Personal Identity and Fractured Selves: Perspectives from Philosophy, Ethics, and Neuroscience

2 Toward a Neurobiology of Personal Identity

chapter two Toward a Neurobiology of Personal Identity Peter V. Rabins, M.D., M.P.H., and David M. Blass, M.D. Because mental experiences derive from the brain, it has long seemed likely that neuroscientists could make progress in understanding the neural basis of specific mental experiences. In this chapter, we review three approaches to advancing our knowledge of how and which brain structures and functions might contribute to the experience of individual or personal identity. One approach, neuropsychiatry, is clinical and is derived from “accidents of nature,” that is, injuries to and diseases of the brain. The second approach, experimental neuropsychology, stems from the study and manipulation of normal (intact-brain) and brain-injured individuals. The third, developmental psychology, is both descriptive and experimental and primarily uses information gathered from the study of normal infants, although some data from adults and from individuals with impaired function are also cited. Such approaches can illuminate plausible central nervous system underpinnings of an experience such as personal identity, but they are unlikely to explain the construct and experience as a whole. Whether other approaches, in addition to these, can do so is beyond the scope of this chapter, but concepts such as personal identity have an ine≠able aspect that will never be fully encompassed in a strictly neurobiological model. Neuropsychiatry is the discipline that seeks to learn about brain-behavior relationships by studying diseases with identifiable brain pathology that are associated with changes or impairments in mental life. This method has been used to study the neural basis of human cognition and behavior since Broca’s demonstration in 1869 that left-hemisphere stroke is associated with the development of a language disorder. An example of its application to a noncognitive sphere is the study of major depression in persons with stroke (Robinson 1998), multiple sclerosis (Rabins et al. 1986), and Parkinson disease (Marsh 2000). The term personal identity is used here to refer to the reports of individual human beings that they experience themselves as unique individuals and to others’ observations of the unique characteristics of that individual. We distinguish personal identity from personhood, which refers to the concept that the individual life has a value vis-à-vis its status as a living being. We note that there is overlap between the constructs of personal identity and self, but suggest that self refers primarily to the experience of the individual, whereas personal identity also includes the views/observations of others. The question of whether a construct such as personal identify or selfawareness can be localized to the brain reflects a two-century-old argument about whether specific cognitive, emotional, and behavioral capacities are located in identifiable regions of the brain or brain functions are di≠usely distributed. There is now evidence that both sides are correct and that the answer di≠ers by construct. For example, language is primarily subserved by neurons located in the left frontal lobe and left temporal lobe in most (approximately 95% of) individuals. Remote or long-term memory, by contrast , seems to be more di≠usely stored. Thus, this chapter examines not only whether there is evidence that personal identity is located in the brain but also, if it is, whether there is evidence of focal and/or di≠use structural and functional contributions. In fact, the data support the intermediate position that some aspects of personal identity can be localized but that personal identity as a whole requires the interaction among a distributed set of brain locations and systems. A Neurobiology of Personal Identity 39 Neuropsychiatric Evidence Impairment of Self-perception (Agnosia) Several abnormalities of self-recognition have been known to clinicians for more than 100 years. In 1891, Sigmund Freud used the term agnosia to refer to a person’s inability to recognize something that is familiar, in spite of the intactness of the sensory system (sight, touch, smell, taste, sound) that underlies the observation. An example is the inability to recognize by touch a familiar object, such as a coin or keys placed in the palm, when the eyes are closed. The agnosia relevant to this discussion is anosagnosia or somatophrenia. These terms refer to a condition in which an individual who has sustained a brain injury resulting in paralysis is unaware of the weakness or loss of function in the paralyzed body part. This symptom is associated with injuries of the right parietal cortex...