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. Author manuscript; available in PMC: 2022 Jun 21.
Published in final edited form as: J Cogn Dev. 2021 Jun 21;22(5):678–694. doi: 10.1080/15248372.2021.1926254

Eye Tracking Lateralized Spatial Associations in Early Childhood

Eloise West 1,1, Koleen McCrink 1
PMCID: PMC8942349  NIHMSID: NIHMS1722326  PMID: 35330699

Abstract

This experiment tests the age at which left-to-right spatial associations found in infancy shift to culture-specific spatial biases in later childhood, for both numerical and non-numerical information. Children ages 1 to 5 years (N=320) were tested within an eye-tracking paradigm which required passive viewing of a video portraying a spatial transposition. In this video, an item was hidden in a vertical set of locations, which were then surreptitiously rotated 90°. There were several conditions, which varied in the degree to which the locations were presented alongside ordinal (numerical, alphabetical) or non-ordinal (nonsense label) information. After transposition, a narrator prompted the child to visually search the array. The amount of time spent fixating in a location consistent with a left-to-right mapping or a right-to-left mapping was measured to gauge the degree and laterality of spatial associations. Overall, children looked more towards locations consistent with a left-to-right mapping. This effect fluctuated with age, dipping as children entered toddlerhood, increasing in 3- and 4-year-olds, and then disappearing at age 5. The ordinal nature of the stimuli (e.g., numerical or non-numerical) did not influence the laterality of the spatial associations. A follow-up experiment confirms that, like older preschoolers, adults (N=66) also exhibit no spontaneous left-to-right mapping bias in this paradigm, with no fluctuation as a result of condition. These data support the presence of a decrease in left-to-right processing around the age of two, as children recede from infantile spatial biases and progress to exhibiting culture-specific spatial biases in early childhood.

Keywords: spatial associations, space, number, laterality, toddlerhood, eye-tracking

Humans use space as an organizational scaffold to facilitate cognitive processing. Spatial associations occur when ordered information, which is not intrinsically spatial, is mapped onto an internal linear spatial continuum. Spatial associations in which ordered information is organized in a structured, directional, and relational fashion increase encoding and memory in infants (Bulf, de Hevia, Gariboldi, & Macchi Cassia, 2017), children (Thompson & Opfer, 2016), and adults (McCrink & Shaki, 2016). This ordered information includes time or future events (Boroditsky, 2000; Tillman, Tulagan, Fukuda, & Barner, 2018), magnitudes of power (Schubert, 2005), months of the year, letters of the alphabet (Gevers, Reynvoet, & Fias 2003), pitches of sound (Rusconi, Kwan, Giordano, Umilta, & Butterworth, 2006), newly ordered sequences of random words (Previtali, de Hevia, & Girelli, 2010), and perhaps most prominently, numbers (Dehaene, Bossini, & Giraux, 1993; Moyer & Landauer, 1967).

Some spatial associations are driven by evolutionary factors. Human populations with little or no environmental experience such as neonates and preverbal infants (Bulf, de Hevia, & Macchi Cassia, 2016; de Hevia, Addabbo, Girelli, & Macchi Cassia, 2014; de Hevia, Izard, Coubart, Spelke & Streri, 2014; de Hevia & Spelke, 2013; de Hevia, Veggiotti, Streri, & Bonn, 2017; DiGiorgio, Lunghi, Rugani, Regolin, Dalla Barba, Vallortigara, & Simion, 2019), as well as non-human animals such as macaques (Drucker & Brannon, 2014), chimpanzees (Adachi, 2014), and chicks (Gallus gallus; Rugani, Kelly, Szelest, Regolin, & Vallortigara, 2010; Rugani, Vallortigara, Priftis, & Regolin, 2015), all exhibit a spontaneous penchant for associating “few” with the left side of space and “many” with the right. These early untrained lateralized spatial mappings emerge primarily when the to-be-mapped dimension contains discrete magnitude information (Bulf et al., 2016; de Hevia & Spelke, 2013; de Hevia et al., 2017). For example, discrete numerical quantities such as dot configurations, but not continuous magnitudes like area, orient infants’ attention towards the region of space congruent with the quantity’s relative position on a left-to-right oriented spatial continuum (Bulf et al., 2016). The left-few/right-many lateralization is likely due to right-hemisphere dominance in spatial-attentional processes (Lourenco & Longo; 2010; Vallortigara, 2012), which is associated with the initial processing of the left visual field, resulting in leftward visual biases for the initial stimulus in a series. In humans, this dominance is coupled with a right-hemisphere processing bias for number early in infancy; the neural regions that encode magnitude are located in the right parietal cortex of infants’ brains, before becoming more bilateral later in development (Cantlon, Brannon, Carter, & Pelphrey, 2006; Hyde, Boas, Blair, & Carey, 2010; Izard, Dehaene-Lambertz, Dehaene 2008; Libertus, Pruitt, Waldorf, & Brannon, 2009).

