Episodic memory across the life span : evidence from infancy, childhood, adults and amnesia

Abstract

It is vital to examine changes in hippocampal-dependent memory across the life-span, in order to understand both its ontogeny and subsequent decline with healthy aging. To the authors’ knowledge, earlier research has not used the same methodology to assess episodic memory processes in children and adults, yet comparisons in performance between these groups have been made regardless. Equally, there is a lack of robust evidence to indicate that episodic memory paradigms employed with young children are hippocampal-dependent. In this thesis, I aimed to address these important issues by assessing memory performance across the lifespan in children aged 7.5-months-old to 8-years-old (n= >500), young adults aged 18-25 years (n= >60), older adults aged 54-77 years (n= >60) and patients with selective hippocampal damage aged 52-75 years (n=5). Two tasks were used; 1) a deferred imitation task which measured memory for action sequences and 2) a faces and places task which measured memory for face-scene associations via eye-tracking and/or explicit recall. Comparisons between patients and adult controls permitted me to infer whether these paradigms are measuring hippocampal-dependent processes. Both tasks contained conditions that did not rely on instructions, in order to permit valid comparisons to be made between pre-verbal infants and adults and determine at what age task performance becomes adult-like and exceeds that of patients. When patient performance was examined on both tasks relative to adult controls, patients demonstrated significantly poorer memory for the action sequence (deferred imitation task) and significantly worse recall for face-scene associations (faces and places task). These findings suggest that both tasks appear to index hippocampal-dependent memory processes and the integrity of the hippocampus is needed to support successful performance. Subtle distinctions were found between memory for action information and memory for temporal order information across childhood. While both types of memory became adult-like by 4- years-old and remained relatively stable from this age onwards, memory for actions increased more incrementally with age from approximately 2-years-old whereas temporal order memory emerged more sharply around 4-years-old. Alongside supporting episodic memory, the hippocampus plays a specialised role in the processing of scenes (Hassabis & Maguire, 2007) and spatial memory (O’Keefe & Nadel, 1979). In order to examine whether hippocampal scene processes may be influencing memory development, memory for face-scene associations was assessed in all age groups when scene viewing perspective either remained the same or was shifted slightly between learning and test. We examined whether participants could tolerate the change in scene perspective, i.e. recognise that it is the same place albeit the view of the scene has shifted slightly, to retrieve the previously formed association between that scene and a face. While all groups aged between 7.5-month-old to 4-years-old, with the exception of 3-year-olds, demonstrated eye movements veridical of remembering face-scene pairs when scene view remained constant within a trial, this behaviour was eradicated when scene perspective was shifted between learning and test in all groups with the exception of 4-year-olds. Shifting scene perspective between learning and test had a detrimental effect on memory for previously presented facescene pairs in older adults and to a more significant extent in patients with selective iv hippocampal damage. In contrast, shifting scene perspective between learning and test did not impact on recall for face-pairs in young adults and children aged 5-8 years. In addition to age-related increases in memory across childhood, the acquisition of developmental milestones may also facilitate memory development. Previous literature has tentatively linked the attainment of independent locomotion (IL) with greater memory retrieval flexibility in infancy (Herbert et al., 2007), with suggestion that the greater experience in varying spatial contexts that accompanies this milestone may be providing scaffolding to support episodic memory processes (Rovee-Collier & Cuevas, 2009). Therefore, I aimed to not just explore age-related differences in memory performance within my tasks, but assessed whether attaining IL in early infancy provides mnemonic benefits compared to peers who develop this ability later in the first year. Performance was compared between infants who had achieved IL and age-matched non-locomotive peers (NIL) at 7.5- months-old. A sub group of these infants returned to participate when aged 9-months-old and performance was compared between infants who had acquired IL by 7.5 months of age compared to age-matched peers who only recently acquired this milestone. DI performance in 9-month-olds who had acquired IL by 7.5 months of age significantly outperformed their agematched peers who only recently acquired IL (i.e. IL was acquired between 7.5-9 months of age). Furthermore, only those infants who had acquired IL by 7.5-months-old demonstrated eye-movements veridical of remembering previously presented face-scene pairs. This collection of findings are discussed in terms of how using the same hippocampaldependent memory task across the life-span can inform current understanding of the developmental trajectory of this specific form of memory. I reflect upon how the additional onus of the hippocampus in spatial processing may be fundamentally intertwined with episodic memory development and how the acquisition of spatial knowledge through attaining IL may be providing a scaffold for this type of memory development in early childhood.