ABSTRACT | PDF
SEMINAR
Perception
Debjit Roy
Postgraduate Trainee, Department of Psychiatry, Silchar Medical College and Hospital
Introduction Definition: It is the conscious awareness of elements in the environment by the mental processing of sensory stimuli. Perception refers to the way the world looks, sounds, feels, tastes or smells. For much of what we perceive the sensory input provides only raw materials for experience but the perceptual system searches for the percept which is most consistent with the sensory data, thus making perception an active process rather than a passive one.
Many years ago, the famous American Psychologist William Jones put it this way: “Part of what we perceive comes through the senses from the object before us; another part . . . always comes . . . Out of our own Head.” The “out of our own head” refers to the active processing of sensory input that makes our experience of the world what it is.
Out of the different types of perception, vision is our preferred spatial sense, giving us variegated patterns of form and colour in three dimension, also it is a good time sense because we see succession, movement and change.
History The theoretical significance of the pattern of stimuli producing a perceptual experience was recognised early by proponents of Gestalt Psychology, a school of psychology that arose in Austria and Germany. According to Gestalt Psychology perception acts to draw sensory data in a wholistic pattern. For that reason it was sometimes said “The whole is different from the sum of its parts.”
Perceptual processes 1. Form Perception - Perhaps the most fundamental process in form perception is the recognition of a figure on a backgound.
a) Contours in visual form perception: We are able to separate forms from background in our visual perception only due to contours. Contours are formed whenever a marked difference occurs in the brightness or colour of background.
b) Organization in form perception: When several objects are present in the visual field we tend to perceive them as organized into patterns or groupings.
Laws of perceptual organization: Proximity - This law states that the items which are close together in space or time tend to be perceived as belonging together or forming an organized group.
Similarity - Similar objects tend to be grouped together in perceptual process.
The law of good figure - It says that there is a tendency to organize things to make a balanced or symmetrical figure that includes all parts.
Continuation: This is the tendency to perceive a line that starts in one way as continuing in the same way.
Law of closure: Refers to the perceptual processes that organize the perceived world by filling in gaps in stimulation.
2. Depth perception is a complicated process which can be explained on the basis of monocular and binocular cues.
a) Monocular cues - These are cues which work when only one eye is looking. They are the ones used by the painters to give us a three dimensional experience from a flat painting.
Linear perspective: the distances separating the images of far objects appear to be smaller.
Clearness: The more clearer we see an object the nearer it seems.
Gradients of texture: The continuous gradation of texture in the visual field also acts as a cue for depth perception.
Interposition: When one object is completely visible, while another is covered by it, the first object is perceived as nearer.
Shadow: The way that light falls on an object and reflects off its surfaces, and the shadows that are cast by objects provide an effective cue for the brain to determine the shape of objects and their position in space.
b) Binocular cues: The retinas receive slightly different or disparate views of the world. This disparity increases for close objects and decreases for far objects. This correspondence between distance and amount of disparity is used as a depth cue.
3. Constancy
a) Size constancy: The tendency to see an object as the same regardless of the distance is called size constancy. Factors for size constancy are idea about the distance of the object, background information, and knowledge about the size of a familiar object.
b) Brightness constancy: Visual objects appear constant in their degree of whiteness, grayness, or blackness irrespective of the illumination. This is due to constant or unchanged brightness ratio between the object and the surrounding.
4. Movement perception
a) Real motion: We perceive movement due to stimulation of successive parts of retina as the object moves, and also because of the relationship between the object and it’s surroundings.
b) Apparent motion: It is the movement perceived in the absence of physical movement of an image across retina.
Types of apparent motion
Stroboscopic motion - this is created when separated stimuli, not in motion, are presented in succession e.g. the kind seen in cine films.
Autokinetic effect: If a person stares at a stationary spot of light, in a dark room, the spot will eventually appear to move.
Induced movement: A stationary spot or object is perceived as moving when its frame or background moves e.g. the moon is often perceived as “racing” through a thin layer of clouds.
5. Individual differences: Though perceptual process are much the same for everyone, but we know that people differ in the way they process sensory inputs to give rise to what they experience. Individual differences in learning, sets, motives, and perceptual styles gives rise to these differences.
a) Perceptual learning: It is defined as an increase in the ability to extract information from the environment as a result of experience or practice e.g.
i) Distinguishing the calls of birds by an ornithologist.
ii) Remarkable feat of the blind people.
b) Set: Set refers to the idea that we may be “ready” and “primed for” certain kind of sensory inputs e.g. a husband who is expecting an important phone call more easily hears the phone ring in the night while his wife does not. The wife, on the other hand, may be more likely to hear the baby crying than the telephone ring.
c) Motives and needs: Motives and needs also influence perception e.g. people who are hungry, thirsty, sexually aroused are likely to pay attention to events in the environment which will satisfy their needs.
d) Perceptual cognitive styles: People are said to differ in the ways they typically and characteristically process information. The general processing strategies that characterise different people are known as perceptual cognitive styles. Two dimensions along which people vary in perceptual-cognitive styles are (i) the degree to which their perceptions are flexible or constricted, and (ii) their field dependence or independence.
