Somatosensation, Body Representation Disorders PDF

**Somatosensation** Body representations include a set of interconnected functions which support: - The perception and localization of somatic stimuli (somatosensation) - Motor planning and execution - Body awareness body representations depend on multisensory integration of visual, somat...

**Somatosensation** Body representations include a set of interconnected functions which support: - The perception and localization of somatic stimuli (somatosensation) - Motor planning and execution - Body awareness body representations depend on multisensory integration of visual, somatosensory, proprioceptive, and vestibular information according to the classical definition by Head & Holmes, 1911 our \"body representations\" is constituted by: - BODY SCHEMA (postural schema): A postural-proprioceptive implicit representation, the individual is not aware of - BODY IMAGE (superficial schema): **The somatosensory system** = postcentral gyrus, posterioir parietal lobe, cerebellum The somatosensory pathway carries information about - Exteroceptive information = originated outside the body → touch, pressure, vibration, etc. - Interoceptive information = originated within the body → the relative position of body parts, the position of the body in space, etc. - Temperature, pain,... **Somatosensory pathway**: skin/muscles/tendons/joints → brainstem and cerebellum → thalamus → primary somatosensory cortex (SI) in parietal lobes → posterior parietal areas (associative areas) \- - \> there are specific regions related to specific body images, and they are: the motor cortex and the sensory cortex (divided by the central sulcus) -\> sensory cortex is also called S1, and it is somatotopically organized, which means that body areas are represented on the cortex corresponding to the receptor density (somatosensory homunculus) **Proprioception** Proprioception carries information regarding the relative and reciprocal position in space of body parts, allowed by mechanosensory neurons (receptors) located within muscles, tendons, and joints both conscious and unconscious information: for example, if you focus on force, you are conscious about it, but as for posture, it's usually unconscious until you willingly put focus on it **The vestibular system** Creates the sense of balance and spatial orientation for the purpose of coordinating movement with balance Vestibular pathway: ears → vestibular nuclei (brainstem) → thalamus and cerebellum → posterior insula and secondary somatosensory cortex in the parietal lobe \- - \> at the cortical level, vestibular information is integrated with somatosensation and proprioception, contributing to body representations **Neuroplasticity** Neuroplasticity is the brain\'s ability to change and adapt in response to sensory input from touch, pain, temperature, or body position E.G: if a person loses sensation in one part of the body, the brain can reorganize itself to focus more on other sensory areas, improving their sensitivity → it's how the nervous system adjusts to new experiences or injuries **Body illusions** Demonstrate the importance of multisensory integration to maintain a 'healthy (useful) representation of our body / body parts when a conflict between sensory information from different sensory channels is generated, representations get disorganized (artificially/as a consequence of a sensory loss/brain damage) E.G: Pinocchio illusion, in which there is a conflict between tactile and proprioceptive information E.G: rubber hand illusion, in which there is a conflict between vision and proprioception generated by a synchronous visuo-tactile stimulation on one hand (participants report experiences of ownership over a fake hand → subjective ownership over one\'s body can be easily disrupted) **Body sense of ownership and sense of agency** 1. SENSE OF OWNERSHIP = impression/sensation that I own a specific body part it depends on the spatial contiguity between that part and the rest of the body can be extended to instruments (e.g., tennis racket) and protheses \- - \> [Embodiment]: the phenomenon that enables to the brain to consider an external object "as if" it is a body part (Vignemont, 2011) a. I experience sense of ownership and agency over it b. It becomes part of my body schema c. The space around it becomes part of my peri-personal space: peri-personal space is the space we are able to reach with our hand, coded in specific fronto-parietal networks able to integrate sensory stimuli in different modalities (visual, auditory, and tactile) depending on their location 2. SENSE OF AGENCY = impression/sensation that I control that specific body part it depends on the temporal relationship (causality) between motor command and sensory feedback can be extended to any instruments (e.g., tennis racket) and protheses  **Full body illusion** The conflict between proprioceptive, tactile, visual and vestibular information can even generate 'full body illusions', i.e., [mis-location of one's body in space] a study says: humans normally experience the conscious self as localized within their bodily