The pathology of experience

The fundamental assumption of cognitive neuroscience is that the way we behave and the way we experience the world is determined by the way our brains work. Pathological cases provide the most stringent tests for this assumption. From our knowledge of the way our brains work it should be possible to predict what kind of behaviour or experience will occur as a result of damage to a specific brain region or system. Lichtheim (1885) was the first to describe this approach. In his simple model of how the brain processes speech Lichtheim pointed out that there were seven possible ‘interruption points’ in this system. He then specified the different kinds of aphasia that should result from damage at each of these points. A striking and erroneous early example of the approach concerns cerebral achromatopsia. For many years neurologists refused to accept the existence of this disorder on the basis of the mistaken belief that it was incompatible with the way the brain worked (Zeki, 1990). Today the demonstration of segregation in the visual system predicts the existence of many specific disorders including achromatopsia (impaired colour perception), akinetopsia (impaired visual movement perception) and prosopagnosia (impaired face recognition).These disorders are all negative in the sense that the patients lack a particular aspect of normal experience. It is not too difficult to understand how damage to the brain could prevent some behaviour or experience. It is much more difficult to understand how damage to the brain can create a positive symptom in which the patient experiences something, such as an hallucination, which most of us don’t experience. A second problem is that there is a long tradition of treating hallucinations not as false, but as veridical perceptions from a spiritual world; telepathy or a voice from beyond the grave. This tradition gave …