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Chapter 13: The Body Senses (Tactile Perception: Perceiving Mechanical…

- - - - produce signals in afferent nerve fibers in response to changes in muscle length
      - also produce signals in response to isometric forces on muscle (weight on hand)
      - generally believed to be most important for proprioception, including information about limb position and limb motion
        
        SAII receptors also provide information about skin stretch, which contributes to perception of limb position/movement
    - - often involves changes in length, convey information about joint angle
      - muscles stretch/contract when angle of joint changes, which changes both muscle force and muscle length
- - - - generally persists until tissue damage is healed
    - - often chronic as opposed to acute
      - typically results in sensation of pain from part of body that hasn’t been damaged; technically a side-effect of damage to pain-signaling system
    - - acute pain
      - nociception:
        the perception of nociceptive pain
        
        body’s early earning system, motivating you to take immediate action to make the pain stop, along with any consequent tissue damage
  - - - may also be involved in transmission of ‘erroneous’ signals associated with neuropathic pain
      - normally have relatively high threshold; respond only to stimuli intense enough to make tissue damage likely
      - sensitization:
        a mechanism that decreases the response threshold of nociceptors so that even very low-level stimulation of an injury site can cause pain
        
        often associated with inflammation
        
        can lead to favoring a damaged part of the body, which can make healing occur more quickly and more completely
  - - - produce rapid response to potentially damaging mechanical stimuli and to excessive heat
    - - produce slower response to a wide range of pain stimuli
- - - - minimum distance at which person’s judgements are 75% correct is two-point threshold for that area of skin
      - threshold is smallest (spatial resolution greatest) on fingertips and lips and larger (lower resolution) on arms, legs, torso
    - - as pattern moved across receptive fields of SAI fibers, generated depiction known as spatial event plot showing how pattern of action potentials produced by an individual SAI fiber matched up with entire pattern of the stimulus
      - respond with single action potential when pass edge of one dot over receptive field and more action potentials when more dots
    - - FAI fibers provide somewhat blurry representations of Braille patterns but much less precise than those produced by SAI fibers
    - - moved letters across fingertips who had to guess which letter it was and same letter moved across fingertips of monkeys
      - construct confusion matrix- shows how often people confused each stimulus letter with every other letter
      - neural response patterns for B and D very similar and both look much more like D, accounts for 27% of responses to B being reported as D and only 2% of responses to D being reported as B
      - spatial event plots for responses of monkey SAI fibers match up very well with patterns of errors shown in the confusion matrix
    - - difference in response rates of monkey SAI fibers to textures with different roughness almost perfectly matched the difference in human judgments about the roughness of the same textures
      - study of neural responses produced by different-sized spheres pressed against the skin with different degrees of force showed that SAI mechanoreceptors provide reliable information about aspects of surface shape such as curvature (shall sphere has greater curvature than large sphere)
      - although individual SAI fibers don’t respond selectively to the orientation of edges pressed into the skin, the responses of a population of many SAI fibers to provide reliable information about orientation
  - - - ideal for perceiving slip and maintaining control over force of one’s grip on object
    - - during load phase when fingers touch object and then drop it with enough force to lift it off the surface without having it slip through fingers
        
        reflects microscopic slippages that indicate more grip force is needed to keep object from slipping further
      - during the unload phase, when fingers release the object after setting it back down
        
