Illusion and hallucination

Types of illusion and hallucination. difference between illusion and hallucinaion.

1. Illusion & Hallucination Dr. Sushma Rathee Assistant Clinical Psychologist , PGIMER, Chandigarh Email: [email protected] 1

2. What is Illusion An illusion is a perception that does not correspond to reality: People think they see something when the reality is quite different. Another way of thinking of illusions is as visual stimuli that “fool” the eye. Illusion came from the Latin word “ illusio ” which means to mock (fake).

3. An  illusion  is a distortion of the senses, which can reveal how the human brain normally organizes and interprets sensory stimulation. Though illusions distort our perception of reality, they are generally shared by most people. Illusions may occur with any of the human senses, but visual illusions (optical illusions) are the best-known and understood. The emphasis on visual illusions occurs because vision often dominates the other senses.

4. Types of illusion: Optical Illusions: An optical or visual illusion is a kind of illusion in which the images perceived through the sense of sight tend to be misleading or deceptive, causing errors in perception. An optical illusion is based on the process through which the brain creates a visual world in one's mind using either or both these two sources: previous memory stored in it and the current presentation of the object in the environment .

5. 1. Muller Lyer Illusion: The  Müller - Lyer illusion  is an optical   illusion  consisting of three stylized arrows . The distortion happens when the viewer tries to determine if the two lines are exactly the same length

6. 2. THE MOON ILLUSION Another common illusion is the moon illusion , in which the moon on the horizon appears to be much larger than the moon in the sky.

7. 2. Auditory Illusions: While optical illusions deceive the eyes through visual images, auditory illusions mislead the ears through sounds. These sounds are usually those that are not really present in the physical stimulus, but is heard by the ears and perceived as a sound related to the stimulus in the environment. There are also auditory illusions that come from "impossible sounds", such as hearing a missing fundamental frequency, provided that there are other portions of the harmonic series, and different psychoacoustic tricks of lossy audio compression.

8. 3. Tactile Illusion While optical and auditory illusions are common manifestations of several psychological disorders such as schizophrenia and psychosis, tactile illusion is experienced by patients who have undergone amputation. The phantom limb is a tactile illusion wherein the patient still 'feels' pain on the leg, arm, or digit that has already been removed.

9. 4. ILLUSIONS OF MOTION Sometimes people perceive an object as moving when it is actually still. A utokinetic effect: In this effect, a small, stationary light in a darkened room will appear to move or drift because there are no surrounding cues to indicate that the light is not moving . Stroboscopic motion: seen in motion pictures , in which a rapid series of still pictures will seem to be in motion. Many a student has discovered that drawing little figures on the edges of a notebook and then flipping the pages quickly will also produce this same illusion of movement. Phi phenomenon: in which lights turned on in sequence appear to move.

10. What is hallucination: A hallucination is a perception in the absence of external stimulus that has qualities of real perception. Hallucinations are vivid, substantial, and are perceived to be located in external objective space. The word "hallucination" itself was introduced into the English language by the 17th-century physician Sir Thomas Browne in 1646 from the derivation of the Latin word  alucinari  meaning to wander in the mind.

11. Types of hallucination: Hallucinations can occur in any sensory modality — visual ,  auditory ,  olfactory ,  gustatory ,  tactile ,  Hallucinations can be associated with drug use, sleep deprivation, psychosis, neurological disorders, and delirium tremens.

12. 1. Visual A   visual hallucination  is "the perception of an external visual stimulus where none exists ". A separate but related phenomenon is a  visual illusion , which is a distortion of a real external stimulus. Visual hallucinations are classified as simple or complex: Simple visual hallucinations  (SVH) are also referred to as non-formed visual hallucinations and elementary visual hallucinations. These terms refer to lights, colors, geometric shapes, and indiscrete objects. Complex visual hallucinations  (CVH) are also referred to as formed visual hallucinations. CVHs are clear, lifelike images or scenes such as people, animals, objects, places, etc.

13. 2. Auditory : Auditory  hallucinations are the perception of sound without outside stimulus. Auditory hallucinations are the most common type of hallucination. Elementary hallucinations: are the perception of sounds such as hissing, whistling, an extended tone, and more. Complex hallucinations: are voices, music, or other sounds that may or may not be clear, may be familiar or completely unfamiliar, and friendly or aggressive, among other possibilities.

14. 3. Command : Command hallucinations are hallucinations in the form of commands; they can be auditory or inside of the person's mind or consciousness .  The contents of the hallucinations can range from the innocuous to commands to cause harm to the self or others . Command hallucinations are often associated with schizophrenia. People experiencing command hallucinations may or may not comply with the hallucinated commands, depending on the circumstances. Command hallucinations are sometimes used to defend a crime that has been committed, often homicides .  In essence, it is a voice that one hears and it tells the listener what to do. Sometimes the commands are quite benign directives such as "Stand up" or "Shut the door ."  Whether it is a command for something simple or something that is a threat, it is still considered a "command hallucination."

15. 4. Olfactory : Phantosmia (olfactory hallucinations ), smelling an odor that is not actually there, and parosmia (olfactory illusions), inhaling a real odor but perceiving it as different scent than remembered ,  are distortions to the sense of smell (olfactory system) that, in most cases, are not caused by anything serious and usually go away on their own in time . It can result from a range of conditions such as nasal infections, nasal polyps, dental problems, migraines, head injuries,  seizures , strokes, or brain tumors . Environmental exposures are sometimes the cause as well, such as smoking, exposure to certain types of chemicals (e.g., insecticides or solvents), or radiation treatment for head or neck cancer . It can also be a symptom of certain mental disorders such as  depression,  bipolar disorder, intoxication or withdrawal from drugs and alcohol, or  psychotic disorders (e.g ., schizophrenia ).   The perceived odors are usually unpleasant and commonly described as smelling burned, foul spoiled, or rotten.

16. 5. Tactile Tactile hallucinations are the illusion of tactile sensory input, simulating various types of pressure to the skin or other organs. One subtype of tactile hallucination, formication, is the sensation of insects crawling underneath the skin and is frequently associated with prolonged  cocaine  use .   However , formication may also be the result of normal hormonal changes such as  menopause, or disorders such as peripheral neuropathy, high fevers, Lyme disease, skin cancer, and more . The most common tactile hallucination in patients with schizophrenia is a sensation in which a patch of their skin is stretched elastically across their head .  They vary in intensity, range and speed at which they feel this stretching painful sensation. Also known as cutaneous hallucination.

17. Gustatory : This type of hallucination is the perception of taste without a stimulus. These hallucinations, which are typically strange or unpleasant, are relatively common among individuals who have certain types of focal epilepsy, especially temporal lobe epilepsy. The regions of the brain responsible for gustatory hallucination in this case are the insula and the superior bank of the sylvian fissure

18. Other types of hallucination: Hypnagogic hallucinations: can occur as one is falling asleep and Hypnopompic hallucinations: occur when one is waking up. Hypnagogic hallucinations and hypnopompic hallucinations are considered normal phenomena.

19. Difference between illusion and hallucination: The term  illusion  refers to a specific form of sensory distortion. Unlike a hallucination, which is a distortion in the absence of a stimulus, an illusion describes a misinterpretation of a true sensation. Both hallucination and illusion are related with perceptual processes . Hallucinations are false perceptions while illusions are misperceptions as the former merely reacts to nonexistent external stimuli while the latter involves actual stimuli . Unlike illusions, it is difficult to conduct researches on hallucinations due to its uniquely experienced nature . As hallucinations are generally abnormal, they receive more negative connotations as compared to illusions which is largely associated with the arts and entertainment.

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