Original Research Paper
Signature dynamics in Alzheimer's disease

https://doi.org/10.1016/j.forsciint.2019.109880Get rights and content

Highlights

  • Dynamic analyses of signatures revealed normal temporal, spatial and fluency features in AD.

  • Feature variability over repetitive signatures in AD fell within 10% of the variability of control signatures.

  • Variability in stroke amplitude and speed for non-text-based signatures was associated with dementia severity.

  • Dynamic aspects of signature formation in AD remained stable over 1 year.

Abstract

Forensic document examiners are often called upon to opine on the authenticity of handwritten signatures by individuals with diminished mental capacity. Legal arguments surrounding the decisional capacity of an individual with dementia can be found in many cases involving wills, deeds, trusts, and contracts. The purpose of this study was to provide estimates of feature variability derived from dynamic analyses of signatures written by individuals with dementia of the Alzheimer type (AD) compared with age-comparable healthy individuals. Dynamic features of digitally captured signatures were analyzed to test the hypothesis that AD signature features will show greater variability compared with signatures from age-comparable healthy subjects. The study enrolled 69 AD and 74 age comparable healthy subjects. Results revealed four main findings from AD signatures: (1) that the temporal, spatial and fluency characteristics of signature formation did not differ from signatures of healthy writers; (2) variability in dynamic features over a series of repetitive signatures fell within 10% of the natural variation of healthy subjects; (3) there was a significant association between increased dynamic signature feature variability and increased dementia severity for stylized and mixed signatures only; and (4) despite significant decline in cognitive status over a 1-year period, dynamic signature features remained stable. Overall, these results suggest that signature writing is preserved in AD. The association between dementia severity and dynamic feature variability among AD subjects with stylized or mixed signatures warrants further research.

Introduction

Forensic document examiners (FDEs) are often tasked with determining if a questioned signature is genuine or non-genuine. Such signatures may appear on documents such as wills, deeds, trusts, and contracts and the health of the signatory can be a pertinent factor [1]. In cases of suspected dementia, two questions are usually asked: (1) did the signatory execute the questioned signatures? and (2) did the signatory have the mental capacity to understand what they were signing? Given an adequate number of contemporaneous specimens, the FDE can answer the first questioned to some degree of certainty [2], [3], [4]. However, uncertainty creeps into this process in cases where the signatory's mental status declines or fluctuates making it difficult for the FDE to estimate the individual's natural variation [5].

Walton [6] cautioned that FDEs must be careful in the examination of writers with neurological diseases. She noted that line quality, tremor, retouching, pen pressure, and speed may be erratic, but this is not necessarily due to simulation, but may result from the effects of normal aging. Her study found that some people in their 90s and even centenarians wrote with normal speed and exhibited little deterioration in their handwriting. Earlier, Behrendt [7] suggested that the FDE should be aware of the writer's medical history and be cognizant of the effects of medication (especially on tremor) on a patient with AD. He warned that contemporaneous standards were a necessity and in late-stage AD, specimens written on the same day as the questioned signature may be necessary to prevent error.

Questionable levels of signature variation in a writer often raise concern of health change, reaction to prescribed medication, or substance use. Of concern to document examiners preparing testimony on signature authorship, the presence of features outside the range of natural variation may be indicative of a simulation. Documents such as wills that bear disputed signatures are frequently submitted to FDEs for examination. Dementia is not an uncommon explanation that is proffered to explain the presence of poor line quality in the testator's signature. However, data are lacking to inform the FDE on the impact of a dementing illness on a writer's natural signature variability. The purpose of this study was to provide estimates of feature variability derived from dynamic analyses of signatures written by individuals with AD compared with age-comparable healthy individuals.

Section snippets

Participants

Study subjects were recruited from volunteers participating in a large clinical research program at the University of California, San Diego Alzheimer’s Disease Research Center. Sixty-nine subjects meeting DSM-IV criteria for dementia as well as the Alzheimer's Disease and Related Disorders Association (ADRDA) criteria for probable AD [8] and 74 age-comparable healthy control (HC) subjects were included in the study. AD subjects had a mean (standard deviation) age of 75.56 (9.44) years; while HC

Results

The majority of subjects from both groups wrote signatures using text-based style, where every allograph was discernable from the signature. Ten AD (14.5%) and two HC subjects (2.7%) wrote signatures using mixed (8/10 AD) or stylized (2/10 AD and both HC) styles. The difference in proportion of non-text-based signatures between the two groups was statistically significantly (χ2 = 6.46; p = 0.01). For this reason, we report group differences in kinematic features and variability for text-based

Discussion

Several new findings emerged from the present study. First, dynamic signature features (including stroke duration, amplitude, velocity, smoothness, and pen pressure) for AD subjects did not differ from age-comparable healthy subjects. These findings from a relatively large sample of subjects suggest that the presence of moderate levels of dementia severity associated with AD appeared not to impact signature dynamics. For the FDE, this means that parameters such as stroke height, inferred speed

Summary and conclusions

Previously published empirical research on the impact of dementia on linguistic and motor aspects of handwriting offers little help to the forensic document examiner tasked with evaluating signature specimens. This study is one of few and perhaps the largest to address signature writing in individuals with Alzheimer's disease, the most common form of dementia. The study found that signatures written by individuals with AD show normal temporal, spatial, and fluency characteristics when subjected

Author contributions

Caligiuri: conceptualization; data curation; formal analysis; funding acquisition; investigation; methodology; project administration; resources; software; supervision; validation; visualization; roles/writing – original draft; writing – review & editing.

Mohammed: roles/writing – original draft; writing – review & editing.

Funding

This research was supported by a grant from National Institute on Aging (AG05131).

Acknowledgments

The author wishes to acknowledge Kelly Landy and Chi Kim for their assistance in data collection and Doug Galasko, M.D. and David Salmon, Ph.D., for their contribution to the diagnosis and scoring of the clinical assessments and leadership in making the ADRC facilities and patients available to this research.

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