Lecture 11: Somatosensation and Vestibular Sense

Somatosensation & vestibular sense

• are active, and they are for action.

• They are multi-sensory.

• Work to provide an integrated percept of the body and its position & movements in space.

Touch:

Mechanical displacements and other physical impacts on the skin.

Proprioception

muscle sense.

Somatosensation:

A collective term for sensory signals; touch, proprioception, pain and temperature, from the body: 4 (or more) different senses!

Hold a pen. What do you feel?

• => combining info from hand and arm

• => subtracting (less interesting) sensation of the pen

Cutaneous rabbit

• 2-3 separate locations stimulated multiple times in quick succession feels like a rabbit, hopping across the skin in a continuous manner.

• Bayesian priors

  • In the rabbit illusion, the brain assumes that tactile stimuli usually move smoothly and continuously across the skin.

  • So when presented with fast stimuli at separate locations, it fills in the blanks to match this prior belief — hence perceiving movement where there is none.

• Illusion reflected in somatosensory cortex that activates even though it shouldn’t (top down affect)

Cutaneous rabbit Investigated with fMRI

• Middle location equally activated as stimulated location

• This graph shows % of global brain signal in three different skin locations:

  • P1 = First stimulated location

  • P2 = Middle location (where no touch occurred)

  • P3 = Final stimulated location

• Two conditions are shown:

  • Illusory-Rabbit minus Control (white bars): Real taps only on P1 and P3 (wrist and elbow), but illusion of tapping at P2

  • Veridical-Rabbit minus Control (gray bars): Real taps at all three locations (P1, P2, P3)

• Key result:

  • The P2 activation in the illusory condition (white bar) is just as strong as in the real stimulation condition (gray bar).

  • This means the brain "believes" it was touched at P2 — even though no physical stimulus occurred there.

Somatosensory receptors have three attributes:

1. Type of stimulation the receptor responds to

   • Mechanical displacement for touch perception

   • Temperature

2. Size of the receptive field

   • Big or small

3. Rate of adaptation

   • Measured in Hz

   • Slow adaptation rate (low numbers) and high adaptation rate (high numbers)

Touch receptors:

• Embedded in outer layer (epidermis) and underlying layer (dermis)

• Multiple types of touch receptors

  • Mechanoreceptors

  • Meissner corpuscles

  • Merkel cell neurite complexes

  • Ruffini endings

  • Pacinian corpuscles

Mechanoreceptors

respond to mechanical stimulation or pressure

Messier corpuscles

Sit on top of the dermis close to surface

Merkel cell neurite complexes

Sit on top of the dermis close to surface

Ruffini endings

Deeper in dermis

Pacinian corpuscles

• Deeper in dermis

• Orange part is a nerve ending but embedded in what looks like multiple areas of a onion

• Mechanical impact → nerve endings will be bent and mechanical force will be absorbed by the onion structure

• Very quickly adapting

Which tactile receptors are deeper in dermis

• Pacinian corpuscles

• Ruffini endings

Which tactile receptors Sit on top of the dermis close to surface

• Merkel cell neurite complexes

• Messier corpuscles

Different tactile receptor cells have different properties based on

How deep they are embedded in skin but also on morphology

Response characteristics of 4 mechanoreceptor populations

• Memorize chart

• l for small recpetive field and ll for large receptive fields

• FA: fast adapting and SA: slow adapting

FA I:

• Meissner

• Good for

  • edge contours (tracing edges),

  • Braille (moving fingers over elevated surfaces

  • low- freq vibration (3-40 Hz)

SA I:

• Merkel

• Good for

  • local spatial discontinuities,

  • static texture

  • sustained pressure (when you hold on to something and keep touching, at some point the meissner cells stop responding but the merkel receptors continue responding)

  • spatial deformations (<5 Hz)

FA II:

• Pacinian

• mechanical transients;

• high freq vibration (50-700 Hz)