Despite the left-to-right spatial associations observed in infants and non-human animals, adults’ spatial associations are not identical across cultures. Adults enculturated in a society where language is scripted from left-to-right preferentially organize ordered information from left-to-right (Dehaene et al., 1993; Vallesi, Weisblatt, Semenza, & Shaki, 2014). Adults and children enculturated in a society where language is scripted from right-to-left exhibit mixed or reversed spatial mappings (Dehaene et al., 1993; Shaki, Fischer, & Petrusic, 2009; Vallesi et al., 2014), and illiterate adults exhibit no asymmetrical spatial biases (Zebian, 2005). Bilingual Hebrew and English speakers show varied spatial biases modulated by exposure, exhibiting left-to-right spatial associations following presentation of the English alphabet, and right-to-left associations following presentation of the Hebrew alphabet (Shaki & Gevers, 2011). These cultural differences suggest that asymmetric spatial associations observed in infancy eventually shift in a way that reflects the learner’s dominant language. This shift appears to start around the age of three or four; at which point children exhibit culturally-dependent counting behaviors (Shaki, Fischer, & Göbel, 2012), and encode ordered information more readily when it is spatially presented in a culturally-consistent manner (McCrink, Shaki, & Berkowitz, 2014; Opfer & Furlong, 2011).

The field has yet to fully document the developmental transition from universal infantile biases to culturally-consistent directional organization of ordered information. Logically, this change must occur during toddlerhood; we theorize that toddlers are temporarily agnostic in their spatial biases, allowing cognitive flexibility as they become attuned to cultural norms (McCrink & de Hevia, 2018; McCrink, Perez, & Baruch, 2017). Toddlers may become sensitive to cultural spatial conventions due to increased spatial activities and social scaffolding of cultural customs as the child’s play becomes more vigorous and structured. Possible mechanisms include parental gesture and children’s observations of reading (Göbel, McCrink, Fischer, & Shaki, 2018; McCrink, Caldera, & Shaki, 2017; McCrink & de Hevia, 2018; Patro, Nuerk, Cress, & Haman, 2014). For example, McCrink et al. (2017a) found that Hebrew-speaking and English-speaking parents gestured and laid out photos in a culturally-consistent direction when telling a story or explaining a video, especially when the stimuli contained numerical information. McCrink, Perez, & Baruch (2017) found that toddlers exhibit left-to-right spatial-numerical mappings, but only when provided numerical labels for locations in the visual array, and not when locations were unlabeled nor when they had alphabetical labels. Even in this ‘agnostic’ period of toddlerhood, then, numerical information prompts more rigid left-to-right spatial biases than other types of ordered information, due to the strong environmental spatialization of numerical order.

The present study sought to document the laterality of spatial associations throughout late infancy and early childhood, and whether these spatial associations fluctuate as a function of the type of information to be processed. Although there is a good deal of research into this area, there exists no study which uses an identical task across infancy, toddlerhood, and the preschool years to directly compare spatial associations during this critical period. In this study, children ages 1 to 5 years were presented with a spatial transposition task adapted from the non-human animal literature (Drucker & Brannon, 2014; Rugani, Kelly, Szelest, Regolin, & Vallortigara, 2010), and previously used in live-action form with toddlers (McCrink, Perez, & Baruch, 2017), wherein the participant is trained to learn that one spatial location in a vertical set is associated with an object. Then, the set is surreptitiously transposed, such that the participant can search with a left-initial or right-initial mapping strategy at either of the correct locations. This paradigm does not manipulate the distance between the locations, and therefore both spatial and object order cues relating to the position number are conflated. Thus, we use the term spatial location to refer to the target object’s location during training, which combines both the discrete nature of the object position as well as the spatial distances between the locations.

