Tracking the eyes to see what children remember

Episodic memory development: Bridging animal and human research

Human episodic memory is not functionally evident until about 2 years of age and continues to develop into the school years. Behavioral studies have elucidated this developmental timeline and its constituent processes. In tandem, lesion and neurophysiological studies in non-human primates and rodents have identified key neural substrates and circuit mechanisms that may underlie episodic memory development. Despite this progress, collaborative efforts between psychologists and neuroscientists remain limited, hindering progress. Here, we seek to bridge human and non-human episodic memory development research by offering a comparative review of studies using humans, non-human primates, and rodents. We highlight critical theoretical and methodological issues that limit cross-fertilization and propose a common research framework, adaptable to different species, that may facilitate cross-species research endeavors.

Attentional Measures of Memory in Typically Developing and Hypoxic-Ischemic Injured Infants

Hypoxic–ischemic injury (HII) at birth has been found to relate to differences in development, including decreased memory performance. The current study assessed recognition memory in 6- and 12-month-old HII infants and typically developing (TD) infants using two eye-tracking paradigms well suited to explore explicit memory processes early in life: visual paired comparison (VPC) and relational memory (RM). During the VPC, infants were familiarized to a face and then tested for their novelty preference immediately and after a two-minute delay. At 6 months, neither HII nor TD showed a VPC novelty preference at immediate delay, but at 12 months, both groups did; after the two-minute delay, no group showed a novelty preference. During RM, infants were presented with blocks containing a learning phase with three different scene–face pairs, and a test phase with one of the three scenes and all three faces appearing simultaneously. When there was no interference from other scene–face pairs between learning and test, 6-month-old TD showed evidence of an early novelty preference, but when there was interference, they revealed an early familiarity preference. For 12-month-old TD, some evidence for a novelty preference during RM was seen regardless of interference. Although HII and TD showed similar recognition memory on the VPC, when looking at RM, HII infants showed subtle differences in their attention to the familiar and novel faces as compared to their TD peers, suggesting that there might be subtle differences in the underlying memory processing mechanisms between HII and TD. More work is needed to understand how these attentional patterns might be predictive of later memory outcomes.

Eye Movements in Real-World Scene Photographs: General Characteristics and Effects of Viewing Task

The present study examines eye movement behavior in real-world scenes with a large (N = 100) sample. We report baseline measures of eye movement behavior in our sample, including mean fixation duration, saccade amplitude, and initial saccade latency. We also characterize how eye movement behaviors change over the course of a 12 s trial. These baseline measures will be of use to future work studying eye movement behavior in scenes in a variety of literatures. We also examine effects of viewing task on when and where the eyes move in real-world scenes: participants engaged in a memorization and an aesthetic judgment task while viewing 100 scenes. While we find no difference at the mean-level between the two tasks, temporal- and distribution-level analyses reveal significant task-driven differences in eye movement behavior.

The "eyes have it," but when in development?: The importance of a developmental perspective in our understanding of behavioral memory formation and the hippocampus

Lynn Nadel has been a trailblazer in memory research for decades. In just one example, Nadel and Zola-Morgan [Infantile amnesia, In Infant memory, Springer, Boston, MA, 1984, pp. 145-172] were the first to present the provocative notion that the extended development of the hippocampus may underlie the period of infantile amnesia. In this special issue of Hippocampus to honor Lynn Nadel, we review some of his major contributions to the field of memory development, with an emphasis on his observations that behavioral memory assessments follow an uneven, yet protracted developmental course. We present data emphasizing this point from memory-related eye movements [Hannula & Ranganath, Neuron, 2009, 63(5), 592-599]. Eye tracking is a sensitive behavioral measure, allowing for an indication of memory function even without overt responses, which is seemingly ideal for the investigation of memory in early childhood or in other nonverbal populations. However, the behavioral manifestation of these eye movements follows a U-shaped trajectory-and one that must be understood before these indictors could be broadly used as a marker of memory. We examine the change in preferential looking time to target stimuli in school-aged children and adults, and compare these eye movement responses to explicit recall measures. Our findings indicate change in the nature and timing of these eye movements in older children, causing us to question how 6-month-old infants may produce eye movements that initially appear to have the same properties as those measured in adulthood. We discuss these findings in the context of our current understanding of memory development, particularly the period of infantile amnesia.