SA II:

• Ruffini

• Good for

  • lateral stretch (e.g., grasping)

  • hand shape

  • static force

  • low sensitivity to vibration

Other types of mechanoreceptors within muscles, tendons, and joints:

• Proprioceptive receptors

• Spindle

• Receptors in tendons

• Receptors in joints

Proprioceptive receptors:

• Necessary to sense

• where limbs are / posture

• what kinds of movements are made

Spindles:

Convey the rate at which the muscle fibers are changing in length

2 muscles fibers in spindles

• Extrafusal muscle fibers

• Intrafusal muscle fibers

4 neurons in spindle

• alpha motor fibers

• Delta motor fibers

• Type la sensory fiber

• Type ll sensory fiber

Extrafusal muscle fibers

• Normal muscle fibers

• When you flex muscles those are the muscle fibers being used

Intrafusal muscle fibers

• Inside in the spindles, that are so thin, they can not contribute to any force

• They are meant for fine tuning for Proprioceptor cells

  • fibers are connected to sensory neurons that detect changes in muscle length and rate of stretch.

  • They fine-tune proprioceptive feedback by adjusting the sensitivity of the muscle spindle, helping the nervous system precisely monitor muscle status, even during voluntary movement.

Efferent neurons in spindle

• Control muscles

• alpha motor fibers

• Delta motor fibers

Afferent neurons in spindle

• Sensory cells attached to different parts of intrafusal muscle fibers and they sense how the fibers are being stretched.

• Used to estimate the length and change of length of the muscles

• Type la sensory fiber

• Type ll sensory fiber

alpha motor fibers

• Control muscles (efferent) → contraction

• Control extrafusal muscle fibers

Delta motor fibers

• Control muscles (efferent) → contraction

• Control intrafusal muscle fibers

Receptors in tendons

• Golgi tendon organs

  • provide signals about tension in muscles attached to tendons

Example of Receptors in tendons

Play a role in arm wrestling. The loser usually losses very suddenly. When the loser’s hand suddenly hits the table, thats a reflex. That’s the golgi tendon organs inhibiting contraction of the muscle because their is too much tension

Receptors in joints

react when joint is bent to an extreme angle

Proprioceptive illusion I

• E.g., vibration of the biceps (arm perceived as being extended more than it actually is

• When you vibrate the tendon of the biceps muscle, you stimulate the muscle spindles unnaturally.

• This mimics the signal the brain would normally get if the muscle were being stretched (i.e., the arm is extending).

• But in reality, the arm isn’t moving — the illusion is that the arm is extending or moving away from the body, even though it’s still.

• The brain interprets the artificial spindle activation as real muscle lengthening, causing a false sense of movement.

Proprioceptive illusion II Somatosensory Physiology Pinocchio illusion

• actually a multisensory illusion

• A vibration is applied to the biceps tendon while a person is touching their nose with their finger, eyes closed.

• The vibration stimulates muscle spindle receptors, just like in the first proprioceptive illusion.

• This creates the illusion that the arm is extending — even though it’s not.

• However, since the fingertip is still in contact with the nose, the brain tries to make sense of these conflicting signals: If the arm feels like it’s extending, but the finger is still touching the nose... then maybe the nose is growing!

• The result: The person feels like their nose is getting longer — hence the name "Pinocchio Illusion."

Why Is Proprioceptive illusion II Somatosensory Physiology Pinocchio illusion Important

• This illusion reveals how the brain resolves sensory conflicts:

  • Proprioceptive input says the arm is moving,

  • Tactile input says the finger is stationary on the nose,

  • The brain blends the two, producing a bizarre but coherent body perception (a growing nose).

• It’s called a multisensory illusion because it involves integration of both touch and proprioception — not just one sensory system.

Example for Pinocchio illusion without proprioceptive stimulation

• This version mimics the Pinocchio illusion but doesn't involve vibrating the muscle or stimulating proprioceptors.