        respond at moment object comes back into contact with table, exerts an upward ‘slip’ force on the skin, indicating that the grip can now be released
    - - SAI and SAII active at about same level throughout process, indicating their role in signaling the surface texture of the object and perhaps its corners/edges (SAI) and in signaling the conformation of the hand (SAII)
  - - - provide important information about skin stretch but little about patterns/textures
    - - perception of stretch of skin when open/close hand gives you information about hand conformation
      - closely associated with conformation of hand during grasping and the ability to perceive how the hand is configured is critical for maintaining precision grips and for recognizing object shape
    - - object pulling the skin slightly as it glides across it contributes to the perception of motion, including the direction of motion, across the skin
  - - - many nested capsules surrounding a central capsule where the end of FAII fiber resides
      - layered tissue surrounding the end of the fiber is what gives the fiber a fast-adapting response profile
    - - sensitivity along with fact that vibrations are transmitted across large distances within the dermis, results in the late receptive field that characterizes FAII fibers
    - - can sense vibrations conveyed through pen as write with it, as if awareness has migrated out to tip of pen itself
    - - in experiments, finely textured surface is surreptitiously vibrated while someone is touching it, and surface is perceived as being tougher than it would be otherwise
  - - - C-tactile mechanoreceptors (CT mechanoreceptors):
        mechanoreceptors that are a type of free nerve endings only present in hairy skin; they respond to slow, gentle touch and send signals to the insular cortex
      - respond to slow, gentle touch, sending signals relatively slowly to the insular cortex, an area of the brain that is also involved in the perception of the pleasant-unpleasant dimension of taste and smell
    - - in experiment, rated pleasantness of having skin on forearm gently stroked by mechanical device that maintained a constant pressure but varied the speed and temperature of the stimulating
      - CT mechanoreceptors responded most strongly to neutral temperatures and intermediate speeds, correlated with pleasantest ratings
      - firing rates of myelinated mechanoreceptors increased as the speed of the stimulation increased, regardless of temperature
      - confirm CT mechanoreceptors are tuned to pleasant stimulation and suggest that the temperature and speed of mechanical stimulation are important factors in determining the strength of their response
  - - - skin deformation (tactile perception, or what is commonly meant by ‘touch’)
      - muscle stretch and joint angle for motoring limb position/movement (proprioception)
      - pain, for detecting actual or potential tissue damage (nociception)
      - temperature, of something contacting the skin (thermoreception)
      - object shape, perceived through touch and proprioception together (haptics)
      - balance and acceleration of the body (vestibular senses)
  - - - skin with hairs (hairy skin)
      - skin without hairs (glabrous skin)
      - consists of two main layers- epidermis and dermis
        
        within these layers (mainly in the dermis), in both glabrous and hairy skin, are a variety of sensory receptors, including four main types of mechanoreceptors
        
        mechanoreceptors:
        sensory receptors that transduce mechanical deformations of the skin into neural signals that are sent to the brain
        
        four types differ in the timing of their responses
        
        SAI mechanoreceptors:
        slow-adapting mechanoreceptors with Merkel cell endings; they have relatively small receptive fields and are relatively densely arranged near the surface of the skin
        
        Merkel cells:
        specialized endings of SAI mechanoreceptors, where transduction takes place
        
        SAII mechanoreceptors:
        slow-adapting mechanoreceptors; they have relatively large receptive fields and are relatively sparsely distributed relatively deeply in the skin
        
        SAI and SAII produce a burst of action potentials at onset of skin deformation but then a lower/sustained response until stimulus removed from skin
        
        unknown ending associated with SAII
        
        FAI mechanoreceptors:
        fast-adapting mechanoreceptors with Meissner corpuscle endings; they have relatively small receptive fields and are relatively densely arranged near the surface of the skin
        
        Meissner corpuscles:
        specialized endings of FAI mechanoreceptors, where transduction takes place
        
        FAII mechanoreceptors:
        fast-adapting mechanoreceptors with Pacinian corpuscle endings; they have relatively large receptive fields and are relatively sparsely distributed relatively deeply in the skin
        
        FAI and FAII produce a burst of action potentials only at the onset and offset of skin deformation
        
        sensitivity of FA to abrupt changes makes them well suited for detecting vibrations and motion
        
        Pacinian corpuscles:
        specialized endings of FAII mechanoreceptors, where transduction takes place
        
        also differ in size of receptive fields and typical position/depth within layers of skin
        
        SAI and FAI relatively small receptive fields, densely arranged near surface of the skin, giving them relatively high spatial resolution (can respond to fine spatial detail)
        
        SAII and FAII have larger receptive fields and are more sparsely distributed more deeply in the skin, accounts for lower spatial resolution
        