Associative binding in early childhood: Evidence from a preferential looking paradigm

Relational memory processes are responsible for forming representations that bind together the co-occurring elements of an experience. These processes provide a foundation of episodic memory, the capacity to remember specific events about one's past. In the present research, we used a visual comparison paradigm to determine whether toddlers can form memories for the relation between co-occurring items. In Experiment 1, 66 2-year-olds viewed pairs of cartoon faces (AB; CD) and did not show a significant novelty preference for the violated associations (i.e., did not look longer at AD rearranged pair than old AB pair when exposed to AD-AB). However, toddlers looked longest at the individual face (i.e., D) violating a studied pair, which appeared to be supported by visual preference for centrally located stimuli. In Experiment 2, 46 2-year-olds participated in a similar procedure, but faces violating the pair during test were located to the periphery (e.g., AB-AC). Under these conditions, toddlers looked longer at recombined pair AC. Overall, our results show that toddlers show some ability to make item-item associations but may need to overcome preferential looking biases to demonstrate this capacity. Furthermore, looking behaviors beyond overall novelty preference may be informative even when overall novelty preference is not found.

Neural signatures of recognition memory in 10- to 12-month-old infants

Understanding memory mechanisms is crucial in the study of infant social and cognitive development. Here, we show that the Nc ERP component, known to reflect frequency-related attentional and/or memory processes, is a good candidate to investigate infant recognition memory. Previous paradigms have only investigated the effect of frequency during on-line stimulus presentation, but not during stimulus encoding. In this paper, we present a novel method for measuring the neural correlates of recognition memory and the 'degree' of familiarity in 10- to 12-month-old infants. During a familiarization phase, two images were presented frequently, while another two images were presented infrequently to the infants. In the test phase, the infrequent familiar, the frequent familiar, and the novel stimuli, were all presented with equal probability. We found larger Nc amplitudes following the familiar stimuli compared to the novel ones. The 'degree' of familiarity, on the other hand, did not modulate the Nc amplitude. These results can only be explained with memory-related processes, since in our paradigm the on-line presentation frequency did not vary. Furthermore, the lack of familiarization frequency effect suggests that the Nc might be a neural correlate of declarative memory.

Evidence of relational retrieval, even in the absence of the relational eye movement effect (REME)

Eye movements indicate relational memory retrieval by disproportional viewing of a matching stimulus compared to non-matching stimuli after a pair learning study phase (the REME). Experiment 1 showed that the magnitude of the eye movement effect is linked to the subjective confidence level of the responses and we could not find evidence of the REME in incorrect responses, where participants lack conscious recollection. In experiment 2 we used a no-choice task, where participants were instructed to learn the test displays after they have completed a pair learning study phase. The REME only appeared in those participants, who could spontaneously report their relational retrieval (aware group). Meanwhile both aware and unaware participants demonstrated evidence of relational retrieval in a new eye movement measure. Taken together, we demonstrated that the REME is not necessary for retrieval and in both of our experiments the emergence of the REME was associated with conscious access.

The ontogeny of relational memory and pattern separation

Episodic memory relies on memory for the relations among multiple elements of an event and the ability to discriminate among similar elements of episodes. The latter phenomenon, termed pattern separation, has been studied mainly in young and older adults with relatively little research on children. Building on prior work with young children, we created an engaging computer-administered relational memory task assessing what-where relations. We also modified the Mnemonic Similarity Task used to assess pattern discrimination in young and older adults for use with preschool children. Results showed that 4-year-olds performed significantly worse than 6-year-olds and adults on both tasks, whereas 6-year-olds and adults performed comparably, even though there were no ceiling effects. However, performance on the two tasks did not correlate, suggesting that two distinct mnemonic processes with different developmental trajectories may contribute to age-related changes in episodic memory.