• Setup:

  • The person closes their eyes.

  • An experimenter pulls gently on the person’s nose while simultaneously stroking their hand (e.g., finger).

  • The person feels the touch on their hand and the tug on the nose, but their eyes are closed — so the brain lacks visual input.

• What Happens:

  • The brain tries to integrate the tactile sensations.

  • Because the touch on the hand is felt as movement, and the nose is being tugged, the brain constructs a false interpretation:

  • "My hand must be staying still… so my nose must be moving or growing!"

  • This creates a mild illusion similar to the Pinocchio effect, where the nose feels longer than it really is — but no vibration or proprioceptive muscle stimulation is involved.

Proprioceptive illusion III

Press the backs of your hands against the inside of a door frame for 30 seconds—as if you’re trying to widen the frame—and then let your arms down; you’ll feel something odd. Your arms will float up from your sides, as if lifted by an external force. Scientists call this Kohnstamm phenomenon, but you may know it as the floating arm trick. Now, researchers have studied what happens in a person’s brain and nerve cells when they repress this involuntary movement, holding their arms tightly by their sides instead of letting them float up

Proprioceptive illusion III Happens because

• Proprioceptive Receptors are getting tired

• Arm floats up because brain thinks there is an imbalance of how you feel your body posture

• Imbalance of the sensors and brain tries to reestablish equilibrium → arm floats up

Thermoreceptors:

• Sensory receptors that signal information about changes in skin temperature

• Warmth fibers, cold fibers

  • No receptors for something in between. Gap is at body temp so about 37ºC.

  • We don’t need to sense that everything is ok, we need receptors to tell us if something is too cold (initiate shiver) or if something is too hot (initiate sweating)

• Body is consistently regulating internal temperature

• Thermoreceptors kick into gear when you make contact with object warmer or colder than your skin

• Capsaicin

  • Capsaicin is the active compound found in chili peppers that gives them their spicy or hot sensation or pain

Capsaicin

the active compound found in chili peppers that gives them their spicy or hot sensation or pain

At what temp do cold receptors react

Peak at 25ºC

At what temp do warm receptors react

Respond to higher temps at 45ºC

Why no receptors in between warm and cold thermoreceptors

• No receptors for something in between. Gap is at body temp so about 37ºC.

• We don’t need to sense that everything is ok, we need receptors to tell us if something is too cold (initiate shiver) or if something is too hot (initiate sweating)

• We don’t need receptors at 37ºC → everything is ok

Nociceptors:

• Sensory receptors that transmit information about noxious stimulation that causes damage or potential damage to the skin

• Two types of fibers

  • A-delta fibers

  • C fibers

2 phases of pain

1. Sharp pain

2. Throbbing pain

A-delta fibers:

• Wider in diameter than C fibers

• strong pressure, heat; myelinated ➔ fast

C fibers:

pressure, heat, cold, chemicals; unmyelinated ➔ slow

Benefit of pain perception:

• Sensing dangerous objects

• Congenital analgesia

• Leprosy (Hansen’s disease)

Congenital analgesia

• is a rare genetic condition in which a person is born unable to feel physical pain.

• Usually die at early age because they can’t protect themselves

Leprosy (Hansen’s disease)

People with leprosy lose the ability to feel pain due to damage to nociceptive fibers and sensory neurons, similar to congenital analgesia in effect, though acquired rather than genetic

Somatosensory Pathways

• Pathways: up to 2 meters long

  • Information must pass through spinal cord

  • Axons of various tactile receptors combine into single nerve trunks

  • Several nerve trunks from different areas of body

Once in spinal cord (via dorsal root ganglion): 2 major pathways

• Spinothalamic (slower)

• Dorsal column-medial lemniscal (faster)

Spinothalamic pathway

• synapses multiple times within spinal cord: slow, nociception, thermal information

1. Nociceptor in the periphery (e.g., finger)

   • Specialized free nerve endings (pain receptors) detect noxious stimuli (mechanical, thermal, or chemical).

