However, a strict separation of cortical and retinal events does not occur as some degree of processing takes place in the retina and, conversely, certain processes such as instantaneous parallax (see later) occur so quickly that it is difficult to believe they occur exclusively at a cortical level. Visual perception is the end product of the processing or reinterpretation by the cortex of sensory responses made by the retina to visual stimuli. Forrester MB ChB MD FRCS(Ed) FRCP(Glasg)(Hon) FRCOphth(Hon) FMedSci FRSE FARVO, in The Eye, 2021 Visual perception Dyschromatopsia refers to the perversion of colour perception and occurs after unilateral posterior lesions. In organic conditions, achromatopsia, which is the complete absence of colour, has been described after unilateral or bilateral occipital lesions, usually of the lingual and fusiform gyri. These perceptual distortions of colour occur in schizophrenia. ‘this colour looks like an old blue – something horrible’.īleuler (1950) describes ‘one patient sees everything as coloured red another sees everything as white’, and Jaspers ‘I only see black even when the sun is shining, it is still all black’. ‘black looked brown sometimes’, ‘brown looked different trouble with pink as it comes across as green’ and 3. ‘colours are brighter’, ‘colours more vivid – red, yellow, orange stood out’ 2. Cutting (1997) gives several examples: 1. The intensity of the colour ( visual hyperaesthesia), the actual hue and the quality of the colour can all be affected. One of Cutting's patients (1997) said, ‘Man behind a lorry was pulling hideous faces’.ĭifferent aspects of colour perception can be affected. Bleuler (1950) also describes, ‘Wardmates change their faces the very moment that one looks at them’. Schreber (1955) describes his experience as follows: ‘At the same time I repeatedly witnessed that changed heads during their stay in the common room that is to say without leaving the room and while I was observing them, they suddenly ran about with a different head’. Typically, these perceptual distortions of faces are rapidly fluctuant and dynamic. When metamorphopsia affects faces, it is referred to as paraprosopia. Usually, this may involve the appearance of things taking on a different aspect: ‘One woman saw people upside down, on their heads’ ( Bleuler, 1950). Alteration in the customary shape of the perceived object is termed metamorphopsia. These anomalies are common in temporal lobe epilepsy. In some cases, there can be apparent reduction in one hemifield of vision ( hemimicropsia).

The size of the perception can be either larger ( macropsia) or smaller ( micropsia) than expected. Critchley (1951) gave a number of examples: a cat noticed in the street one day kept appearing at various times and various situations over the next few days, and the words ‘Pullman Springs’ noticed on the back of a van kept appearing on other vehicles over the next few months. In visual perception, the recurrence or prolongation of a visual phenomenon beyond the customary limits of the appearance of the real event in the world is termed palinopsia ( Cutting, 1997). Femi Oyebode MBBS, MD, PhD, FRCPsych, in Sims' Symptoms in the Mind: Textbook of Descriptive Psychopathology, 2018 Elementary Aspects of Visual Perception

Perception definition psychology