The extended trajectory of hippocampal development: Implications for early memory development and disorder

Hippocampus has an extended developmental trajectory, with refinements occurring in the trisynaptic circuit until adolescence. While structural change should suggest a protracted course in behavior, some studies find evidence of precocious hippocampal development in the first postnatal year and continuity in memory processes beyond. However, a number of memory functions, including binding and relational inference, can be cortically supported. Evidence from the animal literature suggests that tasks often associated with hippocampus (visual paired comparison, binding of a visuomotor response) can be mediated by structures external to hippocampus. Thus, a complete examination of memory development will have to rule out cortex as a source of early memory competency. We propose that early memory must show properties associated with full function of the trisynaptic circuit to reflect "adult-like" memory function, mainly (1) rapid encoding of contextual details of overlapping patterns, and (2) retention of these details over sleep-dependent delays. A wealth of evidence suggests that these functions are not apparent until 18-24 months, with behavioral discontinuities reflecting shifts in the neural structures subserving memory beginning approximately at this point in development. We discuss the implications of these observations for theories of memory and for identifying and measuring memory function in populations with typical and atypical hippocampal function.

More to it than meets the eye: how eye movements can elucidate the development of episodic memory

The ability to recognise past events along with the contexts in which they occurred is a hallmark of episodic memory, a critical capacity. Eye movements have been shown to track veridical memory for the associations between events and their contexts (relational binding). Such eye-movement effects emerge several seconds before, or in the absence of, explicit response, and are linked to the integrity and function of the hippocampus. Drawing from research from infancy through late childhood, and by comparing to investigations from typical adults, patient populations, and animal models, it seems increasingly clear that eye movements reflect item-item, item-temporal, and item-spatial associations in developmental populations. We analyse this line of work, identify missing pieces in the literature and outline future avenues of research, in order to help elucidate the development of episodic memory.

Episodic memory and future thinking during early childhood: Linking the past and future

Despite extensive examination of episodic memory and future thinking development, little is known about the concurrent emergence of these capacities during early childhood. In Experiment 1, 3-year-olds participated in an episodic memory hiding task ("what, when, where" [WWW] components) with an episodic future thinking component. In Experiment 2, a group of 4-year-olds (including children from Experiment 1) participated in the same task (different objects and locations), providing the first longitudinal investigation of episodic memory and future thinking. Although children exhibited age-related improvements in recall, recognition, and binding of the WWW episodic memory components, there were no age-related changes in episodic future thinking. At both ages, WWW episodic memory performance was higher than future thinking performance, and episodic future thinking and WWW memory components were unrelated. These findings suggest that the WWW components of episodic memory are potentially less fragile than the future components when assessed in a cognitively demanding task.

Eye movements provide an index of veridical memory for temporal order

The present research examined whether eye movements during retrieval capture the relation between an event and its temporal attributes. In two experiments (N=76), we found converging evidence that eye movements reflected the veridicality of memory for temporal order seconds before overt memory judgments, suggesting that these movements captured indirect access to temporal information. These eye movements did not entirely depend on the amount of contextual cueing available (Experiment 1) and reflected the unique ordinal position of an event in a sequence (Experiment 2). Based on our results, we conclude that eye movements reflected the absolute temporal order of past events.

20 years after “The ontogeny of human memory: A cognitive neuroscience perspective,” where are we?