2. Cell bodies of these sensory neurons

   • Located in the dorsal root ganglion (DRG), which lies just outside the spinal cord.

3. Central axons of DRG neurons enter the spinal cord

   • They synapse in the dorsal horn of the spinal cord

4. Second-order neurons

   • Cross to the contralateral side of the spinal cord and ascend via the lateral spinothalamic tract, carrying pain and temperature information up through the brainstem.

5. Third-order neurons

   • These second-order neurons synapse in the ventral posterior nucleus of the thalamus (VP nucleus)

• slow pathway because of so many synapses

Why is pain slow? Shouldn’t it be fast?

Pain reflex arc

Pain reflex arc

is an automatic, spinal reflex that helps protect the body from harm by triggering an immediate movement away from a painful stimulus—before the pain is even consciously felt.

Dorsal-column-medial-lemniscal (DCML) pathway:

• Fast

• Synapse in medulla, near base of brain, then ventral posterior nucleus of thalamus, then somatosensory area 1 (S1), somatosensory area 2 (S2)

1. Fibers for pressure, vibration, joint, position sense

   • These are first-order sensory neurons with their cell bodies in the dorsal root ganglion (not shown).

2. Gracile nucleus (medulla)

   • Receives signals from the lower body.

   • First synapse of the first-order neurons.

3. Cuneate nucleus (medulla)

   • Receives signals from the upper body.

   • Also a first synapse site for first-order neurons.

4. Medial lemniscus

   • After synapsing in the gracile or cuneate nuclei, second-order neurons decussate (cross to the opposite side of the medulla) and ascend through the medial lemniscus.

5. Ventral posterior nucleus of the thalamus (not labeled directly but shown as the relay site before cortex)

   • Third-order neurons start here.

   • This is where second-order neurons from the medial lemniscus synapse.

6. Postcentral gyrus of cortex

   • The destination in the primary somatosensory cortex (S1) for conscious perception of touch, vibration, and position.

Patient with spinal cord injury might have reduced perception of touch vs. for different body sides – why?

• This happens because different sensory pathways travel on opposite sides of the spinal cord.

• Touch, vibration, and proprioception are carried by the dorsal column–medial lemniscal (DCML) pathway, which travels up the same side (ipsilateral) of the spinal cord as the body part sensing the stimulus. So, if the left side of the spinal cord is injured, touch and position sense will be lost on the left side of the body below the injury.

• Pain and temperature are carried by the spinothalamic tract, which crosses to the opposite side (contralateral)within 1–2 spinal levels after entering. So, if the left side of the spinal cord is injured, pain and temperature sensation will be lost on the right side of the body below the injury.

Cranial pathway:

• Somatic sensation for front of the face

• Trigeminal nerve (V): face, teeth etc.

Touch sensations are represented somatotopically

• Analogous to retinotopy found in vision

• Adjacent areas on skin: Connected to adjacent areas in brain, called homunculus

Somatosensory cortex located

• in the parietal lobe of the brain, specifically behind the central sulcus.

• It is primarily found on the postcentral gyrus.

somatosensory cortex responsible for

This area is responsible for processing sensory information from the body's senses, including touch, temperature, pain, and body position

The Sensory Homunculus

• Distorted representation reflects receptor density.

• Brain contains several sensory maps of body, several homunculi, i.e., different subareas of S1, secondary areas as well

• a visual representation of how the human body is mapped onto the primary somatosensory cortex, the part of the brain that processes touch, temperature, and pain.

• It's often depicted as a distorted human figure, with larger areas representing body parts that have a greater density of sensory receptors. This means parts like the lips, hands, and feet are depicted as larger than the torso or legs, reflecting their greater sensitivity

• Top Half: Traditional Model

  • Left Brain Image: Shows the somatosensory strip (orange) in the parietal lobe, responsible for processing sensory input from the body.

