Body schema

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Body schema is an organism's internal model of its own body, including the position of its limbs. The neurologist Sir Henry Head originally defined it as a postural model of the body that actively organizes and modifies 'the impressions produced by incoming sensory impulses in such a way that the final sensation of body position, or of locality, rises into consciousness charged with a relation to something that has happened before'. [1] As a postural model that keeps track of limb position, it plays an important role in control of action.

Contents

It involves aspects of both central (brain processes) and peripheral (sensory, proprioceptive) systems. Thus, a body schema can be considered the collection of processes that registers the posture of one's body parts in space. The schema is updated during body movement. This is typically a non-conscious process, and is used primarily for spatial organization of action. It is therefore a pragmatic representation of the body’s spatial properties, which includes the length of limbs and limb segments, their arrangement, the configuration of the segments in space, and the shape of the body surface. [2] [3] [4] [5] Body schema also plays an important role in the integration and use of tools by humans. [6] [7] [8] [9]

Body schema is different from body image; the distinction between them has developed over time.

History

Henry Head, an English neurologist who conducted pioneering work into the somatosensory system and sensory nerves, together with British neurologist Gordon Morgan Holmes, first described the concept in 1911. [10] The concept was first termed "postural schema" to describe the disordered spatial representation of patients following damage to the parietal lobe of the brain. Head and Holmes discussed two schemas (or schemata): one body schema for the registration of posture or movement and another body schema for the localization of stimulated locations on the body surface. "Body schema" became the term used for the "organized models of ourselves". [10] The term and definition first suggested by Head and Holmes has endured nearly a century of research with clarifications as more has become known about neuroscience and the brain. [2]

A portrait of Henry Head, the pioneering English neurologist who first defined and used the term "body schema". Henry Head.jpg
A portrait of Henry Head, the pioneering English neurologist who first defined and used the term "body schema".

Properties

Neuroscientists Patrick Haggard and Daniel Wolpert have identified seven fundamental properties of the body schema. It is spatially coded, modular, adaptable, supramodal, coherent, interpersonal, and updated with movement. [2]

Spatial encoding

The body schema represents both position and configuration of the body as a 3-dimensional object in space. A combination of sensory information, primarily tactile and visual, contributes to the representation of the limbs in space. [2] [4] This integration allows for stimuli to be localized in external space with respect to the body. [6] An example by Haggard and Wolpert shows the combination of tactile sensation of the hand with information about the joint angles of the arm, which allow for rapid movements of said arm to swat a fly. [2]

Modular

The body schema is not represented wholly in a single region of the brain. [2] Recent fMRI (functional Magnetic Resonance Imaging) studies confirm earlier results. For example, the schema for feet and hands are coded by different regions of the brain, while the fingers are represented by a separate part entirely. [11]

Adaptable

Plastic changes to the body schema are active and continuous. For example, gradual changes to the body schema must occur over the lifetime of an individual as he or she grows and absolute and relative sizes of body parts change over his or her life span. [2] The development of the body schema has also been shown to occur in young children. One study showed that with these children (9-, 14-, and 19-month-olds), older children handled spoons so as to optimally and comfortably grip them for use, whereas younger children tended to reach with their dominant hand, regardless of the orientation of the spoon and eventual ease of use. [12] Short-term plasticity has been shown with the integration of tools into the body schema. [7] [9] The rubber hand illusion has also shown the rapid reorganization of the body schema on the timescale of seconds, showing the high level of plasticity and speed with which the body schema reorganizes. [13] In the Illusion, participants view a dummy hand being stroked with a paintbrush, while their own hand is stroked identically. Participants may feel that the touches on their hand are coming from the dummy hand, and even that the dummy hand is, in some way, their own hand.

Supramodal

By its nature, body schema integrates proprioceptive, (the sense of the relative position of neighbouring parts of one's body), and tactile information to maintain a three-dimensional body representation. However, other sensory information, particularly visual, can be in the same representation of the body. This simultaneous participation means there are combined representations within the body schema, which suggests the involvement of a process to translate primary information (e.g. visual, tactile, etc.) into a single sensory modality or an abstract, amodal form. [2]

Coherent

The body schema, to function properly, must be able to maintain coherent organization continuously. [2] To do so, it must be able to resolve any differences between sensory inputs. Resolving these inter-sensory inconsistencies can result in interesting sensations, such as those experienced during the Rubber Hand Illusion. [13]

Interpersonal

It is thought that an individual's body schema is used to represent both one's own body and the bodies of others. Mirror neurons are thought to play a role in the interpersonal characteristics of body schema. Interpersonal projection of one's body schema plays an important role in successfully imitating motions such as hand gestures, especially while maintaining the handedness and location of the gesture, but not necessarily copying the exact motion itself. [11]

