Prospective Memory: Early Developmental Trajectory and Effects of Paediatric Traumatic Brain Injury on its Functioning

Abstract

Very little is known about the effects of paediatric traumatic brain injury (TBI) on prospective memory, the memory for future intentions such as remembering to post a letter in the morning or do homework. The main aim of this thesis was to redress that shortcoming in the literature. To investigate the effects of paediatric TBI on prospective memory as reliably and fully as possible, the study of children and adolescents with brain injuries was preceded by a developmental study. Given that the process of recovery from brain injury is imposed on the ongoing process of development, it is important to understand more about the normal developmental trajectory of prospective memory first of all. Study 1 compared the prospective-memory performance of 88 normally developing children, adolescents and young adults. The main task was computerised, and its design was influenced by a prefrontal-lobe model because prospective memory is believed to be mediated by the prefrontal regions of the brain. Variables associated with prefrontal-lobe capacity were manipulated: the cognitive demand of an ongoing task, and the importance of the prospective task. Results of Study 1 found that children remembered to respond to fewer prospective cues than adolescents or adults, but that adolescents and adults remembered similarly. Further, the differences between the children's performance and the adolescents' and adults' widened as the cognitive demand of the ongoing task increased. However, the effects of increasing the cognitive demand did not vary between the adolescents and adults. It made no difference to anyone's performance whether the importance of remembering the prospective cues was stressed or not. On the other hand, performance on executive functions, as measured by the Self-Ordered Pointing Task (SOPT), the Stroop Colour Word Interference Test (Stroop), and the Tower of London (TOL), which are also believed to be affected by prefrontal capacity, produced the same age effects as were produced on the computerised prospective-memory task. Further, performance on the SOPT and Stroop predicted performance on the high-demand level of the prospective-memory task. Study 2 compared 34 children and adolescents with TBI with the non-injured children and adolescents from Study 1 on the same tasks. Results revealed that overall those with TBI had poorer prospective-memory performance than their non-injured peers. However, a different pattern of impairment was evident in the children than in the adolescents. Specifically, the children with TBI performed similarly to their non-injured peers, but the adolescents with TBI were significantly worse than the non-injured adolescents. This trend was most noticeable as the cognitive demand of the ongoing task increased. Further, the age and injury effects were reflected in the performances on the executive-function tests, and the TOL predicted performance on the high-demand, prospective-memory task in those with TBI. Study 3 aimed to examine the ecological validity of Study 2, by investigating whether the impairments in prospective memory in young people with TBI measured quantitatively, were matched with qualitative data. Twelve parents of children and adolescents with mild to severe TBI were interviewed about whether or not their children's injuries impacted on their memory (retrospective and prospective) in everyday life. Results showed that in general most children suffered memory losses as a result of their brain injuries, and that prospective-memory loss caused particular hardships for the children and their families. Taken together, the results of the current research revealed that the development of prospective memory reaches a peak of maturity in adolescence, and that adolescents with TBI show greater decrements in prospective memory than adolescents without TBI, but that this pattern is not evident in children, where those with TBI were not significantly different from those without. These findings give support to the prefrontal-lobe model of prospective memory by showing that prefrontal maturity, which reaches a peak during adolescence, reflects the prospective-memory performance of healthy adolescents, and prefrontal injury, which is very common with TBI, shows the effects of deficits more during adolescence than in earlier years when the prefrontal regions are not yet fully developed. Study 3 showed that impairments in prospective memory that result from TBI translate into disabilities in the real world. As a follow up it is recommended that rehabilitation strategies be designed to assist young people with prospective-memory impairments adjust better to school and the demands of everyday living. The prefrontal-lobe model should guide the design of such strategies.