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entists began using computers to model human mental processes. Conversely, those who want to build machines that think are inspired by the science of natural thought, so the problem of consciousness is deeply important for both groups.

The basic issue is that although we know a great deal about the brain, we know far less about its intangible correlate, the mind.The brain, after all, is a working physical part of the body, like the liver or heart, whose physiology and functions can be studied. In a typical adult, it is a 1.3 kg (3 lb) mass of tissue that contains about 100 billion neurons and supporting structures. Like any other part of the body, it uses energy and requires nutrients.Through dissection and other tech- niques, we know its complex anatomy, from the cerebrum with its two walnut-like halves, to the brain stem that exits through the lower skull to become the spinal cord.We know the general functions of its parts, and we can identify areas that control bodily movements, pro- cess visual information, deal with language, and so on.We know the structure of neurons, and something about how they communicate among themselves and their interconnections in the brain, which can change as a result of experience.

Certainly, further insight is needed. That should include, for instance, fuller knowledge of neurotransmittersthe chemicals like serotonin that carry signals among the brains neurons by electro- chemical meansand more extensive mapping of the brains func- tions, especially those like memory that seem to integrate information from different areas. However, scientists firmly believe that their un- derstanding of the brain will steadily grow through the use of electroencephalography (EEG) and the study of the effects of brain damage,and especially through the new techniques of functional mag- netic resonance imaging (fMRI) and positron emission tomography (PET) scanning. Both make it possible to observe something never seen beforethe operations of a living, working brain.

But thats the brain.The human mind, or human consciousness, is different.We know it is correlated with the brain, because if you cut off certain brain functions, consciousness flees, but we do not under- stand its nature and origins, largely because it is a subjective experi-

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