Perception is a complex process which is not restricted to the screening of physical signals by sense organs but implies the processing of these data to represent reality. Ideas from structuralism, constructivism, and the philosophy of the mind have influenced psychiatric concepts of perception and the constitution of reality. Between the 1950s and 1970s Gestalt psychology highlighted the complex moulding of percepts and the disturbance of this process in psychotic phenomena. Archaic 'protopathic' Gestalt and elaborated rational 'epicritical' Gestalt processes were suggested. More recently the distinction between sensory screening and interpretative mentation has been confirmed by neurocognitive research.
Philosphical ideas have also been used. Hundert(4) used the Kantian distinction between a priori categories and a posteriori experiences as a framework for differentiating perception by the sense organs from the secondary evaluation process. Kant's emphasis on the interplay between 'distal' perception and 'proximal' conceptualization can be exemplified by the perception and recognition of faces, which are disturbed in the Capgras syndrome and to a lesser degree in schizophrenia. The processing of visual perception is organized on at least four levels of complexity: the retina, the lateral geniculate body, the occipital visual cortex, and the hippocampus. The third level (the occipital cortex), where we actually 'see', does not contain an image any more than do the preceding levels; rather, it holds a database composed of signals from specific neurones for edges, angles, curves, sudden movements, etc. Compared with the perceptual screen of the retina, these signals are 'scrambled' but even so they form a notion of what we perceive as reality. Recognition of faces needs further processing, probably in the hippocampal area where associations from other cortical fields are integrated with the visual information (e.g. the voice belonging to the face). In psychiatry we deal with very heterogeneous aetiologies and perceptual disturbances which may originate from different levels of processing, usually from a more integrated level than in neurological disease and more distant from the immediate screening of physical stimuli by the sense organs. Put in another way, psychiatric disorders of perception affect different stages of information processing—from disturbances in the sense organs to complex phenomena involving feelings and ideas.
Here we shall mainly focus on hallucinations and some related phenomena which are relevent for psychiatric illnesses.
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