  • The exaggerated body parts around the cortex represent how much brain area is devoted to each body region (e.g., large lips and hands = more sensitivity).

  • Right Brain Image (crossed out): Shows the motor homunculus (in the precentral gyrus), new research found this is wrong

• Bottom Left: Question Mark Overlay

  • This reuses the traditional somatosensory homunculus diagram but adds a question mark, possibly asking how accurate or complete this older model is.

  • It implies there's uncertainty or a need to rethink the traditional sensory mapping.

• Bottom Right: Gordon et al., 2023 – Updated Model

  • This is from a recent study (Gordon et al., 2023) proposing a revised sensory homunculus.

  • It still shows body parts mapped to brain regions but adds:

    • Action-based groupings (e.g., “Actinobody” segments for complex movements),

    • Updated spatial organization of somatosensory input,

    • Emphasis on functional integration, not just anatomical mapping.

Brain contains several sensory maps of body, different subareas of S1, secondary areas as welll

• Nowadays Brodmann Area (BA) 3 considered to be S1.

• BA3a: proprioception, BA3b: touch

• Dense connections from BA3b to BA1 (texture) and BA2 (size, shape, proprioception)

Phantom limb:

• Perceived sensation from a physically amputated limb of the body

• Phantom pain

  • If someone looses a hand, and you touch their shoulder, they’ll feel like their hand is still being touched. That suggests that those paths in S1 that used to respond to hand now respond to shoulder. But they still have the memory of being responsive to the hand

• Parts of brain “listening” to missing limbs not fully “aware” of altered connections, so they attribute activity in these areas to stimulation from missing limb

• Mirror “re-instates” complete body

• Sometimes turn into phantom pain. If you have a cramp in foot you can stretch and get rid of it. But a person who’s foot is gone can’t do that

  • Mirror therapy for phantom pain

Why does mirror help phantom pain

• it tricks the brain into thinking the missing limb is still there and moving normally.

• A mirror is placed vertically in front of the person so that the reflection of the intact limb appears where the missing limb would be.

• When the person moves their intact limb, they see the reflection and it looks like the missing limb is moving.

• This visual illusion can help relieve the mismatch between what the brain expects (a working limb) and what it senses (a missing or painful one).

Pain:

• Pain sensations triggered by nociceptors

• Nociceptive signals arrive at the substantia gelatinosa of the dorsal horn

• Responses to noxious stimuli can be moderated by cognitive, emotional and social factors

• Bottom-up sensation + top-down modulation

• Example: Injury during sports

Analgesia:

• Decreased pain sensation while being conscious

• Sports injury: endogenous opiates released in body to block transmission of pain sensations

• This occurs from an evolutionary point of view because when we are physically active (running away from lion) we don’t worry about minor injury. Body cares about getting somewhere safe and ignores little pains

What do you do when you bump your head?

• Rub head, apply tiny bit of pressure → touch sensation

• We do this because it helps, sort of undos the pain → gate control theory

• Children do this automatically, don’t need to be taught

Gate control theory:

• Describes a pain transmitting system that incorporates modulating signals from the brain

• Feedback circuit located in substantia gelatinosa of dorsal horn of spinal cord

• Gate neurons block pain transmission

• can be activated by noxious stimulation applied to another site distant from source of pain

The gate in gate control theory is located in

spinal cord, specifically in the substantia gelatinosa (SG) of the dorsal horn.

How the Gate Works

• Inputs:

1. Large-diameter A-β fibers

   • Carry non-painful touch or pressure information (e.g. rubbing your skin).

   • Excite inhibitory interneurons (green plus signs) in the SG → this closes the gate.

2. Small-diameter A-δ and C fibers

   • Carry pain signals.

   • Inhibit the inhibitory interneurons → this opens the gate, allowing pain signals to reach the brain.