        ‘adapting’ refers to the fact that sensory fibers tend to reduce their firing rate during prolonged, unchanging stimulation from an initial high level to either a lower, sustained level (SA) or virtually to zero (FA)
        
        considerable overlap with type of stimuli each respond most strongly to
        
        tactile perception in real world almost always involves input from all four types
- - - - primary somatosensory cortex (or S1):
        a subregion of the somatosensory cortex; the first area to receiving somatosensory signals from the ventral posterior nucleus of the thalamus; divided into four side-by-side strips known as areas 3a, 3b, 1, and 2
        
        strips listed from front to back
        
        strips wrap around side of parietal lobe, from the top all the way down to the lateral sulcus
      - secondary somatosensory cortex (or S2):
        a subregion of the somatosensory cortex; receives signals from area S1
    - - found that when stimulated different brain locations along the postcentral gyrus, patients would report being touched at different locations on body
    - - somatotopic map:
        a mapping of the body surface onto the somatosensory cortex, whereby adjacent locations on the cortex receive somatosensory signals from adjacent locations on the body
      - other studies reported somatotopic representation of proprioception signals from joints in response to limb movements
      - somatotopic map in somatosensory cortex somewhat disarranges/distorted
        
        adjacent regions of cortex don’t necessarily receive signals from adjacent parts of the body
        
        relatively more cortical space devoted to receiving signals from body parts where sensory receptors are densely distributed and have small receptive fields while relatively less space devoted to body parts with fewer receptors that have larger receptive fields
        
        size of cortical representation of a body part corresponds with the spatial resolution of touch on that part
  - - - subregions each contain full somatotopic map of body, all receive signals directly from VP nucleus of thalamus
      - each area receives only certain types of signals and contain neurons specialized for representing the different types of information in those signals
        
        neurons in 3a respond to proprioceptive information carried by signals from muscle spindles and Golgi tendon organs located in tissues below the skin, but not to tactile stimulation of the skin
        
        3a and 3b also receive some signals containing information about discriminative aspects of pain perception
        
        neurons in 3b and 1 respond to tactile information carried by signals from mechanoreceptors in the skin
        
        1 neurons also receive somatosensory signals from area 3b
        
        experiments on areas 3b and 1 with moving embossed letters across fingertips found SAI and FAI mechanoreceptors produced trains of action potentials that were sufficient to discriminate between these different spatial patterns and that corresponded well to the behavioral responses documented in the confusion matrix
        
        in 3b, some neurons showed sustained responses and produced very accurate spatial event plots (some virtually indistinguishable from plots of SAI mechanoreceptors)
        
        other neurons in 3b showed transient responses and produced plots that didn’t correspond as well to shapes of letters
        
        in area 1, neither sustained-response nor transient-response neurons produced plots that corresponded well to the letter shapes
        
        neurons in area 2 respond to proprioceptive information carried by signals from muscle spindles and Golgi tendon organs and to tactile information carried by signals from mechanoreceptors in the skin
        
        also receive signals from areas 3a, 3b, and 1
      - many neurons in S1 respond selectively to specific orientations of tactile stimuli- exhibit orientation tuning
      - many S1 neurons also respond according to the direction of motion of stimuli
        
        in experiment in monkey area 3b, small bar moved in direction over receptive field on fingertip and responses depending strongly on direction of motion
      - specialized involvement of area 3a in proprioception was highlighted by study of human brain activity comparing responses to tactile versus proprioceptive stimuli
        
        for tactile stimulation, touched palm/finger with stiff bristly
        
        for proprioceptive stimulation, have them flex hands as if squeezing ball without allowing fingers to touch palms
        
        areas 3b, 1, and 1 all showed increased activity during both types of stimulation, but area 3a showed increased activity during proprioceptive stimulation only
        
        confirms 3a specialized for processing proprioceptive and not tactile information
      - in area S2, which receives signals from all four areas of S1, many neurons respond selectively based on the orientation of an edge pressed against the skin on multiple fingers
        
        orientation tuning in S2 could provide part of the basis for judging the shape of a complex object held in the hand and touching several fingers
        
