Forensic applications of atomic force microscopy

The first project undertaken was to develop a currently non-existent forensic technique -- data recovery from damaged SIM cards. SIM cards hold data valuable to a forensic investigator within non-volatile EEPROM/flash memory arrays. This data has been proven to be able to withstand temperatures up to 500°C, surviving such scenarios as house fires or criminal evidence disposal. A successful forensically-sound sample extraction, mounting and backside processing methodology was developed to expose the underside of a microcontroller circuit's floating gate transistor tunnel oxide, allowing probing via AFM-based electrical scanning probe techniques. Scanning Kelvin probe microscopy has thus far proved capable of detecting the presence of stored charge within the floating gates beneath the thin tunnel oxide layer, to the point of generating statistical distributions reflecting the threshold voltage states of the transistors. The second project covered the novel forensic application of AFM as a complimentary technique to SEM examination of quartz grain surface textures. The analysis and interpretation of soil/sediment samples can provide indications of their provenance, and enable exclusionary comparisons to be made between samples pertinent to a forensic investigation. Multiple grains from four distinct sample sets were examined with the AFM, and various statistical figures of merit were derived. Canonical discriminant analysis was used to assess the discriminatory abilities of these statistical variables to better characterise the use of AFM results for grain classification. The final functions correctly classified 65.3% of original grouped cases, with the first 3 discriminant functions used in the analysis (Wilks' Lambda=0.336, p=0.000<0.01). This degree of discrimination shows a great deal of promise for the AFM as a quantitative corroborative technique to traditional SEM grain surface examination

Healthy ageing and binding features in working memory: measurement issues and potential boundary conditions

Accurate memory for an object or event requires that multiple diverse features are bound together and retained as an integrated representation. There is overwhelming evidence that healthy ageing is accompanied by an associative deficit in that older adults struggle to remember relations between items above any deficit exhibited in remembering the items themselves. However, the effect of age on the ability to bind features within novel objects (for example, their colour and shape) and retain correct conjunctions over brief intervals is less clear. The relatively small body of work that exists on this topic to-date has suggested no additional working memory impairment for conjunctions of features beyond a general age-related impairment in the ability to temporarily retain features. This is in stark contrast to the feature binding deficit observed in the early stages of Alzheimer’s disease. Nevertheless, there have been reports of age-related feature binding deficits in working memory under specific circumstances. Thus a major focus of the present work was to assess these potential boundary conditions. The change detection paradigm was used throughout this work to examine age-differences in visual working memory. Despite the popularity of this task important issues regarding the way in which working memory is probed have been left unaddressed. Chapter 2 reports three experiments with younger adults comparing two methods of testing recognition memory for features or conjunctions. Contrary to an influential study in the field, it appears that processing multiple items at test does not differentially impact on participants’ ability to detect binding changes. Chapters 3, 4, and 5 report a series of experiments motivated by previous findings of specific age-related feature binding deficits. These experiments, improving on previous methodology where possible, demonstrate that increasing the amount of time for which items can be studied (Chapter 3) or mixing feature-conjunction changes in trial-blocks with more salient changes to individual features (Chapters 4 and 5) does not differentially impact on healthy older adults’ ability to detect binding changes. Rather, the argument is made that specific procedural aspects of previous work led to the appearance of deficits that do not generalise. Chapter 5 also addresses the suggestion that healthy ageing specifically affects the retention of item-location conjunctions. The existing evidence for this claim is reviewed, and found wanting, and new data are presented providing evidence against it. To follow-up on the absence of a deficit for simple feature conjunctions, Chapter 6 contrasts two theoretically distinct binding mechanisms: one for features intrinsic to an object and another for extrinsic, contextual features. Preliminary evidence is reported that the cost associated with retaining pairings of features is specifically pronounced for older adults when the features are extrinsic to each other. In an attempt to separate out the contribution of working memory capacity and lapses of attention to age-differences in overall task performance, Chapter 7 reports the results of an exploratory analysis using processing models developed in Chapter 2. Analysis of two data sets from Chapters 4 and 5 demonstrates that lapses of attention make an important contribution to differences in change detection performance. Chapter 8 returns to the issue of measurement in assessing the evidence for specific age-related deficits. Simulations demonstrate that the choice of outcome measure can greatly affect conclusions regarding age-group by condition interactions, suggesting that some previous findings of such interactions in the literature may have been more apparent than real. In closing the General Discussion relates the present work to current theory regarding feature binding in visual working memory and to the wider literature on binding deficits in healthy and pathological ageing