Here, the children passively watched a video with narrated prompts to search for the object at whatever spatial location they think it will appear; this narration also contains information that provides ordinal (numbers, letters) or unordered (nonsense labels) cues for each spatial location. If cultural spatial biases are undergoing development during toddlerhood, then one-year-old infants should show an untrained leftward bias, in parallel with previous work on pre-verbal infants’ spatial associations (Bulf et al., 2016; de Hevia et al., 2014a; de Hevia et al., 2017), which will then fall away in toddlerhood (around 2–3 years), and re-emerge during the preschool years (> 3), consistent with previous findings regarding the development of culturally driven spatial associations (Shaki, Fischer, & Göbel, 2012). Further, we predicted that the degree of ordinality would moderate the strength of spatial associations throughout childhood, as the presence of highly ordered information (number, particularly non-symbolic number which is associated with untrained left-to-right spatial biases in non-human animals and young infants; Adachi, 2014; Bulf et al., 2016; de Hevia et al., 2014a; de Hevia et al., 2017; Drucker & Brannon, 2014; Rugani et al., 2010; Rugani et al., 2015) would prompt a stronger left-to-right mapping strategy than ordered but non-quantitative information (letters), which in turn would be more lateralized than non-ordinal information (nonsense words). Specifically, it has been theorized that, in a left-to-right scripted culture, non-symbolic number is mapped from left-to-right from infancy throughout adulthood. Then, children begin mapping symbolic number from left-to-right during toddlerhood when they learn to count, and go on to map other ordinal information from left-to-right once they become more enculturated during the pre-school/kindergarten years (McCrink & de Hevia, 2018).

Method

Participants

Three hundred and twenty children between the ages of 12 and 72 months (Mage= 42 months) were recruited at two local children’s museums in a large urban city. An additional 65 participants were excluded due to inability or refusal to complete the task (8), a poor gaze track (36), equipment malfunction (14), experimenter error (4), or exposure to a language that is scripted from right-to-left (3). The final sample included 64 one-year-olds between the ages of 12.1–23.9 months (Mage =1 year, 6 months; 26 females), 64 two-year-olds between the ages of 24–35.9 months (Mage =2 years, 6 months; 26 females), 64 three-year-olds between the ages of 36–47.9 months (Mage =3 years, 6 months; 31 females), 64 four-year-olds between the ages of 48–49.8 months (Mage =4 years, 6 months; 30 females), and 64 five-year-olds between the ages of 60–71.5 months (Mage=5 years, 5 months; 33 females) (see Table 1 for more demographic information). This sample size was calculated by a power analysis beforehand in G*Power (Faul, Erdfelder, Lang, & Buchner, 2007; small overall predicted effect size of partial n2 = .05, α error prob = .05, Power = .85, after McCrink et al., 2017b, which utilized a similar paradigm in live-action form with preschoolers). We included side-of-screen, age, condition, and gender as variables in our power analyses. Gender was included as a variable as a requirement from our funding source, and also in a more exploratory fashion as the developmental trajectory of gender differences in spatial cognition remains controversial (see Levine, Foley, Lourenco, Ehrlich, & Ratliff, 2016 for a review).

Table 1.

Participant demogaphics by age group. Parentheses indicate standard error.

Age Group Parental Education Level Vocabulary Level (as assessed by the DVAP; Libertus et al., 2013) Exposed to a language other than English (scripted from left-to-right)
1-year-olds Some High School: n = 0
GED/HS Diploma: n = 2
Some College: n = 2
College Graduate: n = 12
Masters/Doctoral Degree: n = 17
N/A: n = 31		 10.8
(2.92)
 n = 20
2-year-olds Some High School: n = 0
GED/HS Diploma: n = 1
Some College: n = 6
College Graduate: n = 15
Masters/Doctoral Degree: n = 20
N/A: n = 22		 53.24
(7.40)		 n = 25
3-year-olds Some High School: n = 1
GED/HS Diploma: n = 1
Some College: n = 4
College Graduate: n = 21
Masters/Doctoral Degree: n = 23
N/A: n = 14		 88.32
(9.14)		 n = 35
4-year-olds Some High School: n = 0
GED/HS Diploma: n = 1
Some College: n = 3
College Graduate: n = 12
Masters/Doctoral Degree: n = 30
N/A: n = 18		 116.21
(7.14)		 n = 26
5-year-olds Some High School: n = 0
GED/HS Diploma: n = 2
Some College: n = 6
College Graduate: n = 26
Masters/Doctoral Degree: n = 15
N/A: n = 15		 115.46
(4.67)		 n = 20
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We subsequently tested a sample of college-aged adults to determine whether the observed pattern of results extends to adulthood (66 undergraduate students; N=17 in the Non-Symbolic Sets and Symbolic Number condition, and N=16 in the Letter and Nonsense Labels conditions; Mage=20 years).

Design and Procedure

Each participant was randomly assigned to one of four conditions, which differed in the information available during the initial vertical mapping portion of the experiment: Symbolic Number, Non-Symbolic Number, Letter, or Nonsense Label. The placement of the object during the vertical training trials was counterbalanced; it was hidden in the second spatial location from the bottom for half the sample, or fourth spatial location from the bottom for the other half of the sample (see Figure 1).

Figure 1.

Figure 1.

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Schematic of the scene progression and script for each condition and trial; in this example the target (the ‘baby chick’) is located in the second pond from the bottom during training. The scenes surrounded by the green border indicate when the visual search occurs. The dotted lines indicate the Area of Interest (AOI) for computing the participants’ gaze, with the yellow box indicating the AOI for Correct / Left-to-right mapping in the vertical and horizontal test trials, and the red box indicating the AOI for Incorrect / Right-to-left mapping.