        supported by studies showing many S2 neurons do not respond to stimulation of multiple fingers, suggesting they integrate information from large regions of the fingers and hand
    - - from area S1, signals travel along two pathways
        
        a dorsal pathway carries information used to guide actions that require tactile and proprioceptive input
        
        goes from S1 to the posterior parietal cortex (including specifically to the anterior and lateral intraparietal areas which are involved in perception of action) and from there into the premotor cortex
        
        ventral pathway carries tactile and proprioceptive information used in perceiving and remembering object shape and identity
        
        begins in S1 areas 3b and 1, with signals carrying information about tactile features such as texture, curvature, and edge orientation, and in 3a, with signals carrying proprioceptive information
        
        then travel to area 2 of S1, where tactile information combined with proprioceptive information, which get to area S2, where representations of 3D shape are created
        
        from S2, sent to prefrontal cortex for perceptual decision making and to hippocampus for storage in long-term memory
    - - thermoreceptive signals carry information about temperature from the skin into the spinal cord and then along the spinothalamic pathway to the VP nucleus of Thalamus
        
        from there, travel to S1 and to other regions of the cortex
      - when cooled hand to different degrees from neutral temperature, found brain activation in the left posterior insular cortex (contralateral to the hand that was cooled) and varied with the temperature applied to the hand
        
        in the right ipsilateral anterior insular cortex, the intensity of activation was also correlated with the stimulus temperature and even more strongly correlated with temperature ratings
        
        conclude contralateral insular cortex is a site where nonpainful temperature sensations are represented in a somatotopic map and used for maintaining homeostasis
        
        ipsilateral insular cortex is site where the relative intensities of temperature sensations are evaluated
    - - signals carrying tactile information from mechanoreceptors flow mainly to areas 3b and 1
      - proprioceptive signals flow to areas 3a and 2
      - from S1, tactile and proprioceptive signals travel along two pathways
        
        a dorsal pathway that carries tactile information used to support touch-based action
        
        a ventral pathway carries both tactile and proprioceptive information used to support somatosensory object recognition in the formation of new somatosensory memories
      - signals carrying discriminative information about the location and intensity of painful stimuli also flow to areas 3a and 3b and then to area S2
      - signals carrying information about the unpleasantness of pain don’t flow to either S1 or S2 but to several other regions of the brain
    - - pain experiences on discriminative and affective dimension
        
        discriminative dimension consists of perceptions that enable the person to locate the pain and to classify it in terms of its intensity and type
        
        affective dimension consists of unpleasant emotional responses to pain
      - neural signals related to both dimensions follow the spinothalamic pathways from the spinal cord to the VP nucleus of the thalamus
        
        from thalamus, discriminative signals flow to areas 3a and 3b within S1 and then to area S2, as well as to posterior insular sorted
        
        affective dimension flow to anterior cingulate cortex (ACC) to the amygdala and other emotion-mediating structures, and to the anterior insular cortex
      - thought that information in S1 and S2 somatotopic pathways provide information about the discriminative dimension but not the affective dimension
      - in experiment, participants who perceived differences in unpleasantness showed changes in activity in ACC while those who perceiving differences in pain intensity showed changes in activity in S1
      - distinction between two dimensions of pain also appears to relate to feelings of empathy
        
        experiencing pain or watching loved one experience it exhibited brain activity in the ACC and anterior insular cortex, but only group experiencing pain exhibited activity in posterior insular cortex as well as in somatosensory cortex
  - - - deployment of attention away from painful stimulus reduces perceived intensity of pain and level of activity in parts of brain associated with discriminative dimension of pain
    - - placebos can lessen both experience of pain and associated pain-related brain activity
      - placebo effect:
        pain reduction due to a fake treatment, with no actual therapeutic value; occurs because the person believes the treatment is real and expects to benefit from it
    - - reduces sensory to pain
    - - endogenous opioids belong to class of substances called opiates, which includes analgesic drugs like morphine/heroin
        