• Central Control (e.g. PAG — Periaqueductal Grey):

• The brain can send signals back down to influence the gate.

• Can enhance inhibition (via descending pathways) → close the gate and reduce pain.

When the Gate is Closed in Gate control theory

• Less Pain

• You bump your shin and start rubbing it.

1. Rubbing your skin activates A-β fibers — these are large-diameter fibers that carry non-painful touch sensations.

2. These A-β fibers activate inhibitory interneurons in the Substantia Gelatinosa (SG).

3. These inhibitory interneurons block the T cell (Transmission cell).

4. If the T cell is blocked, the pain signal cannot travel up to the brain.

5. As a result, you feel less pain.

• This is why rubbing a painful spot makes it feel better — it’s literally “closing the gate” in your spinal cord!

When the Gate is Open in Gate control theory

• More Pain

• Example: You step on a nail (ouch!).

1. The injury activates A-δ and C fibers — these are small-diameter pain fibers.

2. These fibers turn off the inhibitory interneurons in the SG.

3. Without those interneurons, the T cell becomes active.

4. The T cell sends the pain signal to the brain.

5. You now feel sharp or burning pain — the gate is open.

• The more active your pain fibers are, the wider the gate opens, and the more pain you feel.

Shining light on pain Hu et al., 2022

• Bright light can have antinociceptive effects

• Visual pathway via portion of the LGN (ventral lateral geniculate nucleus and intergeniculate leaflet) ➔ PAG ➔ regions involved in pain regulation (locus Coeruleus, LC; rostral ventromedial medulla, RVM)

• Bright light can reduce pain (antinociception):

  • Exposure to bright light can dampen or block pain signals — this is called an antinociceptive effect (anti = against, nociception = pain sensing).

• How light reduces pain — the brain pathway:

1. Light is detected by your eyes, and the signal travels along your visual pathways.

2. Instead of going to the parts of the brain that process images, some light signals are sent to a special part of the thalamus called the ventral lateral geniculate nucleus (vLGN) and the intergeniculate leaflet (IGL).

3. From there, the signal is passed on to the periaqueductal gray (PAG) — an area deep in the brain that’s known for controlling pain.

4. The PAG then sends signals to other key pain control centers:

   • The locus coeruleus (LC) — involved in alertness and stress responses.

   • The rostral ventromedial medulla (RVM) — which can inhibit pain signals in the spinal cord.

Cognitive aspects of pain

• Pain: Generally subjective experience, two components: Sensation of painful source, emotion that accompanies it

• Areas S1 and S2: Responsible for sensory aspects of pain

• Areas of brain that correspond to more cognitive aspects of painful experiences

experiment that used hypnosis to get people to put their hand in “different temperatures of water “

• Hypnosis: Experiment with lukewarm and hot water: water was actually lukewarm, but hypnosis made them think it was hot water → painful response

• Anterior cingulate cortex responded differentially to two hypnotic suggestions, by increasing or decreasing its activity

• Emotional response associated with prefrontal cortex

How sensitive are we to mechanical pressure?

• Von Frey hair: 19th century: carefully calibrated stimuli: Horse & human hairs

• Today: nylon monofilaments: no matter how much force you put on it pressure stays the same (make sure the pressure is consistent)

• obviously lips have less threshold than calf

How finely can we resolve temporal details?

• Look at absolute threshold not just by force applied, but also depends on frequency of vibration

• Higher rates of vibration, better sensitivity, lower thresholds.

• Vibration sensitivity on finger tip: Max. at ~300 Hz (~ FA II)

• Only low-freq. notes can be sensed on skin (E4 ~330Hz)

How finely can we resolve spatial details?

Two point touch threshold

Two-point touch threshold:

• The minimum distance at which two stimuli (e.g., two simultaneous touches) are just perceptible as separate

• Ex.

  • If someone lightly touches your fingertip with two toothpicks that are 1 mm apart, you might feel two distinct points.