Introduction trial.

The participants were presented with an animated visual scene of a vertical rectangle with one blue circle in the middle, and a chick at the base. The narration introduced the chick as “mommy chick”, who asked where her hiding baby is. The baby chick then popped out from the blue circle (labeled as a pond), before the rectangle was occluded by an animated box coming down from the top of the screen. The narration explained that “she is being covered and turned”. When the occluder was lifted, the baby chick was still in the pond. The total length of the trial was 15.1 seconds, the baby chick appeared at 6.1 seconds, and the occluder was lifted after 11.3 seconds.

Vertical trials.

After the introduction trial (15.1 seconds), the children received four vertical trials, in which they learned the consistent location of the hidden chick (one of 5 possible vertical locations). During each trial, the baby chick was revealed to be in the same pond. In the first vertical trial, the mommy chick asked where her baby is hiding. The target pond was marked with a pink star. The narrator then introduced labels in a serial order, from the bottom of the array to the top. The ponds blinked when the narrator presented their label. For the Nonsense Labels condition, the narrator stated: “She’s not in pond dax. Oh! She’s in pond ziff. Not in pond blick, not in pond mott, and not in pond wug.” In the Symbolic Numbered condition, the narrator labeled the ponds as pond one, two, three, four, and five. In the Non-Symbolic Numbered condition, the narration stated: “She’s not in this pond” as each pond blinked, and “Oh, she’s in this pond!” when the target pond was reached. For this condition, the labels were provided visually as the ponds had a set of dots depicted visually on each pond location, from one to five, respectively (see Figure 1). In the Lettered condition, the narration labeled the ponds as pond A, B, C, D, and E. The second and third vertical trials were nearly identical, without the target pond being marked by a star. The narration differed slightly to maintain interest, utilizing the aforementioned labels. The first, second, and third vertical training trials were 18 seconds, 13 seconds, and 14 seconds long, respectively. The fourth and final vertical trials began with the vertical array, which was then covered by an occluder. After two seconds of occlusion, the ponds were then uncovered, and the narrator asked the participant where they think the chick is hiding. After seven seconds post-occlusion, the chick was then revealed to be in the second/fourth pond from the bottom, where it had been hiding in the previous trials. The final vertical trial was 17 seconds. See Figure 1 for a schematic of the trials and the supplemental materials (S1) for the videos of each condition.

Horizontal test trial.

The final trial began with the vertical array of five ponds, which were occluded while the narrator stated: “Now they’re covering and turning the ponds.” After two seconds, the ponds were uncovered to reveal that the vertical array of five ponds had been rotated 90°. The mommy chick stated that she does not know where her baby is, and asked the participant where they think it is. After ten seconds, the chick was revealed to be in the pond consistent with left-to-right mapping (the second/fourth pond from the left). The horizontal test trial was 21.3 seconds. The counterbalancing of hiding the to-be-encoded item in either the second or fourth location from the bottom during training ensured that any overall bias to scan the visual scene from left-to-right could not drive a main effect of side of screen.

Stimuli

The rectangle containing the ponds was 15×3 cm, and each pond measured 2.5 cm in diameter. The mommy chick was 4×3 cm, and the baby chick was 1.5×2 cm. Stimuli were made in Keynote and exported to a movie file, presented on a 15.6 Inch HP Z-Book within the Tobii Pro Studio software program, approximately 75 cm away from the child. A Tobii Pro Eye-tracker X3–120 was used; this device utilizes binocular tracking with a gaze sampling frequency of 120 Hz. Participants were calibrated using Tobii Studio’s infant calibration tool, which presents geometric animated visuals and associated sounds at five fixed locations on the screen. During calibration, the experimenter observed the participant attend to each location, and after judging that the participant was looking toward the stimulus for approximately two seconds, presented the stimulus in a novel location by key press. If the participant looked away from the screen during calibration, the experimenter presented a different animation and associated sound to direct the participant’s attention toward the screen. When the participant fixated on the novel stimulus, the calibration visual was resumed. Participants viewed the calibration visual until their eyes were reliably tracked at each of five locations on the screen, determined by Tobii Studio’s calibration report.

