Hallucination: Philosophy and Psychology

3. The Hallucinating Brain: Neurobiological Insights into the Nature of Hallucinations

3 The Hallucinating Brain: Neurobiological Insights into the Nature of Hallucinations Dominic H. ffytche Abbreviations CBS Charles Bonnet Syndrome AMD Age-related macular degeneration Abstract Charles Bonnet’s eighteenth-century philosophy of hallucinations was ahead of its time. Bonnet argued that hallucinations were caused by activity within specialized functional units serving both normal perception and hallucinations, a view largely supported by recent imaging evidence. Here I describe Bonnet’s theory and the brain activity underlying visual hallucinations that occur in a condition named after him—the Charles Bonnet syndrome. The brain activity challenges our current view of hallucinations. It reveals similarities between the neural substrate of hallucinations, illusions, and afterimages, but differences from that of imagery, a finding seemingly inconsistent with current philosophical and clinical classificatory schemes. The activity also casts doubt on the assumption that hallucinations are reactivated memories. Finally, the wider clinical perspective questions our view of hallucinations as a single category of experience. Future theories of hallucinations need to be sufficiently broad to encompass a range of phenomena traditionally held as distinct, while sufficiently narrow to refer to specific neurobiological mechanisms. A single neurophilosophical account of hallucinations will not suffice; we need a family of theories. 1 Introduction Hallucination on Crete marked the 250th anniversary of an important event in the history of scientific and philosophical inquiry into hallucinations. In the autumn of 1758, Charles Lullin, a retired Genevan magistrate, experienced the last of the visual hallucinations that had fascinated him since February of that year. His grandson Charles Bonnet, recently turned philosopher from biologist, recognized the significance of the experiences and encouraged Lullin to document them in a diary. Bonnet 46 D. H. ffytche understood that hallucinations provide important evidence as to the nature of perception and how it relates to brain activity, and he derived a theoretical account of hallucinations that differs little from our current view. Here I describe Bonnet’s theory and the clinical disorder referred to today as the Charles Bonnet syndrome. I then compare the brain activity underlying this type of hallucination to activity associated with other categories of visual perceptual experience in an attempt to reveal something of what hallucinations are and what they are not. 2 Charles Bonnet Although Charles Bonnet (1720–1793) received a doctorate in law, his main early interest was the natural sciences. He became interested in entomology at age sixteen, and by twenty he had become the youngest corresponding member of the French Academy of Sciences for his description of parthenogenesis in aphids. A decade later, at the height of his natural science career, he was forced to abandon his studies because of an eye condition that made it increasingly difficult for him to use a microscope. He turned to philosophy, focusing first on psychology and the relationship between mind, brain, and soul. Later works placed greater emphasis on religion and a classificatory theory of past and future life on earth and heaven. 2.1 Eighteenth-Century Sensationism Charles Bonnet’s philosophical works fall in the tradition of sensationism, an extension of Locke’s seventeenth-century empiricism championed in eighteenth-century French philosophical circles by Étienne Bonnot, Abbé de Condillac (1715–1780). For Locke (1690), sensation was the source of knowledge of external objects, while reflection was the source of knowledge of internal mental operations, a hierarchical taxonomy of ideas deriving from these two primary sources. Sensationists argued that Locke’s reflection could be derived from sensation alone, with all higher functions founded in sensory experience, succinctly summarized in the axiom “to feel is to think” (Carr, 1930). In his 1754 “Treatise on the Sensations,” Condillac used a hypothetical cognizant marble statue whose exterior could be opened one sense at a time and through which the perceptual consequences of each sensory system, either in isolation or combined with other senses, could be examined philosophically. For this purpose we imagined a statue constructed internally like ourselves, and animated by a mind which as yet had no ideas of any kind. We supposed the marble exterior of the statue to prevent the use of its senses, and we reserved to ourselves the right to open them at will to the different impressions of which they are susceptible. (Condillac, 1754, “Dedication to Madame la Comtesse de Vassé,” xxxi) Condillac’s aim was to demonstrate that much of our experience and knowledge could be derived from a single sense. He first gave the statue a sense of smell and [18.217.220.114] Project...