Updated with movement

A working body schema must be able to interactively track the movements and positions of body parts in space. [2] Neurons in the premotor cortex may contribute to this function. A class of neuron in the premotor cortex is multisensory. Each of these multisensory neurons responds to tactile stimuli and also to visual stimuli. The neuron has a tactile receptive field (responsive region on the body surface) typically on the face, arms, or hands. The same neuron also responds to visual stimuli in the space near the tactile receptive field. For example, if a neuron's tactile receptive field covers the arm, the same neuron will respond to visual stimuli in the space near the arm. As shown by Graziano and colleagues, the visual receptive field will update with arm movement, translating through space as the arm moves. [14] [15] Similar body-part-centered neuronal receptive fields relate to the face. These neurons apparently monitor the location of body parts and the location of nearby objects with respect to body parts. Similar neuronal properties may also be important for the ability to incorporate external objects into the body schema, such as in tool use.

Extended body schema

The idea of the extended body schema is that, aside from the proprioceptive, visual, and sensory components that contribute to making a mental conception of one's body, the same processes that contribute to a body schema are also able to incorporate external objects into the mental conception of one's body. [16] Part philosophical and part neuroscience, this concept builds upon the ideas of plasticity and adaptation to attempt to answer the question of where the body schema ends.

There is debate as to whether this concept truly exists, with one side arguing that the body schema does not extend past the body and the other side believing otherwise. [17] [18]

Supporting arguments

The perspective shared by those who agree with the theory of the extended body schema follow reasoning in line with such that supports theories on tool use.

In some studies, attempts at understanding tool assimilation are used to argue for the existence of the extended body schema. In an experiment involving the use and interaction with wool objects, subjects were tested on their ability to perceive afterimages of wool objects in varying contexts. Subjects accustomed their eyes to a dark room and then were shown a brief (1 millisecond) flash of light, intending to produce an afterimage effect of their arms which they held out in front of them during the experiment. Moving an arm afterwards would make the afterimage "fade" or disappear as it moved, thus indicating that the feature (the arm) was being tracked and integrated into the person's body schema. To test integration of the meaningless wool objects, subjects experienced four different contexts.

  1. Subjects held the wool objects in each hand and one hand (the active hand) would move, still holding the object (the active object).
  2. Using the active hand, the active wool object would be dropped once an afterimage was perceived.
  3. Using the active hand, one would grab the active wool object once an afterimage was perceived.
  4. The subjects were to hold onto a mechanical device which held the wool object. Once an afterimage was perceived, a subject's active hand would cause the mechanical device to drop the wool object.

In all situations but the fourth, the subjects experienced the same "fading" effect as they did with their arm alone. This would thus indicate that the wool objects had been integrated into their body schema and contributes support towards the idea of the body's using proprioceptive and visual elements to create an extended body schema. The mechanical device acted as an intermediate between the subject and the active object, and the subjects' failure to detect an afterimage in that context indicates that this concept of extension is limited to being sensitive to only what the body is directly in contact with. [7]

Dissenting arguments

The alternate perspective is that the body is the limit of any sort of body schema.

An example of this division is found in a study and discussion on personal and extrapersonal attention, where personal relates to the body's sense of itself (the body schema) and extrapersonal relates to all external of such. Some research supports the claim that these two categories are purely distinct and do not intermingle, contrary to what the extended body schema theory describes. Evidence for such is primarily found in subjects with unilateral neglect, such as in the case of E.D.S., who was a middle-aged man with right hemisphere brain damage. When he was tested for hemispatial neglect using traditional measures such as sentence reading and cancellation tests, E.D.S. showed few signs and upon later examination showed no signs whatsoever, leading doctors to believe he was normal. However, he constantly had issues with physical therapy because he would claim to not be able to see his left leg; upon further examination, E.D.S. was known to have a particular type of hemispatial neglect that only affected the perception of his body. The motor function of the left side of his body was negatively affected though not totally compromised, yet when attempting tasks such as shaving, he would invariably not shave the left side of his face. This led some researchers to believe that there is a distinction between personal and extrapersonal neglect, which would thus reflect a similar distinction with body schema itself. [19]

Associated disorders

Deafferentation

The most direct of related disorders, deafferentation occurs when sensory input from the body is reduced or absent, without affecting motor neurons. The most famous case of this disorder is "IW", who lost all sensory input from below the neck, resulting in temporary paralysis. He was forced to learn to control his movement all over again using only his conscious body image and visual feedback. As a result, when constant visual input is lost during an activity, such as walking, it becomes impossible for him to complete the task, which may result in falling, or simply stopping. IW requires constant attention to tasks to be able to complete them accurately, demonstrating how automatic and subconscious the process of integrating touch and proprioception into the body schema actually is. [20]