borders BUT it was found that, during multisensory conflict, participants felt as if a virtual body seen in front of them was their own body and mislocalized themselves toward the virtual body, to a position outside their bodily borders The integration between multisensory information supporting an integrated bodily self depends on the activity of the bilateral temporo-parietal junction the angular gyrus seems to enable the integration of tactile, visual, vestibular and proprioceptive information: lesion in these areas generates phenomena as autoscopy and out-of-the-body experience (i.e., illusion of body duplicates and mis-location of one's body → visuo-proprioceptive dissociation) **Impact of sensory deprivation in patients** **The effect of deafferentation: without perception** STORY: *In 1971 Ian Waterman (I.W.) was a butcher. Then suddenly he went down with* *what seemed to be gastric flu. Ian had contracted a disease of the nervous system so rare that the doctors were unable to diagnose it. It had destroyed all the sensory nerves responsible for touch, and for conveying information about muscle and joint position, senses so fundamental to our capacity to move in the world that they have been called a sixth sense.* without this \"proprioception\" we can have no inner sense of posture or limb position and cannot initiate or control movement I.W. was unable to perceive any tactile or proprioceptive sensations below the neck. He was unable to tell where his limbs were in space in the absence of vision (e.g., in the dark he was unable to stand) although his brain was completely spared (viral infection only destroyed the afferent pathways): the deafferentation disorganizes the body schema **In patients without brain lesions** Amputees also suffer from altered body representations **Phantom limb experience**: patients with an amputation refer somatosensory (and [painful]) sensations localized in the "missing" limb \- - \> the representation area of the removed limb is progressively occupied by the representation of neighboring areas \- - - \> peripheral lesions lead to a reorganization of cortical maps: a patient felt his fingers (which he was "missing" due to an amputation) in his face, so the cortical area of the face expanded **The body integrity dysmorphia (BID)** A disorder of body ownership in patients without a brain lesion (apparently): BID is a long-lasting desire for the amputation of physically healthy limbs, associated with reduced fMRI resting-state functional connectivity of somatosensory cortices this sensation is usually present from early childhood and in these patients the refusal is so strong that they ask for (and often obtain) the amputation it was considered a psychopathology, but is has a neuropsychological basis: a possible dysfunction in parietal (SII and posterior parietal) and insular regions that impairs body representations thanks to a study, it was concluded that BID is associated with altered integration of somatosensory and motor signals, involving limb-specific cortical maps and brain regions where the first integration of body-related signals is achieved through convergence **In patients with brain lesions** A brain lesion can affect body representations, usually, because it damages specific brain regions responsible for multisensory integration - Supernumerary limbs - Somatoparaphrenia - Personal neglect - Autotopoagnosia - Finger agnosia - Left-right disorientation \*may be concomitantly present in patients with Gerstmann syndrome (when combined with agraphia and acalculia)  **Supernumerary phantom limb** **Somatoparaphrenia** Delusion regarding a contralesional limb (leg or arm) → usually following a right-hemispheric damage -\> left hemi-body \- - \> it usually occurs after a brain damage comprising the parietal and temporal lobes and the insula -\> crucial network for the multisensory integration of vestibular, proprioceptive and visual information → often in patients also suffering from (personal) neglect → the patient: - Does not recognize the limb as his/her own (asomatognosia = disturbance of the body ownership) - Attributes the limb to someone else (doctor / relative, e.g. the wife, sister...) \- - \> the patient may even perform aggressive act towards the limb (misoplegia)  **To sum up** - Somatoparaphrenia, supernumerary (phantom) limbs, personal neglect as well as autoscopy and out-of --the-body experiences → are associated with a lesion in the right hemisphere - Autotopoagnosia, finger agnosia, and left-right disorientation **Can eating disorders be related to body representation disorders?** **Body representation disorder in anorexia nervosa** A core symptom of Anorexia Nervosa (AN) is a severe alteration of body representations - Researchers hypothesized that AN patients perceive and represent their body larger in only one dimension - ⁠Researchers used tactile tasks across the bodies (horizontally) and along the bodies (vertically) - ⁠AN patients judged horizontal stimuli significantly wider than the same stimuli oriented vertically - ⁠AN patients perceive and represent their body larger but not bigger