientation of gaze in the marked compared to the unmarked version of the preferential-looking test. When the images were unmarked infants oriented to the novel face, but when the images were marked infants made no discrimination. This difference in looking patterns was not evident in the current study. The only session in which the PTLTs differed significantly between conditions was at 12 months where the infants showed a significant preference for the marked self-image (PTLT = 0.45) but did not discriminate between the unmarked images (PTLT = 0.53).10 The preference for the self-image at 12 months was also the only session in which infants showed such a preference in the marked condition. As with the results reported above for the unmarked condition, in the absence of any reliable pattern of looking preference across sessions, the lone significant orientation to the marked self-image remains challenging to interpret and should be treated with caution. The looking preferences observed in the current study suggest that the processes driving gaze orientation in the studies with younger infants (Bahrick, 1995; Bahrick et al., 1996; Legerstee et al., 1998; Rochat & Striano, 2002) are distinct from those driving the preferences in the older infants tested here. However, these apparent differences may be a function of assessing older infants with a test designed to evaluate development in younger infants.11 Moreover, in order to maintain the interest of the older infants tested here, it was necessary to use a
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shorter presentation time than has been previously used. The current investigation was also the first in which the same infants were assessed on both unmarked and marked conditions of the preferential-looking test, and it remains possible that assessing the gaze orientation of the same infants in both conditions altered their regular patterns of looking preference. Together, these methodological differences may have contributed to the lack of correspondence between the findings reported here and those reported in the studies cited above. Further research is needed to disentangle which of these methodological differences account for the present results. Upon cursory observation of the behavior of the infants studies here, it was notable that no infant (at any age) responded to seeing his or her marked image in the preferential-looking test by exploring the marked region on his or her face. This observation is in line with the finding that young 2 year olds fail video or photograph analogues of the mark test (Povinelli et al., 1996; Suddendorf, 1999; Zelazo, Somerville, & Nichols, 1999). That is, in the absence of contingency cues, young children do not appear to connect images of the self to their current reality, and this is something they need to transcend if they are to bind time into one conceptual present (Suddendorf & Corballis, 1997). In stark contrast to the vast corpus of research conducted using the traditional mirror test, the use of the preferential-looking paradigm to investigate self-recognition has been minimal. Therefore, continued refinement of the preferential-looking test is warranted. In so doing, preferential-looking may prove to be a valuable adjunct to the standard mirror test in the investigation of self-recognition. There is a continued importance for assessing the development of how human and non-human individuals deal with external representations of themselves. Such research will enrich our understanding of what may prove to be a crucial process in both ontogenetic and phylogenetic development. Notes 1. Young infants use contingency information to determine the relatedness of external representations to the self (e.g., Bahrick & Watson, 1985). 2. More detail on the cohort can be obtained from the first author. 3. The infants were also assessed on a range of tasks that are not discussed here. 4. It is relevant to note that the video used as the 15-s ‘self’ stimuli was same video used as the 15-s ‘peer’ stimuli for the following infant. 5. Previous research with the preferential looking task has used presentation times ranging from 30 s (Bahrick, 1995; Bahrick et al., 1996) to 1 min (Legerstee et al., 1998; Rochat & Striano, 2002). However, pilot testing indicated that a 15-s presentation time was sufficient to assess discrimination while maintaining the interest of the older infants studied here. 6. Note that this provided tactile cues. 7. The AOE measure indicates the approximate age at which the infants first developed MSR. It is acknowledged that MSR may have emerged in the 3 months preceding the AOE session. 8. The alternative to calculating PTLTs is to compare actual looking times for each stimulus. Thus, all analyses reported here were also conducted using actual looking times. As this
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alternative analysis did not effect the substantive interpretations made (and were identical with regard to the onset of MSR), in order to maintain consistency with Bahrick (1995) and Bahrick et al. (1996) and for the sake of parsimony all analyses are reported using PTLTs. 9. An alternative approach would be to credit these seven infants with an arbitrarily determined AOE that is greater than 24 months. However, this approach is rendered problematic by reports that, for example, some children as old as 4 years of age still fail to pass the mark test (Rochat, MacGillivray, Callaghan, Genest, & Lewis, 2002). Thus, the relevant measure, individual differences in the AOE of MSR, would potentially be confounded by such an approach. 10. t (75) = 2.98, p < .01. 11. However, the preferential looking task has been successfully used with older infants in studies of language acquisition (e.g., Bavin & Growcott, 2000; Naigles & Gelman, 1995).
Acknowledgments The work described in this article was funded by a La Trobe University Postgraduate Research Scholarship to the first author and formed part of his doctoral thesis under the supervision of the second author. The authors would like to thank the carers and children who gave their time to participate in this study. The authors also thank Virginia Slaughter and Thomas Suddendorf for their comments and suggestions on this article.
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