Autotopagnosia

Autotopagnosia typically occurs after left parietal lesions. Patients with this disorder make errors which result from confusion between adjacent body parts. For example, a patient may point to their knee when asked to point to their hip. Because the disorder involves the body schema, localization errors may be made both on the patient’s own body and that of others. The spatial unity of the body within the body schema has been damaged such that it has incorrectly been segmented in relation to its other modular parts. [21]

Phantom limb

Phantom limbs are a phenomenon which occurs following amputation of a limb from an individual. In 90–98% of cases, amputees report feeling all or part of the limb or body part still there, taking up space. [22] The amputee may perceive a limb under full control, or paralyzed. A common side effect of phantom limbs is phantom limb pain. The neurophysiological mechanisms by which phantom limbs occur is still under debate. [23] A common theory posits that the afferent neurons, since deafferented due to amputation, typically remap to adjacent cortical regions within the brain. This can cause amputees to report feeling their missing limb being touched when a seemingly unrelated part of the body is stimulated (such as if the face is touched, but the amputee also feels their missing arm being stroked in a specific location). Another facet of phantom limbs is that the efferent copy (motor feedback) responsible for reporting on position to the body schema does not attenuate quickly. Thus the missing body part may be attributed by the amputee to still be in a fixed or movable position. [2]

Others

Asomatognosia, somatoparaphrenia, anosognosia, anosodiaphoria, allochiria and hemispatial neglect all involve (or in some cases involve) aspects of impaired body schema. Hemispatial neglect is not uncommon because strokes sometimes cause it.

Tool use

Rhesus macaques are able to be trained to use rudimentary tools, but have never been proven to use tools spontaneously in the wild. Rhesus Macaques - cropped.jpg
Rhesus macaques are able to be trained to use rudimentary tools, but have never been proven to use tools spontaneously in the wild.

Not only is it necessary for the body schema to be able to integrate and form a three-dimensional representation of the body, but it also plays an important role in tool use. [9] Studies recording neuronal activity in the intraparietal cortex in macaques have shown that, with training, the macaque body schema updates to include tools, such as those used for reaching, into the body schema. [9] In humans, body schema plays an important role in both simple and complex tool use, far beyond that of macaques. [6] [8] [9] Extensive training is also not necessary for this integration. [11]

The mechanisms by which tools are integrated into the body schema are not fully understood. However, studies with long-term training have shown interesting phenomena. When wielding tools in both hands in a crossed posture, behavioral effects reverse in a similar way to when only hands are crossed. Thus, sensory stimuli are delivered the same way be it to the hands directly or indirectly via the tools. These studies suggest the mind incorporates the tools into the same or similar areas as it does the adjacent hands. [9] Recent research into the short term plasticity of the body schema used individuals without any prior training with tools. These results, derived from the relation between afterimages and body schema, show that tools are incorporated into the body schema within seconds, regardless of length of training, though the results do not extend to other species besides humans. [6]

Confusion with body image

Historically, body schema and body image were generally lumped together, used interchangeably, or ill-defined. In science and elsewhere, the two terms are still commonly misattributed or confused. Efforts have been made to distinguish the two and define them in clear and differentiable ways. [24] A body image consists of perceptions, attitudes, and beliefs concerning one's body. In contrast, body schema consists of sensory-motor capacities that control movement and posture.

Body image may involve a person’s conscious perception of his or her own physical appearance. It is how individuals see themselves when picturing themselves in their mind, or when perceiving themselves in a mirror. Body image differs from body schema as perception differs from movement. Both may be involved in action, especially when learning new movements.

See also

Related Research Articles

<span class="mw-page-title-main">Parietal lobe</span> Part of the brain responsible for sensory input and some language processing

The parietal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The parietal lobe is positioned above the temporal lobe and behind the frontal lobe and central sulcus.

Stimulus modality, also called sensory modality, is one aspect of a stimulus or what is perceived after a stimulus. For example, the temperature modality is registered after heat or cold stimulate a receptor. Some sensory modalities include: light, sound, temperature, taste, pressure, and smell. The type and location of the sensory receptor activated by the stimulus plays the primary role in coding the sensation. All sensory modalities work together to heighten stimuli sensation when necessary.

<span class="mw-page-title-main">Sensory neuron</span> Nerve cell that converts environmental stimuli into corresponding internal stimuli

Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded receptor potentials. This process is called sensory transduction. The cell bodies of the sensory neurons are located in the dorsal ganglia of the spinal cord.

