Do Theoretical Bridges Exist Between Perceptual Experience and Neurophysiology?

DO THEORETICAL BRIDGES EXIST BETWEEN PERCEPTUAL EXPERIENCE AND NEUROPHYSIOLOGY? WILLIAM R. UTTAL * Introduction In this article I would like to consider one of the most interesting, challenging , and certainly controversial issues in modern neuroscience: the relation between perceptual and neurophysiological findings. The literature is now flooded with a variety of different efforts to reductively explain perceptual phenomena in terms of the underlying neurophysiological mechanisms . These efforts are imaginative and sometimes ingenious efforts to cross the empirical and conceptual gap between two very different experimental approaches. Conceptual bridge building of this kind is a highly regarded goal for several reasons. First, there is no question that perceptual phenomena, like all other mental events, are the products of the nervous system. This psychobiological monism is what philosophers would call a metaphysical premise—a statement of our belief about the fundamental reality of minds and brains. It is something that most psychologists and neurophysiologists implicitly accept. To do otherwise would be to reject the very foundations of this branch of biological science. The question asked in my title is, however, not the metaphysical one about the monistic nature of psychobiological reality that most of us take for granted. Rather, it is an epistemological one that is concerned with what we can know. It is concerned with the possibility that there are limits to our ability to reductively explain psychophysics in neurophysiological terms. I want to convince you that it is entirely possible to accept both the premise that there are intractable problems that will ultimately have to be faced by neuroreductionism and the premise that all phenomena are ultimately neural. It is important that my readers do not incorrectly infer that the argument presented here is some kind of a crypto-dualism. From my * Perception Laboratory, Department of Industrial and Management Systems Engineering, Arizona State University, Tempe, AR 85287-5906© 1996 by The University of Chicago. All rights reserved. 0031-5982/97/4003-0991101.00 280 William R. Uttal ¦ Perceptual Experience point of view, such a crypto- or explicit dualism would be an unacceptable metaphysical alternative to psychobiological monism. What is acceptable, however, is a nonreductive monism, a point of view that accepts epistemologica ! constraints on a metaphysical monism. So, accepting the premise that all perceptual processes are neural, this article asks: Can we build bridges between perceptual phenomena and neural mechanisms? There is, of course, avast difference of opinion regarding this issue. It does not take too extensive a reading of current journals to appreciate that many perceptual neuroscientists enthusiastically support the idea that neuroreductionism in both directions (top-down and bottomup ) is likely to succeed without limit. Given enough time, many ofus argue, whatever has not yet been accomplished is constrained only by the absence of a better microelectrode, an improved computational engine, or some other eventually-to-be-developed tool. Only pessimists suggest that there are fundamental barriers to knowledge without limit. On the other hand, there is a smaller number of perceptual neuroscientists, among whom the author must be included, who would argue that perceptual neuroreductionism is limited by major logical, theoretical, and conceptual difficulties, as well as by some mathematical and technical computational ones. The perceptual neuroreductionism issue is also clouded by the fact that there are several questions concerning the relationship between mental and neural functions that sometimes overlap, are often confused, and in which different amounts of progress are likely to be made. For example, currently there is a substantial amount of excitement surrounding the use of Positron Emission Tomography (PET) and Functional Magnetic Resonance Imaging (fMRI) devices to examine the locus of brain activity in an observer who is performing some perceptual, cognitive, or motor task. Many experiments using this tool are asking, and seem to be answering an important question. That question is: What parts of the brain are active during particular mental processes? Such questions are examples of what has previously been referred to as the localization question [I]. The quest to answer this question is a respectable scientific enterprise. It follows in the great tradition of the brain extirpators , who cut, poisoned, and froze portions of the brain in attempts to find out what parts were centers for various perceptual-motor functions...