From cs240@cs.rochester.edu Tue Nov 4 18:00:06 2003 Date: Tue, 4 Nov 2003 17:58:06 -0500 From: cs240 To: tetreaul@vienna.cs.rochester.edu Phantoms in the Book

Book Review: Phantoms in the Brain by V. S. Ramachandran

Jonathan Tomer (email), University of Rochester

What is self? Why is it that we perceive ourselves as indivduals, persisting throughout space and time? What is the ineffable quality that makes our experiences seem "real"? These questions have plagued philosophers for millennia. More recently, it has been generally accepted that all experience must somehow be occuring within the brain -- but the workings of that organ are still largely unknown, and what we do know about it has, if anything, made the mystery of consciousness even deeper. How do the random-seeming propagations of electrical signals in minuscule cells in the inside of our heads cause us to see, hear, feel, and think? Furthermore, how is it that even when we know that our senses are really just the firings of neurons in our brain, we still persist in perceiving a world external to us?

In Phantoms in the Brain, V. S. Ramachandran takes an unusual approach to these questions. By examining what happens to conscious experience when certain subsystems of the brain become damaged, he is able to form some conclusions about the roles these systems play in consciousness in general. From the "phantom limbs" of amputees (the phenomenon for which the book is named) to temporal lobe epilepsy, Ramachandran explores a variety of pathological conditions and provides evocative descriptions of their effect on the world as perceived by his patients. The conclusions he draws are always interesting, usually insightful, often surprising, and sometimes even believable.

The least controversial phenomena in Phantoms are naturally described at the beginning. Ramachandran examines the "phantom limb" syndrome, first described in detail during the American Civil War and immortalized in Oliver Sacks' A Leg to Stand On. In this syndrome, amputees (and occasionally even people with congenitally missing limbs) can actually feel a limb that they do not have. These phantom limbs can usually move, and can even experience sensations of touch, cold, warmth, itching, or pain. These sensations -- especially phantom pain -- are often very disturbing to the phantom's owner, since with no actual limb there is no apparent way to eliminate unpleasant sensations. A popular theory of the reason for phantom limb syndrome is that nerves that once innervated the missing extremity come to re-innervate the stump, and that messages from the nerves' new destination are simply misinterpreted by the brain as coming from the old, missing lib. Ramachandran, however, had a different theory. He narrates how he came upon the possibility that what is occurring in phantom limb patients is a "remapping" of touch-sensitive nerves in the brain from areas that normally innervated the missing limb to neighboring areas, which respond to sensation in other parts of the body.

Ramachandran was able to test his new theory by noting that the maps of touch-sensitivity in the brain are not entirely continuous -- for instance, regions responding to the face are right next to those responding to the fingers. Lo and behold, he discovered that when the face of a patient with a phantom arm was touched, the patient felt the sensations in his phantom hand! This discovery led to further research on phantom limbs, and some important insights into the brain structures that give us our "body image." These structures are both more and less malleable than one might think: we are capable of reassigning sensations from one region to another, or even to inanimate objects, but apparently the brain cannot simply decide that a limb does not exist. The theories brought up in these first few chapters are surprising, but credible. Since the publication of Phantoms it was discovered that, contrary to initial belief, individual neurons can spontaneously create new synaptic connections. Furthermore, instances of remapping like those postulated by Ramachandran have been demonstrated in ferrets [1]. This discovery confirms Ramachandran's suspicions about the cause of phantom limbs, and cast in its light even his speculative arguments about body image seem very plausible.

Next, Ramachandran tackles the visual systems of the brain. Human vision is one of our most powerful senses, responsible for a large portion of what we think of as conscious being, and discoveries about it are of great value to the study of consciousness. Some of the best of subjects for the study of visual perception are, paradoxically, blind. People with failing vision, or who have large blind spots (scotomas) due to retinal damage, often experience a surprising syndrome in which they hallucinate vividly in their blind areas. The nature of these hallucinations varies wildly from patient to patient, but they always seem very real -- indeed, sometimes more real than things actually seen. As Ramachandran argues, these patients are living proof of the strength of the often-underestimated "back propagation" channels in the visual system. It has long been known that the visual cortex is divided into many smaller areas, each of which performs a specific kind of analysis of the image seen by the retina. These areas are connected by millions of synapses, and it is generally considered that most of the communication is in the "upward" direction -- that is, the earlier stages of vision perform simple analysis like edge detection, and pass this information to more complicated systems that, for instance, recognize objects, which in turn pass this information to areas responsible for higher brain function. Hallucinations in blind spots, Ramachandran claims, show that the connections in the other direction -- from the areas responsible for complex thinking down through the visual pathways to the simplest areas of vision -- are at least as important: we decide that something is there and proceed to construct an image of it to "see".

Another type of patient Ramachandran found useful for studying the visual systems had a different kind of blindness. These patients had parts of their visual cortex damaged, and though their eyes were fully functional, they could not consciously perceive or recongize any objects. However, they could instantly and accurately perform complicated tasks involving vision, such as orienting an envelope to fit it in a mail slot, though they were unable to "see" either envelope or slot, and could not tell the experimenter whether the slot was horizontal or vertical! These results come as a great shock, and provide a critical insight into the way vision works. It was already well known there are separate neural pathways in the brain for recognizing objects and for determining and acting on their spatial characteristics. However, it seems that conscious perception of vision arises only from the "what" pathways in the temporal lobe. The parietal "how" pathways, which are used when trying to figure out where something is (in order to grab a seen object, for example, or correctly orient an object in hand with a seen object), are unconscious. We can use them without realizing that we are even doing so. This discovery helps explain why many precise activities, such as target shooting, are easier when we do not attempt to guide them consciously.

Ramachandran explores several other disorders. "Hemineglect", in which a person with damage to the right cerebral hemisphere fails to notice that the left-hand portion of the world even exists, provides some clues about the division of labor between the two hemispheres of the cortex. So does the extraordinary syndrome experienced by some stroke victims who, paralzyed on their left-hand side, deny the paralysis entirely and claim that their paralyzed limbs are fully movable. Capgras' syndrome, a curious delusion that certain emotionally close people have been replaced by impostors, may be the result of damage to a connection between the visual face-recognition system and the limbic system, responsible for emotions. However, the inferences Ramachandran makes from these cases about the study of consciousness are tenuous at best. The speculations made even later in Phantoms -- that laughter evolved as a means of communicating "false alarms" or that a propensity for religion can be found in the temporal lobes -- are even sketchier, and Ramachandran exercises an unacceptable amount of leeway in describing the phenomena he is trying to explain.

The conclusion of the book presents an interesting theory of what consciousness is, neurologically speaking. Though difficult or impossible to test, Ramachandran's "three laws of consciousness" are a somewhat intellectually satisfying set of criteria, and are successful in the fairly limited sense that they explain all the pathological phenomena described in Phantoms. However, such hypothetical, untestable descriptions can never be truly satisfying for the very reason of their untestability: Ramachandran's ideas may be appealing, but there is no real reason to believe that they are accurate. Philosophers will be left with the same questions about the nature of self after reading Phantoms as they had before picking it up -- but if one is sure that, after reading the book, he is the same individual he was before, then perhaps they are not such critical problems after all.

References


[1] von Melchner, L., Pallas, S. L., & Sur, M.    Visual behavior mediated by retinal projections directed to the auditory pathway.    Nature 404, 871--876 (2000).
Jonathan Tomer
Last modified: Fri Oct 10 20:31:04 EDT 2003