Lost in trauma: post-traumatic stress disorder, spatial processing and the brain derived neutrophic factor gene.

Abstract

This study enquired into a puzzling feature of Post-Traumatic Stress Disorder (PTSD); a loss of wayfinding ability (Osofsky et al., 2010; Ehringa et al., 2006; Lubit et al., 2003; Kowitz, 2011; Adler et al., 2009; Handley et al., 2009; Butler et al., 1999). Previous research by Smith et al. (2015) demonstrated that in cases of PTSD allocentric processing was impaired. This thesis pursued this line of enquiry and assessed the impact of PTSD and of any trauma exposure on navigation performance using a static perspective taking task and a more ‘active’ navigation paradigm. The study also introduced navigation questionnaires to these assessments, to see how accurate individuals with different experiences of trauma (including combat) were in their perceptions of their own navigation competence (or indeed impairment). Finally, the thesis approached the issue of genetics and explored the influence of the Brain-Derived Neurotrophic Factor (BDNF) gene on experiences of PTSD and on navigation behaviour. In summary, the study’s findings confirmed those of Smith et al. (2015) that PTSD impaired allocentric processing. What is more, this thesis revealed that PTSD also impaired egocentric navigation and that allocentric navigation performance was also impaired in healthy trauma exposed individuals who reported no ill-effects from their trauma. The thesis demonstrated for the first time that PTSD brought with it an associative bias which was transferable to navigation behaviour. This was interpreted as being the consequence of a competition for hippocampal resources between trauma processing and navigation in otherwise healthy individuals (Vasterling & Brewin, 2005). When it came to perceptions of navigation competence, healthy trauma exposed participants were accurate in their self-reported competence, but those with PTSD-related navigation impairment (including those who had been military trained) were not. Notably, the correlation between self-reported and actual navigation competence was limited to allocentric (not egocentric) navigation competence. This was explained using models of neural processing which present hippocampal dependent memory systems as being more declarative than associative memory systems (e.g. Morris in Andersen et al., 2007). In the final chapter, the explorative analysis of the BDNF gene produced some noteworthy findings. Zhang et al. (2014) speculated that the relationship between BDNF and PTSD is likely confounded by environmental conditions (i.e. the diversity and extent of trauma exposure and opportunities individuals have to process it). BDNF did not influence the PTSD prevalence or severity in this study which did not control for such conditions. In terms of navigation, there were no distinct performance disadvantages from carrying the met allele and this is in line with many similar studies (e.g. Sanchez et al., 2011, etc.). Nonetheless, BDNF met carriers showed different patterns of egocentric performance to valval homozygotes. What is more, met carriers showed an inability to accurately describe their competence at allocentric navigation and observations were made of data that indicated a delay in their uptake of allocentric strategy during navigation (similar to significant findings of Banner et al., 2011). The observations were consistent with Lövdén et al.’s (2011) suggestion that met carriers may require more ‘obvious’ cues to apply allocentric processing to a given task than valval homozygotes do. The implications of these genetic differences in approach to allocentric processing are considered in terms of both trauma processing and navigation training interventions.