I) People whose perceptions are at the flexible end of are said to have a wider focus of attention, to be less affected by interfering influences and to be less dominated by internal motives or needs than are people at the constricted end.
II) A field dependent person perceives things as whole whereas a field independent person will perceive it in parts.
Disorders of perception Classification: 1. Sensory distortion
i) Changes in intensity
ii) Changes in quality
iii) Changes in spatial form
iv) Distortion of experience of time.
2. Sensory deception
A) Illusions
B) Hallucinations
C) Distortions of body image.
1. Sensory distortion: In distortion there is constant real perceptual object, which is perceived in a distorted way.
i) Changes in intensity - The intensity of perception may be altered so that it may be either heightened or diminished e.g.
Hyperacusis: sounds of normal intensity are perceived abnormally loud. Seen in migraine, hangover from alcohol excess, depressive disorders.
Visual hyperaesthesia: Colours look more intense or vivid. Seen in hypomanics, epileptic aura, under effect of lysergic acid diethylamide (LSD), and situation of intense emotion like religious fervour or being in love.
ii) Changes in quality - It is mainly visual perception that are affected by this, brought about by toxic substances e.g. colouring of yellow, green, or red. These are mainly effects of drugs such as poisoning with mescaline or digitalis.
iii) Changes in spatial form (dysmegalopsia) - Micropsia, macropsia, porropsia, metamorphosia. These are most commonly seen in temporal and parietal lobe lesions, also in retinal disease, disorders of accomodation and convergence.
iv) Distortion of experience of time - There are two varieties of time: physical and personal. It is the latter that is affected by psychiatric disorders e.g. in severe depression patient may feel time passes slowly and even stands still. By contrast the manic patient feels that time speeds by.
2. Sensory deception: These are of three types - A) Illusions B) Hallucinations C) Body image distortion.
A) Illusions: In illusions, stimuli from a perceived object are combined with a mental image to produce a false perception.
Types of illusion: Completion illusion - They depend on inattention for their occurrence e.g. misreading words or missing misprints.
Affect illusions - They arise in the context of a particular mood state e.g. bereaved person may momentarily believe that they see the deceased person.
Pareidolia - In these type vivid illusions occur without the person making any effort. This occurs as a result of vivid visual imagery and excessive fantasy thinking e.g. person sees vivid pictures in fire or in clouds without any conscious effort on his part and sometimes even against his will.
B) Hallucinations: Hallucination is a sensory percept experienced in the absence of external stimulus to the corresponding sense organ.
Causes of hallucinations
Emotion or psychiatric disorders e.g. very depressed patient experience auditory hallucinations which are disjointed and fragmentary, uttering single words or short phrases which are in contrast to the continuous persistent hallucinatory voices of schizophrenia.
Suggestion: Several experiments have shown that normal people can be persuaded to hallucinate e.g. when asked to walk down a dimly lit corridor and stop when they see faint light over the door at the end, most subjects stopped walking at some point saying they could see a light though none was switched on.
Disorders of peripheral nervous system e.g. Charles Bonnet syndrome.
Sensory deprivation.
Disorders of the central nervous system (CNS): lesions of diencephalons or cortex.
Hallucination of individual senses
a) Auditory - These are also called phonemes. They are of two types.
Elementary and unformed: They occur mainly in organic states e.g. simple noises, bells, whispers or Voices.
Organized: These can be partly organized as music or completely organized as hallucinatory voices, in schizophrenia.
b) Visual: These can be elementary as flashes of light or organized in the form of visions of people, objects or animals. They characteristically occur in organic states than in functional psychosis e.g. delirium tremens, occipital lobe tumours, post concussional state, temporal lobe epilepsy, hepatic failure, Charles Bonnet syndrome.
c) Olfactory: Seen in patients of schizophrenia, epileptic aura, other organic states.
d) Gustatory: They occur in schizophrenia (they coexist with delusion of being poisoned), depression (flavor of food may disappear altogether or become unpleasant), temporal lobe epilepsy.
e) Tactile hallucination: It is of three types.
i) Superficial - Thermic, haptic, hygric, paraesthetic.
ii) Kinaesthetic: This affects the muscle and joints of the patient and he feels that limbs are being twisted, pulled or moved.
iii) Visceral: These are bizarre hallucinations in which patient might complaint that his organs are being torn out or flesh ripped apart from his body.