Coding

Eye-tracking data were gathered from the final vertical trial and the horizontal test trial. The amount of time spent fixating in the half of the screen that contained the incorrect and correct hiding location was analyzed for the final vertical trial (total search window: 7 seconds), and the amount of time spent fixating in the half of the screen consistent with a left-to-right or right-to-left mapping strategy was analyzed in the horizontal trial (total search window: 10 seconds), split down the center of the middle pond (See Figure 1). (This large Area of Interest (AOI) reflected a lack of surety with the fine-tuning of the tracker, given how inattentive children are during this developmental phase, and the imprecise nature both of the track and children’s ability to organize their looking patterns (e.g., fixating on irrelevant parts of the scene on their way to a specific location). Our central questions revolve around the general laterality bias, not the children’s precise ability to recall the location of an object. Thus, the large split screen analysis could capture any considerations of the left locations vs. the right locations. We coded as well for a restricted AOI around a very narrow region around the 2nd or 4th pond in the array; the overall patterns in the data are similar, but variability is higher. These data, alongside other exploratory and demographic variables such as handedness, maternal education, and vocabulary level, can be found in Supplemental Material S2.)

With regard to the accuracy of the eye-tracker, eye-gaze data were collected from children during an average of 74.73% of the total duration of the experiment, (SD=17.29), comparable to other studies using this technology with preschoolers (Skibbe, Thompson, Plavnick, 2017). Data were collected during an average of 89.53% of the total duration of the experiment for adults (SD=8.25), within the theoretical ideal range (85–95%) and in line with previous adult studies (Hvelplund, 2014).

Results

Our main question of interest was the degree to which children searched leftward vs. rightward after the vertical array was transposed. A repeated-measures ANOVA over the sum of fixation time, computed within Tobii Studio software, was conducted with side of screen (left-to-right mapping, right-to-left mapping) as a within-subjects factor, and age (1,2,3,4,5), condition (Non-Symbolic Sets, Symbolic Number, Lettered, Nonsense Labels), and gender as between-subjects factors. There was a main effect of side of screen (F(1,280)=5.91, η2p =0.21, p=.016); overall, children spent more time searching the side of the screen consistent with left-to-right mapping (M=2.0 sec, SE=.09) than the side of the screen consistent with right-to-left mapping (M=1.7 sec, SE=.08). There was no main effect of gender, age, or condition, nor any interpretable interaction between these variables (see Tables 2 and 3 for means by age and condition). Note that, although there was a significant four-way interaction between side of screen x age x gender x condition, follow-up analyses suggest this interaction is not indicative of overall lateralization tendencies, and no clear pattern emerges across the data.

Table 2.

Results of a repeated-measures ANOVA over the sum of fixation time, with side of screen (left, right) as a within-subjects factor, and age (1,2,3,4,5), condition (Non-Symbolic Sets, Symbolic Number, Lettered, Nonsense Labels), and gender as between-subjects factors.

Source Df F η2 p.
Side of screen 1 5.91 0.21 0.016
Side of screen x age 4 1.19 0.02 0.314
Side of screen x gender 1 0.00 0.00 0.996
Side of screen x condition 3 1.10 0.01 0.351
Side of screen x age x gender 4 1.91 0.03 0.108
Side of screen x age x condition 12 0.32 0.01 0.986
Side of screen x gender x condition 3 0.87 0.01 0.457
Side of screen x age x gender x condition 12 1.80 0.07 0.048
Error (side of screen) 280
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Table 3.

Mean fixation duration (seconds) for each age group and condition. Parentheses indicate standard error.