<span class="mw-page-title-main">Hemispatial neglect</span> Medical condition

Hemispatial neglect is a neuropsychological condition in which, after damage to one hemisphere of the brain, a deficit in attention and awareness towards the side of space opposite brain damage is observed. It is defined by the inability of a person to process and perceive stimuli towards the contralesional side of the body or environment. Hemispatial neglect is very commonly contralateral to the damaged hemisphere, but instances of ipsilesional neglect have been reported.

Multisensory integration, also known as multimodal integration, is the study of how information from the different sensory modalities may be integrated by the nervous system. A coherent representation of objects combining modalities enables animals to have meaningful perceptual experiences. Indeed, multisensory integration is central to adaptive behavior because it allows animals to perceive a world of coherent perceptual entities. Multisensory integration also deals with how different sensory modalities interact with one another and alter each other's processing.

Sensory processing is the process that organizes and distinguishes sensation from one's own body and the environment, thus making it possible to use the body effectively within the environment. Specifically, it deals with how the brain processes multiple sensory modality inputs, such as proprioception, vision, auditory system, tactile, olfactory, vestibular system, interoception, and taste into usable functional outputs.

<span class="mw-page-title-main">Premotor cortex</span>

The premotor cortex is an area of the motor cortex lying within the frontal lobe of the brain just anterior to the primary motor cortex. It occupies part of Brodmann's area 6. It has been studied mainly in primates, including monkeys and humans. The functions of the premotor cortex are diverse and not fully understood. It projects directly to the spinal cord and therefore may play a role in the direct control of behavior, with a relative emphasis on the trunk muscles of the body. It may also play a role in planning movement, in the spatial guidance of movement, in the sensory guidance of movement, in understanding the actions of others, and in using abstract rules to perform specific tasks. Different subregions of the premotor cortex have different properties and presumably emphasize different functions. Nerve signals generated in the premotor cortex cause much more complex patterns of movement than the discrete patterns generated in the primary motor cortex.

Autotopagnosia from the Greek a and gnosis, meaning "without knowledge", topos meaning "place", and auto meaning "oneself", autotopagnosia virtually translates to the "lack of knowledge about one's own space," and is clinically described as such.

Eye–hand coordination is the coordinated motor control of eye movement with hand movement and the processing of visual input to guide reaching and grasping along with the use of proprioception of the hands to guide the eyes, a modality of multisensory integration. Eye–hand coordination has been studied in activities as diverse as the movement of solid objects such as wooden blocks, archery, sporting performance, music reading, computer gaming, copy-typing, and even tea-making. It is part of the mechanisms of performing everyday tasks; in its absence, most people would not be able to carry out even the simplest of actions such as picking up a book from a table.

Tactile discrimination is the ability to differentiate information through the sense of touch. The somatosensory system is the nervous system pathway that is responsible for this essential survival ability used in adaptation. There are various types of tactile discrimination. One of the most well known and most researched is two-point discrimination, the ability to differentiate between two different tactile stimuli which are relatively close together. Other types of discrimination like graphesthesia and spatial discrimination also exist but are not as extensively researched. Tactile discrimination is something that can be stronger or weaker in different people and two major conditions, chronic pain and blindness, can affect it greatly. Blindness increases tactile discrimination abilities which is extremely helpful for tasks like reading braille. In contrast, chronic pain conditions, like arthritis, decrease a person's tactile discrimination. One other major application of tactile discrimination is in new prosthetics and robotics which attempt to mimic the abilities of the human hand. In this case tactile sensors function similarly to mechanoreceptors in a human hand to differentiate tactile stimuli.

<span class="mw-page-title-main">Proprioception</span> Sense of self-movement, force, and body position

Proprioception is the sense of self-movement, force, and body position.

Extinction is a neurological disorder that impairs the ability to perceive multiple stimuli of the same type simultaneously. Extinction is usually caused by damage resulting in lesions on one side of the brain. Those who are affected by extinction have a lack of awareness in the contralesional side of space and a loss of exploratory search and other actions normally directed toward that side.

Sensory maps are areas of the brain which respond to sensory stimulation, and are spatially organized according to some feature of the sensory stimulation. In some cases the sensory map is simply a topographic representation of a sensory surface such as the skin, cochlea, or retina. In other cases it represents other stimulus properties resulting from neuronal computation and is generally ordered in a manner that reflects the periphery. An example is the somatosensory map which is a projection of the skin's surface in the brain that arranges the processing of tactile sensation. This type of somatotopic map is the most common, possibly because it allows for physically neighboring areas of the brain to react to physically similar stimuli in the periphery or because it allows for greater motor control.