Special kinds of hallucinations
a) Functional hallucinatons: This is a strange phenomenon in which an external stimuli is necessary to provoke hallucination, but the normal perception of the stimulus and hallucination in the same modality are experienced simultaneously e.g. a schizophrenic patient heard hallucinatory voices only when water was running through the pipes.
b) Reflex hallucinatons: In this type a stimulus in one sensory modality produces a hallucination in another e.g. a patient felt pain in her head (somatic hallucination) when she heard other people sneeze (the stimulus).
c) Autoscopy or phantom mirror image: It is the experience of seeing one’s own body projected into external space, usually in front of oneself for short periods. This type of hallucination is seen in acute and subacute delirious state, epilepsy, focal lesions affecting parietooccipital regions. In internal autoscopy in which subject sees their own internal organs. In negative autoscopy patient looks at the mirror and sees no image at all.
d) Extracampine hallucinations: The patient has a hallucination that is outside the limits of sensory field e.g. a schizophrenic patient hears voices talking in London when they are in Liverpool.
e) Hypnagogic and hypnopompic hallucinations: These are normal phenomenon in which people experience hallucinations mostly in the auditory sense modality, when falling asleep or just during awakening from sleep respectively.
Hallucinatory syndromes: They are also termed hallucinosis, characterised by persistent hallucination in any sense modality in the absence of any psychotic feature.
i) Alcoholic hallucinosis
ii) Organic hallucinosis: Present in 20-30% of dementia patients, especially of Alzheimer’s type.
Pseudohallucinations: A pseudohallucination is an involuntary sensory experience vivid enough to be regarded as a hallucination, but recognized by the patient not to be the result of external stimuli.
Difference with hallucinations and similarities with imagery - experience is figurative, not concretely real; located in inner subjective space.
Similarity with hallucination and difference with imagery - definite edges, vivid, coloured; constant over time; not created voluntarily.
C) Body image distortion: Body image includes both perceptional and ideational components and may reflect primarily perceptual distortions or combination of disturbed perception and self appraisal or both. This may be seen in:
Phantom limb - normal response to abrupt body change.
Duplication phenomenon - complex partial seizure, schizophrenia and migraine.
Agnosia, anosognosia, hyperschemazia, hyposchemazia - damage of nondominant parietal lobe.
Koro - culture bound syndrome.
Some recent advances 1. Neurobiology of halucinations: Most consistent finding of structural neuroimaging studies of patients with hallucinations is reduced grey matter volume in superior temporal gyrus, including primary auditory cortex.
Functional magnetic resonance imaging (fMRI) studies in patients with schizophrenia with auditory hallucinations have shown altered connectivity among temporal, prefrontal and anterior cingulate regions.
Failure of corollary discharge mechanism - Hallucinating individuals often misinterpret internally generated speech as coming from outside.
fMRI studies of auditory hallucinations have shown increased activity in the ventral extrastriate visual cortex.
2. Treatment resistant auditory hallucinations: Can be treated by slow repetitive transcranial magnetic stimulation (rTMS) of the right temporoparietal cortex.
References
1. Sims A. Pathology of perception. In Sims A. Symptoms in the mind. 3rd ed. Philadelphia: Saunders, An Imprint of Elsevier;2003. pg 90-117.
2. Casey A, Kelly B. Disorders of perception. In Casey A, Kelly B. Fish’s clinical psychopathology. 3rd ed. London: The Royal College of Psychiatrists;2007. pg 14-31.
3. Yager J, Gitlin MJ. Clinical Manifestation of Psychiatric Disorders. In Sadock BJ, Sadock VA. Comprehensive Textbook of Psychiatry. 8th ed. Philadelphia: Lippincott Williams & Wilkins; 2004. pg 964-1002.
4. Hilgard ER, Atkinson RC, Atkinson RL. Perception. In Hilgard ER, Atkinson RC, Atkinson RL. Introduction to Psychology. 6th ed. New Delhi OXFORD & IBH PUBLISHING CO. PVT. LTD.; pg 129- 151.
5. Morgan CT, King RA, Weisz JR, Schopler J. Sensory Processes and Perception. In Morgan CT, King RA, Weisz JR, Schopler J. Introduction to Psychology. 7th ed. New Delhi: Tata Mcgraw Hill Education Private Limited; 1993. pg 80-135.
6. Internet sources.
This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.