Condition Age Mean Cumulative Fixation Duration; LR mapping (seconds) Mean Cumulative Fixation Duration; RL mapping
(seconds)
Non-Symbolic Number 1-year-olds 1.88
(0.46)		 1.71
(0.40)
2-year-olds 1.68
(0.39)		 1.83
(0.35)
3-year-olds 1.58
(0.40)		 1.58
(0.35)
4-year-olds 2.17
(0.40)		 2.19
(0.35)
5-year-olds 2.31
(0.40)		 2.34
(0.35)
College Students 5.09
(0.72)		 3.31
(0.60)
Symbolic Number
 1-year-olds 1.99
(0.40)		 1.36
(0.35)
2-year-olds 1.34
(0.39)		 1.42
(0.35)
3-year-olds 2.14
(0.43)		 1.74
(0.38)
4-year-olds 2.38
(0.39)		 2.09
(0.35)
5-year-olds 2.49
(0.46)		 2.03
(0.40)
College Students 3.83
(0.55)		 3.96
(0.65)
Lettered 1-year-olds 1.09
(0.43)		 1.19
(0.38)
2-year-olds 1.26
(0.43)		 1.50
(0.38)
3-year-olds 1.59
(0.40)		 1.88
(0.35)
4-year-olds 2.17
(0.43)		 1.44
(0.38)
5-year-olds 2.29
(0.40)		 1.96
(0.35)
College Students 4.02
(0.69)		 3.09
(0.63)
Novel Words 1-year-olds 1.38
(0.39)		 0.70
(0.35)
2-year-olds 1.89
(0.43)		 2.00
(0.38)
3-year-olds 2.45
(0.41)		 1.59
(0.36)
4-year-olds 3.12
(0.39)		 2.09
(0.35)
5-year-olds 2.44
(0.41)		 2.19
(0.35)
College Students 3.78
(0.56)		 4.04
(0.57)
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To determine whether the lateralized nature of the fixation exhibited shifting trends over early development, a proportional laterality score was computed (fixation time LR-mapping/(fixation time LR-mapping + fixation time RL-mapping)). A one-way ANOVA with age (1,2,3,4,5) as the between-subjects factor revealed a cubic developmental trend, with two inflection points at age 2 and age 5 (F(4,315)=7.16, η2 =0.02, p=.008; 1-year-olds: M=.58, SE=.29; 2-year-olds: M=.46, SE=.27; 3-year-olds: M=.53, SE=.27; 4-year-olds: M=.57, SE=.28; 5-year-olds: M=.52, SE=.27). When restricted only to the period of infancy to early preschool (1–4 years), the predicted quadratic trend emerged (F(3,252)=5.58, η2 =0.02, p=.020). One-sample tests against a test value of .50 revealed that 1- and 4-year-olds fixate more to a LR-mapping location than RL (respectively, .58 and .57; ts(1,63)=2.1, 1.9; ps=.019, .031, but not the 2-, 3-, and 5-year-olds (respectively, .45, .53, .52; see Figure 2). It is important to note that the proportional laterality score assesses the strength of this bias across ages. When this proportional laterality score was translated to a binary scale indicating whether children overall spent more time searching the left or right portions of the screen (1 for children with a laterality score above 0.5, 0 for children with a laterality score below 0.5), the number of children who showed a bias was similar across age groups. Slightly more children in all age groups, except for two-year-olds, had a proportional laterality score above 0.5, or looked more toward the side of the screen consistent with left-to-right mapping (n=40 1-year-olds, n=31 2-year-olds, n=39 3-year-olds, n=36 4-year-olds, n=35 5-year-olds).

Figure 2.

Figure 2.

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Proportion of time spent fixating to the side of screen corresponding to the left-to-right mapping of the vertical target to the horizontal array. Error bars indicate +/− 1 SEM.

To determine if this developmental trend reflected poor memory for the initial vertical placement, a proportional correct-mapping score for the final vertical trial was computed (fixation time correct-mapping/(fixation time correct-mapping + fixation time incorrect-mapping)). A one-way ANOVA revealed no significant difference in the proportion of correct looking in the final vertical trial across age groups (F(4,306)=1.78, η2 = .02, p=.132); overall, children spent more time fixating on the correct location (M=.56; one-sample t-test against .50, t(310)=3.81, p = <.001). We then tested for a correlation between the proportion of correct looking (in the final vertical trial) and the proportion of LR-mapping (in the horizontal test trial), controlling for age. The variables were not positively related; indeed, there was a significant negative correlation (r(308)=−.14, p=.016). For 1-year-olds, we see a correlation of r(57)=−.19, p=.161; 2-year-olds r(63)=−.32, p=.011; 3-year-olds r(63)=−.24, p=.057; 4-year-olds r(64)=−.155, p=.222; and 5-year-olds r(64)=.16, p=.198. In combination, these results suggest that children in all age-groups learned the target location during training to a similar extent, but not all children displayed a LR-mapping, and there was no pattern of relationship between the two that could explain the propensity to make a lateralized mapping.

We also measured whether children were more likely to first look toward the location with LR-mapping or RL-mapping. The side of the screen which the child first looked towards was coded on the basis of the time to their first fixation in that Area of Interest. A one-way ANOVA revealed no effect of age on the side of the child’s first fixation: children at any age were equally likely to first look toward the left and right sides of the screen, (F(4,315)=1.76, η2=.02, p=.137). To test whether the counterbalanced baited location (second from the bottom of the array of fourth from the bottom of the array) impacted the laterality of mapping during the transposition task, we ran a univariate ANOVA over the proportion of LR mapping with age (1,2,3,4,5), condition (Non-Symbolic Sets, Symbolic Number, Lettered, Nonsense Labels), and baited location as between-subjects factors. There was no effect of baited location on the proportional laterality score (F(1,280)=1.16, η2=.005, p=.250), nor any interaction between baited location, age, or condition.

Further, as previous studies (McCrink et al., 2017b; Opfer & Furlong, 2011; Opfer, Thompson, & Furlong, 2010) found that children’s counting routine was related to their propensity to exhibit lateralized spatial associations, the present study tested whether children’s degree of left-to-right mapping relating to the direction in which they counted. A one-way ANOVA with laterality difference score as the dependent variable and counter status (organized, disorganized) revealed that non-directional counters (N = 29) exhibited similar left-to-right mapping bias compared to organized counters (N = 184), F(1,212)=1.64, η2=.00, p=.202).