In 1995, Nelson published a paper describing a model of memory development during the first years of life. The current article seeks to provide an update on the original work published 20 years ago. Specifically, we review our current knowledge on the relation between the emergence of explicit memory functions throughout development and the maturation of associated brain regions. It is now well established that the brain regions subserving explicit memory functions (i.e. the hippocampal formation) are far from mature at birth, and exhibit important and gradual structural changes during childhood and beyond. Accordingly, explicit memory functions develop progressively. While some functions are present shortly after birth (formerly proposed as pre-explicit memory), others exhibit protracted developmental profiles during the first years of life. We examine the link between the emergence of different memory functions and the maturation of specific hippocampal circuits.

Learning to remember: the early ontogeny of episodic memory

Over the past 60 years the neural correlates of human episodic memory have been the focus of intense neuroscientific scrutiny. By contrast, neuroscience has paid substantially less attention to understanding the emergence of this neurocognitive system. In this review we consider how the study of memory development has evolved. In doing so, we concentrate primarily on the first postnatal year because it is within this time window that the most dramatic shifts in scientific opinion have occurred. Moreover, this time frame includes the critical age (∼9 months) at which human infants purportedly first begin to demonstrate rudimentary hippocampal-dependent memory. We review the evidence for and against this assertion, note the lack of direct neurocognitive data speaking to this issue, and question how demonstrations of exuberant relational learning and memory in infants as young as 3-months old can be accommodated within extant models. Finally, we discuss whether current impasses in the infant memory literature could be leveraged by making greater use of neuroimaging techniques, such as magnetic resonance imaging (MRI), which have been deployed so successfully in adults.

Age-related differences in memory expression during infancy: using eye-tracking to measure relational memory in 6- and 12-month-olds

Relational memory, or the ability to bind components of an event into a network of linked representations, is a primary function of the hippocampus. Here we extend eye-tracking research showing that infants are capable of forming memories for the relation between arbitrarily paired scenes and faces, by looking at age-related changes in relational memory over the first year of life. Six- and 12-month-old infants were familiarized with pairs of faces and scenes before being tested with arrays of three familiar faces that were presented on a familiar scene. Preferential looking at the face that matches the scene is typically taken as evidence of relational memory. The results showed that while 6-month-old showed very early preferential looking when face/scene pairs were tested immediately, 12-month-old did not exhibit evidence of relational memory either immediately or after a short delay. Theoretical implications for the functional development of the hippocampus and practical implications for the use of eye tracking to measure memory during early life are discussed.

Two rooms, two representations? Episodic-like memory in toddlers and preschoolers

Episodic memory involves binding together what-where-when associations. In three experiments, we tested the development of memory for such contextual associations in a naturalistic setting. Children searched for toys in two rooms with two different experimenters; each room contained two identical sets of four containers, but arranged differently. A distinct toy was hidden in a distinct container in each room. In Experiment 1, which involved children between 15 and 26 months who were prompted with a very explicit cue (a part of the hidden toy), we found a marked shift in performance with age: while 15- to 20-month-olds concentrated their searches on the two containers that sometimes contained toys, they did not distinguish between them according to context, but 21-26-month-olds did. However, surprisingly, without toy cues, even the youngest children showed a fragile ability to disambiguate the two containers by room context. In Experiment 2, we tested 34- to 40-month-olds and 64- to 72-month-olds without toy cues. The 5-year-olds were nearly perfect, and the 3-year-olds showed a significant preference for the correct container given only the context. In Experiment 3, we filled in the age range, and also investigated the effects of the use of labels (i.e. names of experimenters and rooms) and of familiarization time, in groups of 34- to 40-month-olds, 42- to 48-month-olds, and 50- to 56-month-olds. Neither labels nor familiarization time had an effect. Across experiments, there was regular age-related improvement in context-based memory. Overall, the results suggest that children's episodic memory may undergo an early qualitative change, yet to be precisely characterized, and that continuing increments in the use of contextual cues occur throughout the preschool period. A video abstract of this article can be viewed at https://www.youtube.com/watch?v=DkwEFw0UEz4&list=PLwxXcOKHPC0llAPVcJyW4EtzlA934A2Rz&index=1.