        released by the pituitary gland, the hypothalamus, and descending nerve fibers within spinal cord in response to painful experiences
      - endorphins:
        endogenous opioids that have an inhibitory effect on pain-related neural signals in many areas of the central nervous system, reducing the perceived intensity of pain
      - endogenous opioids:
        compounds that belong to a class of substances called opiates; released by the body in response to painful or stressful experiences
  - - - extends into adulthood
    - - people who have had their arm amputated sometimes report that stimulation of their face results in a feeling that both their face and a specific location on the phantom arm are being stimulated
      - region of area S1 devoted to face is very near the extensive region devoted to the arm
      - brain’s circuitry rearranged in such a way that signal from the fact activate not only the region of S1 devoted to the face and also the region formally devoted to the now-missing arm
    - - part devoted to particular fingertip in somatotopic map had expanded after training in activity involving finger tip
    - - for many, illusion occurs after less than a minute, showing how vision can override proprioception in perceiving the location of a body part and how quickly/flexibly the brain can adapt to conflicting information from two senses
  - - - all nerve bundles from each region of skin and each region of muscle tissue enter the spinal cord via one of the spinal nerves
      - cell bodies of all these bipolar neurons are clustered together into a single dorsal root ganglion, refereed to as dorsal root ganglion cells
      - each dorsal root ganglion adjacent to single vertebra, each vertebra has one ganglion on left side and one on right
      - dorsal root (bundled-together axons of dorsal root ganglion cells) enters dorsal part of spinal cord
        
        ventral root consists of axons of motor neurons sending signals from brain to muscles
    - - dorsal column-medial lemniscal pathway (DCML pathway):
        pathway for signals involved in tactile perception and proprioception; travels up the spinal cord on the ipsilateral side, crosses to the contralateral side in the medulla, and then goes through the ventral posterior nucleus of the thalamus and on to the somatosensory cortex
        
        for touch and proprioception
        
        travel up dorsal column of spinal cord on same side as originating sensory neurons, make synapse at medulla in brain stem
        
        ascend via the medial lemniscus
      - spinothalamic pathway:
        pathway for signals involved in nociception and thermoreception; crosses over to the contralateral side within the spinal cord and then goes through the ventral posterior nucleus of the thalamus and on to the cortex
        
        for pain and temperature
        
        ascend via the spinothalamic tract
      - neurons in both pathways cross to the contralateral side after the first synapse, but spinothalamic pathways has its first synapse in the spinal cord while DCML pathway has its first synapse in the medulla
      - both pathways travel through the ventral posterior nucleus of the thalamus and on to the somatosensory cortex in the anterior parietal lobe
        
        ventral posterior nucleus (VP nucleus):
        a nucleus of the thalamus; part of both the DCML pathway and the spinothalamic pathway
        
        somatosensory cortex:
        a region of the cerebral cortex in the anterior parietal lobe; receives signals carrying sensory information via the ventral posterior nucleus of the thalamus
        
        has contralateral organization in the somatosensory cortex
        
        flow through structure in the thalamus (VP nucleus) before traveling to the cortex
- - - - semicircular canals:
        part of the vestibular system; three mutually perpendicular hollow curved tubes in the skull filled with endolymph; responsible for signaling head rotation
        
        at base of canal is small chamber called ampulla, also filled with endolymph and containing specialized structure called a crista that contain hair cells, with stereocilia enclosed within a gelatinous cap called the cupula
      - otolith organs:
        part of the vestibular system; consists of the utricle and the saccule; responsible for signaling when the head is undergoing linear acceleration or being held in a tilted position
        
        utricle and saccule each contain specialized structure with hair cells called macula, which each contain hard cells that are oriented in different directions and are embedded in a gelatinous layer topped by tiny crystals called otoconia
  - - - PIVC thought to provide representation of the position and orientation of the head
      - brain combines signals from the vestibular system with signals from other parts of the body to produce a unified perception of the whole-body position, balance, and movement
- - - - lateral motion- texture; pressure- hardness; static contact- temperature; unsupported holding- weight; enclosure- volume, global shape; contour following- volume, exact shape; function test- specific function; part motion test- part motion