Sensory Distortion

Sensory Distortion in Perceptual Experiences

Disturbance of the mental state, with or without organic brain pathology, may cause sensory distortion. Sensory distortions are phenomena in which a real perceptual object is perceived distorted.

In sensory distortion, the intensity of a perception can become heightened or diminished and qualitative changes can occur.

This commonly affects the visual modality when toxicity from certain drugs alters color vision (e.g. santonin causing violet or yellow vision). Quantitative changes include micropsia, macropsia or dysmegalopsia, in which objects are seen as smaller or larger than they really are, or as altered in shape, respectively. Such phenomena may be due to end organ disease or to acute organic disorders and epilepsy Opens in new window.

Sensory distortion may involve any of the components or elementary aspects of perception, such as uniqueness, size, shape, color, location, motion or general quality.

Elementary aspects of visual perception

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). Critchey (1951) gave a number of examples:

The size of the perception can be either larger (macropsia) or smaller (micropsia) than expected. In some cases, there can be apparent reduction in one hemifield of vision (hemimicropsia).

These anomalies are common in temporal lobe epilepsy. Alteration in the customary shape of the perceived object is termed metamorphopsia. Usually, this may involve the appearance of things taking on a different aspect: ‘One woman saw people upside down, on their heads’ (Bleuler, 1950). This is an example of inversion.

When metamorphopsia affects faces, it is referred to as paraprosopsia. Typically, these perceptual distortions of faces are rapidly fluctuant and dynamic. Schreber (1955) describes his experience as follows:

Bleuler (1950) also describes ‘wardmates change their faces the very moment that one looks at them’. One of Cutting’s patients (1997) said, ‘Man behind a lorry was pulling hideous faces’.

Different aspects of color perception can be affected. The intensity of the color (visual hyperaesthesia), the actual hue and the quality of the color can all be affected. Cutting (1997) gives several examples:

1. ‘colors are brighter’, ‘colors more vivid – red, yellow, orange stood out’;
2. ‘black looked brown sometimes’, ‘brown looked different; trouble with pink as it comes across as green’; and
3. ‘this color looks like an old blue – something horrible’.

Bleuler (1950) describes ‘one patient sees everything as colored red; another sees everything as white’, and Jaspers ‘I only see black; even when the sun is shining, it is still all black’.

These perceptual distortions of color occur in schizophrenia Opens in new window. In organic conditions, achromatopsia, which is the complete absence of color, has been described following unilateral or bilateral occipital lesions, usually of the lingual and fusiform gyri.

Dyschromatopsia refers to the perversion of color perception and occurs following unilateral posterior lesions.

The spatial location of a perceived object may be distorted:

• Teleopsia involves the object appearing far away, and pelopsia the object appearing nearer than it should.
• Allaesthesia is the term for when the perceived object is in a different position from what is expected so that the patient, for example, experiences the transposition of objects from left to right.
• Akinetopsia is the impairment of visual perception of motion in which the individual is unable to perceive the motion of objects. It is very rare and is said to follow bilateral posterior cortical damage. Zeki (1993) quotes Zil’s case: ‘She had difficulty, for example, in pouring tea or coffee into a cup because the fluid appeared to be frozen, like a glacier. In addition, she could not stop pouring at the right time since she was unable to perceive the movement in the cup (or a pot) when the fluid rose’.

The general quality of perception can be affected. This usually involves an indefinable alteration in their visual appearance of the perceived world so that everything seems different from what it used to be:

• ‘People [look] like toys – almost dead and lifeless, carrying out automatic movements with special meaning’ (Cutting, 1997);
• ‘people look dead, pale, cold’ (Cutting, 1997);
• ‘A factory-worker sees a grass-hopper and becomes very disturbed and excited at the sight of this very strange [my emphasis] and unknown animal’ (Bleuler, 1950).

These experiences are examples of derealization Opens in new window.

Normally, perception is accompanied by affect Opens in new window, which may be a feeling of familiarity, of enjoyment, of dislike, of involvement, of proximity and so on. This is usually appropriate and so ignored. However, changes in these feelings may present as symptoms, for example,

There is a feeling of unreality in the perceptual field, an alteration in the feelings associated with the objects of perception.

A patient who exemplified both the loss of intensity of sensation and the change in feelings associated with perception in the context of a depressive illness was a 23-year old Sri Lankan Buddihist priest.

Following a session of meditation, he became very frightened one night after assaulting another young priest, apparently in his sleep. In the next few days, he felt that he had lost all sensation.

Things he saw and heard he could not understand properly. He could see only the things that were nearby. He could not get any sensations from his skin. He said that he could not read nor understand, nor feel sadness or happiness. He said that he could not feel anything: ‘all is numbed, body and mind’. He admitted to feeling low, that life was not worth living and that he had thought of ending his life. There was no neurological or other physical abnormality.

Elementary aspects of auditory perception

The elementary elements of auditory perception that can be disturbed include the uniqueness of the experience, the intensity and the spatial position (Cutting).

• In palinacousis, the uniqueness of a perceptual experience is disturbed and there is persistence of sounds that are heard. A subject returned to answer the door several times during a 30-minute period after the doorbell had actually rung (Jacobs et al., 1973).

• The intensity of auditory perception may be altered so that it is either heightened or diminished. For example, heightening in the auditory modality is called hyperacusis, a symptom in which the patient complains of everything sounding abnormally loud, saying ‘I can’t bear the noise’.

Ordinary conversation may sound intolerably noisy, and even whispering at a distance may be found uncomfortable. There is, of course, no true improvement of auditory perception but simply a lowering of the threshold at which noise becomes unpleasant.

The symptom occurs in depression, migraine and some toxic states, for example the hangover following acute alcohol excess. The spatial position of a sound may be disturbed so that the sound appears as if it was nearer, further or displaced in position.

Splitting of perception

This rather rare phenomenon is described sometimes with organic states and also with schizophrenia: the patient is unable to form the usual, assumed links between two or more perceptions.

A patient watching television experienced a feeling of competition between the visual and auditory perceptions. She felt that the two were not coming from the same source but were competing for her attention and conveying opposite messages.

Splitting of perception occurs when the links between different sensory modalities fail to be made, and so the sensations themselves, although in fact associated, appear to be quite separate and even in conflict.