Neuropsychology of Personality Disorders
Martina M. Voglmaier
Intuitively, one might expect individuals with personality disorders to have neuropsychological deficits. The hypersensitivity of the paranoid and avoidant personalities, the rigidity of the obsessivecompulsive personality, the perceptual distortions of the schizotypal personality, and the limited social and emotional responsiveness of the schizoid personality suggest areas of deficit fascinating to the researcher interested in brain-behaviour relationships. Yet the study of neuropsychological functioning in personality disorders is in its infancy, primarily because of methodological problems associated with studying these disorders. The focus of this chapter is a general review of studies of clinical neuropsychological test performance in DSM-IV personality disorders as well as a description of the methodological problems associated with their study. Information processing, psychophysiological, and structural anatomical studies are dealt with elsewhere in this text.
Before reviewing empirical studies of neuropsychological test performance in personality-disordered individuals, a number of general methodological concerns must be addressed. Typically, in such studies, personality disorders are diagnosed by structured clinical interview, and subjects are administered a battery of neuropsychological tests measuring one or more cognitive domains, such as attention, memory, language, visuospatial perception, executive functions, and motor skills. Test performance of a group of individuals with a personality disorder of interest is then compared to that of one or more groups of subjects with either a different personality disorder, a mixed group of 'other' personality disorders, another psychiatric disorder, or no psychiatric disorder.
Research in this area is fraught with methodological difficulties. Samples differing in selection criteria, inpatient status, medications, and gender may result in inconsistent findings. Moreover, diagnosing personality disorders is a decidedly unclear science (Strack and Lorr, 1997). The clinical definition of character traits, severity, and impact on functional ability can be subjective, and whether a personality disorder can be diagnosed by self-report to an examiner in a preset period of time has been questioned (Oldham and Skodol, 1992). The DSM-IV requires a specific number of character traits to meet clinical diagnostic criteria for a personality disorder, and some studies include individuals who have fewer than the cutoff number of traits, such as those defined as having a 'probable' disorder.
Another concern is determining the specificity of any cognitive deficits given the co-occurrence of the 10 personality disorders with each other (Stuart et al., 1998), the co-morbidity of personality disorders with Axis I disorders (Oldham and Skodol, 1992), and the possibility of misdiagnosis of behavioural traits associated with neurological disorders, such as head trauma, substance abuse, and temporolimbic epilepsy (Devinsky and Najjar, 1999). For example, increased rates of depression have been associated with a number of personality disorders (Table XXVI–5.1). Even with detailed historical and diagnostic information, it could be difficult to determine whether cognitive deficits evinced by a group of subjects are related to the personality disorder under examination, to concurrent symptoms of depression, to cognitive vulnerabilities related to the predisposition to depression, or to treatments for depression. A related concern is that individuals with personality disorders may be vulnerable to neuropsychological dysfunction from secondary sources such as precursors or lifestyle behaviours associated with maladaptive personality traits. For example, risktaking behaviours, substance use, impulsivity, and aggression in antisocial personality disorder (ASPD) may increase the risk of head trauma in this population, and childhood trauma (physical, emotional, or sexual) may predispose an individual to head injury, borderline personality traits, symptoms of post-traumatic stress, and dissociative disorders (Paris, 1997).
The battery of neuropsychological tests employed in research studies is also of importance. Often, only a few tests or cognitive domains are examined, resulting in a limited view of cognitive functioning. For example, if only executive-function tasks are employed, it cannot be determined whether any weaknesses in this domain are due to a generalized deficit associated with having a psychiatric illness, or to a domain-specific deficit associated with the personality disorder being examined. Finally, there is some evidence for sex differences in cognitive functioning in some personality disorders (Voglmaier et al., submitted). Grouping males and females together in such studies may result in inconsistent findings.
Schizotypal, paranoid, and schizoid personality disorders make up this cluster of odd and eccentric personalities. By far, the most research on cognitive functioning in this cluster has been on schizotypal personality disorder (SPD). SPD is characterized by oddities in appearance, perception, and behaviour, as well as marked discomfort in close relationships. On the hypothesis that this disorder is biologically related to schizophrenia, studies have focused on cognitive domains known to be impaired in schizophrenic subjects, that is, abstraction, attention, language, and verbal learning and memory (e.g., Gur et al., 1991; Saykin et al., 1991).