Given questions about the limits of generalizability of results from samples obtained through children’s museums (Callanan, 2012) and the theorized role of experience in the development of spatial-numerical associations (McCrink & de Hevia, 2018), we ran additional exploratory analyses to examine any effects of participant demographics, including parental education level, second language exposure, and vocabulary level, as assessed by the Developmental Vocabulary Assessment for Parents (Libertus, Odic, Feigenson, & Halberda, 2013) (see Table 1 for demographic information by age group). Partial correlations controlling for age revealed no significant relationship between proportional laterality score and parental education level (r(217)=.060, p=.376), second language exposure (r(316) =−.065, p=.249), or vocabulary level (r(158)=.015, p= 85). It should be noted, however, that a large majority of our parents in our sample reported they had obtained advanced degrees (see Table 1 for participant demographics), which may limit the generalizability of these null results.

Due to the unexpected decrease in lateralized mapping at age 5, we tested adults after analyzing these data to determine if this pattern continues into adulthood (66 undergraduate students; N=17 in the Non-Symbolic Sets and Symbolic Number condition, and N=16 in the Letter and Nonsense Labels conditions; Mage=20 years). A repeated-measures ANOVA with side of screen as a within-subjects factor and condition as a between-subjects factor was conducted over adults’ sum of fixation time. There was no main effect of side of screen (F(1,62=1.12, η2 p=0.02, p=.290). There was also no interaction of side of screen with condition (F(3,62)=.76, η2 = 0.03, p=.520) (see table 3) As with the children, the tendency to orient to the correct location during vertical training was not positively correlated with a LR-mapping preference (Pearson correlation=.02, p=.880).

Discussion

The present study examined age-related changes in spatial associations, using a passive visual search task suitable for infants, toddlers, and young children. Overall, we observed a bias in childhood to search the side of screen consistent with left-to-right mapping of a vertical target to a horizontal location, which is consistent with literature that finds multiple types of spatial associations throughout early childhood (Bulf et al., 2016; de Hevia et al., 2014a; de Hevia et al., 2014b; de Hevia & Spelke, 2013; de Hevia et al., 2017; DiGiorgio et al., 2019; McCrink et al. 2014; McCrink et al., 2017b; Opfer & Furlong, 2011; Shaki et al., 2012; Vallesi et al., 2014). However, this bias was not consistent throughout this developmental period: the proportion of time devoted to searching areas corresponding to a left-to-right or right-to-left mapping fluctuated. An initial bias in infancy to spend a greater amount of time searching the side of the screen consistent with left-to-right mapping fell away during toddlerhood and re-emerged around the age of four years. Following this inflection point in toddlerhood, however, the presence of a left-to-right mapping tendency disappeared in five-year-olds and adults. These developmental trends in spatial associations appear to be unaffected by accompanying information; there were no interactions between the spatial biases and whether the information presented was ordinal (symbolic or non-symbolic number, letters) or unordered (nonsense labels).

This study is the first direct evidence for the disappearance and re-emergence of spatial associations before and after toddlerhood, which many theorized to be a critical time of change (McCrink & de Hevia, 2018; McCrink, Perez, & Baruch, 2017). However, this design also revealed that developmental trajectories are not always so tidy as particular theories predict. Instead of a clean, u-shaped, developmental curve that reflects the disappearance of infantile biases and the emergence of mature patterns, we see that even in preschool (from 4 to 5 years of age) spatial associations within the same task are not always stable. Five-year-olds and adults did not have a spontaneous tendency to map initiality with the left side of space, and finality with the right, on this task. If we had not employed a wide range of ages, or used the same task across these ages, these fluctuations would have not been discovered. Further, the present study found age-related changes in the strength of spatial biases, where the proportion of time spent searching the portion of the screen consistent with left-to-right mapping differed with age, however, the overall number of children showing this bias did not. We hope that other researchers, who do not find the “right” pattern of spatial associations in early or middle childhood, are also attentive to their null results and think of them as part of a broader picture of development.

