Chapter 32: sensory and perceptual disorders.

Disorders of perception can be examined appropriately only after the normal operations of the senses have been appreciated. There was a long descriptive history of perceptual phenomena before theories were formed and experiments were performed. The phases through which phenomena pass in progressing from description to dissection are charted. The first stage is a description of phenomena, followed by attempts to incorporate them into the body of extant theory. Finally, the phenomena are accepted and utilized to gain more insights into the functioning of the senses and of the brain. In many cases, the phenomena have been described in the distant past, and no clear origin can be determined. In others, there is an obvious break with the past and a phenomenon is described and investigated for the first time. For most of the history of the senses, interest was usually restricted to illusions or oddities of experience: the commonplace characteristics of constant perception were ignored. These factors are taken into consideration with regard to the classification of the senses, phantom limbs, vertigo, and developmental disorders. Imposing some order on the senses was a long but necessary precursor to examining their disorders. Once order was established then a range of fascinating phenomena came to light (particularly in vision). Others that had long been known became open to more detailed scrutiny.