Amorphosynthesis, also called a hemi-sensory deficit, is a neuropsychological condition in which a patient experiences unilateral inattention to sensory input. This phenomenon is frequently associated with damage to the right cerebral hemisphere resulting in severe sensory deficits that are observed on the contralesional (left) side of the body. A right-sided deficit is less commonly observed and the effects are reported to be temporary and minor. Evidence suggests that the right cerebral hemisphere has a dominant role in attention and awareness to somatic sensations through ipsilateral and contralateral stimulation. In contrast, the left cerebral hemisphere is activated only by contralateral stimuli. Thus, the left and right cerebral hemispheres exhibit redundant processing to the right-side of the body and a lesion to the left cerebral hemisphere can be compensated by the ipsiversive processes of the right cerebral hemisphere. For this reason, right-sided amorphosynthesis is less often observed and is generally associated with bilateral lesions.

Michael Steven Anthony Graziano is an American scientist and novelist who is currently a professor of Psychology and Neuroscience at Princeton University. His scientific research focuses on the brain basis of awareness. He has proposed the "attention schema" theory, an explanation of how, and for what adaptive advantage, brains attribute the property of awareness to themselves. His previous work focused on how the cerebral cortex monitors the space around the body and controls movement within that space. Notably he has suggested that the classical map of the body in motor cortex, the homunculus, is not correct and is better described as a map of complex actions that make up the behavioral repertoire. His publications on this topic have had a widespread impact among neuroscientists but have also generated controversy. His novels rely partly on his background in psychology and are known for surrealism or magic realism. Graziano also composes music including symphonies and string quartets.

Body transfer illusion is the illusion of owning either a part of a body or an entire body other than one's own, thus it is sometimes referred to as "body ownership" in the research literature. It can be induced experimentally by manipulating the visual perspective of the subject and also supplying visual and sensory signals which correlate to the subject's body. For it to occur, bottom-up perceptual mechanisms, such as the input of visual information, must override top-down knowledge that the certain body does not belong. This is what results in an illusion of transfer of body ownership. It is typically induced using virtual reality.

Haptic memory is the form of sensory memory specific to touch stimuli. Haptic memory is used regularly when assessing the necessary forces for gripping and interacting with familiar objects. It may also influence one's interactions with novel objects of an apparently similar size and density. Similar to visual iconic memory, traces of haptically acquired information are short lived and prone to decay after approximately two seconds. Haptic memory is best for stimuli applied to areas of the skin that are more sensitive to touch. Haptics involves at least two subsystems; cutaneous, or everything skin related, and kinesthetic, or joint angle and the relative location of body. Haptics generally involves active, manual examination and is quite capable of processing physical traits of objects and surfaces.

<span class="mw-page-title-main">Tactile hallucination</span>

Tactile hallucination is the false perception of tactile sensory input that creates a hallucinatory sensation of physical contact with an imaginary object. It is caused by the faulty integration of the tactile sensory neural signals generated in the spinal cord and the thalamus and sent to the primary somatosensory cortex (SI) and secondary somatosensory cortex (SII). Tactile hallucinations are recurrent symptoms of neurological diseases such as schizophrenia, Parkinson's disease, Ekbom's syndrome and delirium tremens. Patients who experience phantom limb pains also experience a type of tactile hallucination. Tactile hallucinations are also caused by drugs such as cocaine and alcohol.

<span class="mw-page-title-main">Sensory processing disorder</span> Medical condition

Sensory processing disorder is a condition in which multisensory input is not adequately processed in order to provide appropriate responses to the demands of the environment. Sensory processing disorder is present in many people with dyspraxia, autism spectrum disorder and attention deficit hyperactivity disorder. Individuals with SPD may inadequately process visual, auditory, olfactory (smell), gustatory (taste), tactile (touch), vestibular (balance), proprioception, and interoception sensory stimuli.

Dyschiria, also known as dyschiric syndrome, is a neurological disorder where one-half of an individual's body or space cannot be recognized or respond to sensations. The term dyschiria is rarely used in modern scientific research and literature. Dyschiria has been often referred to as unilateral neglect, visuo-spatial neglect, or hemispatial neglect from the 20th century onwards. Psychologists formerly characterized dyschiric patients to be unable to discriminate or report external stimuli. This left the patients incapable of orienting sensory responses in their extrapersonal and personal space. Patients with dyschiria are unable to distinguish one side of their body in general, or specific segments of the body. There are three stages to dyschiria: achiria, allochiria, and synchiria, in which manifestations of dyschiria evolve in varying degrees.

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