It has long been recognized that individuals who develop psychotic disorders experience and exhibit a range of issues early in life and before illness onset, that are related to risk of developing psychotic disorder. This observation, and related scientific evidence, has led to a view that posits a neurodevelopmental model for development of psychotic disorders. The neurodevelopmental model of psychotic disorders explains that subtle impairments in neurocognitive functioning starts early in life, and develops in a way that implies abnormal maturation of the nervous system. This is in itself a risk factor for developing psychotic disorder. As a result, research into the phase predating psychotic disorder, focusing on identifying factors that precede illness onset and that may be available for intervention that could modify risk and protective factors, have been substantial in the last few decades.
Neurocognitive impairment is one central domain of vulnerability for psychosis that has been quite extensively researched in groups that are defined as high-risk for psychosis. A general conclusion is that people at clinically elevated risk, due to the presence of attenuated or brief psychotic symptoms, or a combination of genetic risk and functional decline, experience general neurocognitive impairment at a level intermediate between healthy controls and individuals with established psychotic disorder. However, the generalizability of these results to younger age-groups who are at-risk is unclear, as most high-risk research includes samples of young adults or mixed groups of adults and adolescents. If the neurodevelopmental model for psychotic disorders is correct, it may be that individuals at risk for psychosis could be identified earlier, and perhaps interventions could be offered that may prevent onset of psychosis or lead to a more favorable outcome. Furthermore, adolescence is a period in life defined by substantial brain development and neurocognitive maturation. It has been pointed out that this period in life could offer a unique possibility for effective preventive interventions.
In addition to including mostly young adults, and not adolescents, high-risk research has predominantly recruited help-seeking participants. Even though about 30% of individuals who are identified through clinical high-risk approaches develop psychosis within 36 months, the majority of people who develop psychosis are not identified as clinical high-risk in the first place. In other words, little is known about the population of individuals who develop psychosis who are not help-seeking, which represents a substantial part of the psychosis population. Also, while much of the focus in high-risk research has been on neurocognitive functioning, the presence of attenuated psychotic symptoms and a range of other clinical and function variables, the low specificity of these variables means that it is still difficult to predict who will develop psychosis. Recently, anomalous self-experiences have been rediscovered as a central feature of both established psychotic disorder and being at risk for psychosis. Anomalous self-experiences include a decline in the sense of subjectivity, hyperreflexivity, meaning disturbed perceptions of normal features of the self and and the world, as well as perplexity. Very little is known about the associations between neurocognitive functioning and anomalous self-experiences in at-risk groups.
In light of the existing evidence, there are still questions that remain unanswered. Specifically, there is a need for knowledge concerning neurocognitive functioning in adolescents at-risk for psychosis. Particularly, little is known about individuals who are not recruited to research based on help-seeking behavior. It could also be useful to widen the scope of mapping domain of vulnerability, like anomalous self-experiences, that could aid in early identification, prediction, and preventive efforts. The main aim of this thesis was to investigate neurocognitive functioning and impairment in individuals at risk for psychosis. This included both a comprehensive review of existing research evidence and specifically investigating neurocognitive functioning in young adolescents at risk for psychosis who are not help-seeking. In addition, an aim was to look at neurocognitive functioning and its associations to anomalous self-experiences. The studies included in this thesis has been made possible because we have included 14-15 year old adolescents who participate in a large, nation-wide population cohort called the Norwegian Mother, Father and Child study. These adolescents have been recruited based on their answers from a large survey, and are not help-seeking.
In Paper I, the aim was to perform a systematic review of existing evidence regarding impairment in specific neurocognitive domains, over and above measures of general intellectual abilities, before onset of psychotic disorder. A secondary aim was to investigate timing of onset of specific neurocognitive impairments: The evidence reviewed in Paper I strongly imply the existence of extensive neurocognitive impairments before onset of psychotic disorder, across a wide scope of samples. Effect sizes were mostly small to moderate. The evidence showed that some verbal impairments can be detected in early childhood. This included impaired language abilities at age 3 and impaired verbal learning and memory at age 7. Conversely, some non-verbal impairments become significant in adolescence or young adulthood. The results are in line with a neurodevelopmental model of psychotic disorders. The results also underline the importance of mapping neurocognitive functioning in both cross-sectional and longitudinal studies of psychosis risk.
In Paper II the aim to investigate specific neurocognitive functions in a group of young adolescents at risk for psychotic disorders who are not help-seeking, compared to age- and gender-matched controls. Analyses found that non-help-seeking adolescents at risk for psychosis had significantly poorer scores than age- and gender-matched controls on measures of general cognitive functioning. Verbal learning was the specific neurocognitive domain in which the at-risk group performed significantly worse on compared to controls, but differences were no longer significant when controlling for symptoms of depression and social phobia. These results suggest that development of neurocognitive impairments starts early in adolescence, also in at-risk groups who are not recruited based on help-seeking behavior. Furthermore, it may be that verbal learning is specifically implicated in groups at risk for psychosis, or it may be a domain of neurocognitive functioning that is vulnerable to symptoms of depression and anxiety.
In Paper III the aim was to examine the associations between neurocognitive functioning in the group of adolescents at risk for psychotic disorder and their self-reported experiences of anomalous self-experiences. Paper III took an exploratory approach asking the question: Is there a relationship between neurocognitive functioning and anomalous self-experiences in non-help-seeking adolescents at presumed enhanced risk of psychosis? Analyses revealed no significant relationship between these two domains of vulnerability to psychosis. Based on this, it could be argued that anomalous self-experiences and neurocognitive functioning both exist and are central in at-risk groups, including adolescent at-risk groups who are not help-seeking, but that these two vulnerability markers are seemingly unrelated. Consequently, both anomalous self-experiences and neurocognitive functioning should be mapped in at-risk groups. More studies are needed, but it may be that anomalous self-experiences could increase specificity of risk criteria and thus enhance their predictive power.