Why, then, do we see a petering out of this left-to-right mapping bias later in development? Although it was unpredicted, we can conceive of several reasons why this may occur. First, the task may be too easy for older participants. With excessive time in the search window, they may grow bored and examine all locations. However, this seems unlikely; adults in our sample were not more likely to make initial gaze orientations to LR-mapping locations (43%). Second, it may be related to metacognitive development. The ability to reflect on one’s knowledge develops around 4 or 5 years and is refined with experience through adulthood (Destan, Hembacher, Ghetti, & Roebers, 2014; Kuhn, 2014; Marulis, Palinscar, Berhenke, & Whitebread, 2016). The diminished left-to-right bias observed in five-year-olds and adults might reflect less certainty about their response in a task where the solution is intentionally vague, and therefore consider alternative solutions more than younger children. Third, it may be that cultural conventions related to verticality interacted with the design of our study (c.f. Göbel, 2015). Specifically, reading, writing, and scanning information on phones and tablets occurs from top-to-bottom. Because we were basing this work off of the non-human animal literature (where training is sagittal), and primarily concerned with the developmental period of before/ during/ after toddlerhood (where independent reading, writing, and screen usage is minimal), we did not account for the potential mismatch between our vertical setup (bottom-initial and top-final) and the ‘real-life’ screen and text experiences of older subjects (top-initial and bottom-final).

In contrast to other work which shows a dissociation between spatial associations for conventionally ordered and unordered stimuli (such as numbers vs. generic labels; McCrink et al. 2017b), the degree of lateralized mapping did not differ if the participant heard or saw numbers compared to nonsense labels. One reason for this difference may be that in McCrink et al. (2017b), the transposition task was live-action and the experimenter was able to better hold the child’s attention during narration. We have no evidence here that the participants attended to the narration; children, particularly pre-verbal infants, may have just fixated on the on-screen action and used visual cues only. Further, visual cues to magnitude were available during the non-symbolic number condition but were removed during the horizontal trial, which may have lessened participants’ tendency to port over what they learned from the vertical trials. While we have evidence that the children learned the location of the object during training, from the final vertical trial, we did not measure whether they also learned the verbally presented labels. Given the wide-age range in the present study, it is possible that attention to the verbal labels relates to age-related changes in linguistic ability. Although such changes in language skills may be considered a limitation of research using samples with a large age-range, wherein infants, toddlers, and preschoolers approach the task in a different manner, we believe it is vital to consider language acquisition in the broader picture of development. These age-related changes in language development may contribute to the development of spatial associations by, for example, driving attention to cultural spatial conventions or allowing children to map nonsymbolic numerical representations onto symbolic number knowledge (McCrink & de Hevia, 2018). It is therefore important to evaluate spatial mappings in those with a wide range of linguistic ability, while acknowledging that this means not all children could glean the same information from the verbal labels provided during training, and as a result, we are limited in our conclusions about the strategies children used to solve the task.

In all, these results shed light on the developmental arc of spatial associations in early childhood. For a simple vertical-to-horizontal transposition task, older infants and younger preschoolers, but not toddlers, associated initial items in a series with the left side of space, and final items with the right. These left-to-right spatial associations were not present in older preschoolers or adults, perhaps as a result of exposure to text and screens. These results provide direct support for developmental theories (McCrink et al., 2017b; McCrink & de Hevia, 2018) that posit toddlerhood as a period of reconciliation between innate and enculturated visuospatial biases.

Future work on this topic is fertile; one clear direction is to run another large sample of children surrounding the period of toddlerhood in a culture where the spatial flow of the dominant language differs from that of the US. Another is to deploy the task with younger infants to detect if the spatial associations observed here in one-year-olds are more pronounced. Further, given the malleability of explicit spatial behaviors like counting (Göbel et al., 2018), future research may explore the stability and malleability of more automatic measures of spatial mapping like eye-tracking, as previous research indicates perceptual biases can be altered with training (Lawson, Fisher, Rakison, 2015). Yet another area for future research is to determine how parents and caregivers mold spatial associations during their child’s toddlerhood, with attention to parental gesture, parental organization of space during a narrative, and children’s observations of parent reading as possible mechanisms for cultural learning (Göbel, McCrink, Fischer, & Shaki, 2018; McCrink, Caldera, & Shaki, 2017). In addition to the advent of new activities and behaviors that involve organizing spatial information, toddlerhood also marks an important developmental milestone where children become increasingly sensitive to cultural conventions. This increased sensitivity is most prominent between three and six years (Legare, Wen, Herrmann, & Whitehouse, 2015). The combination of increased spatial activities and gesture in toddlerhood, as well as the apparent critical period for learning cultural conventions, may underly the development of culturally consistent spatial associations in early childhood. Given that the spatialization of information – especially number – is immensely beneficial for cognitive organization, memory, and numeracy (Gunderson, Ramirez, Beilock, & Levine, 2012; Opfer & Siegler, 2007), these topics are an exciting theoretical and practical avenue for future work.

Supplementary Material

Supplemental Material

Acknowledgements:

This work was supported by award 1R15HD096363–01 from the Eunice Kennedy Shriver National Institute for Child Health and Human Development to K.M.

Footnotes

Conflict of Interest: The authors declare no conflict of interest.

The data that supports the findings of this study are available in the supplementary material of this article.

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