  • But if they do the same on your back, you might not feel them as separate unless they’re 3–5 cm apart.

Correspondence between pattern of two-point thresholds across body & distortions of sensory homunculus

• Fingers have lower thresholds than thigh

  • You can detect two separate touches at a much smaller distance on your fingers than on your thigh.

Two contact points will elicit different responses when

• Sufficient concentration of receptors at the skin

  • More receptors = more detail.

  • Example: The lips and fingertips have a high concentration of receptors, so they can detect very fine touch differences.

• Small receptive fields

  • Small receptive fields = more precise localization of touch.

  • Areas like the lips and fingers have smaller receptive fields, so even small differences in touch location are detected.

• No convergence of neural processing

  • Convergence means multiple sensory neurons send their signals to the same neuron in the brain or spinal cord.

  • When this happens, the brain can’t tell exactly where the signal came from — precision is lost.

  • No convergence = better spatial resolution — each receptor has a direct line to the brain.

  • So in sensitive areas, signals stay separate and are processed individually → you feel two distinct touches.

Methodological drawbacks of 2-point threshold

• Subjective: Depends on the participant's report; prone to bias or inconsistency.

• Response variability: Results can vary depending on attention, fatigue, or understanding of the task.

• Not purely physiological: Influenced by psychological factors, not just receptor density.

• Poor spatial resolution measure: Doesn’t precisely reflect the exact size or spacing of receptive fields.

• Inconsistent pressure: Manual application can lead to varying force between trials.

• Low reliability: Hard to replicate results exactly across sessions or between examiners.

Haptic perception:

Knowledge of the world that is derived from sensory receptors in skin, muscles, tendons, and joints, usually involving active exploration

Exploratory procedure:

• Stereotyped hand movement pattern used to contact objects in order to perceive their properties

• Optimal for obtaining precise details about one or two specific properties, (e.g., to find out how rough object is: Lateral motion)

• Not genetic, but not taught either, we just figure it out

• Consist of

  • Lateral motion

  • Pressure

  • Static contact

  • Unsupported holding

  • Enclosure

  • Contour following

Lateral motion for

• Texture

• Move fingers naturally over surface

pressure for

• Hardness

• Poke something to test rigidness

Static contact for

• Temperature

• Tough surface briefly

Unsupported holding for

• Weight

• Carry something to see how heavy it is while moving up and down

Enclosure for

• Global shape, volume

• Ex. Wrap hand around cup to get a feel for its overall size

contour following

• For global shape, exact shape

• Trace object to get feel for it

The What system of touch:

• Vision: geometric object properties

• Line drawings: Easy to see; difficult to perceive haptically

• Touch: perceiving material properties (cold, smooth, …)

Perception is for Action – Example: Grasping an Object

• Properly lift = pull on your skin

• When FA ll fire, there would be no firing if you underestimated the weight. You would start to squeeze more

• Grasping involves using sensory signals to guide motor actions.

• The tactile system helps detect when and how we interact with objects.

• Tactile afferents (FA-I, SA-I, FA-II, SA-II) respond to different aspects of contact and movement:

  • FA-I and SA-I: Contact timing, friction, and pressure.

  • FA-II: Transient events like lift-off and contact loss; only briefly fires.

  • SA-II: Skin stretch; encodes grip and load forces

Stages of a Grasping Task (From Figure):

1. Reach: Hand moves toward object (no contact yet).

2. Load Phase:

   • Digits contact object (see the tactile signal spike from FA-I/SA-I).

   • Grip force increases to prevent slipping.

   • Load force begins rising (preparing to lift the object).

3. Lift Phase:

   • If the load force matches the object’s weight → lift occurs.

   • FA-II signals a transient spike as contact breaks with the surface.

   • Mismatch (object too heavy/light) triggers corrective actions.

4. Hold Phase:

   • Grip and load force plateau.

   • SA-II may still fire due to skin stretch.

5. Replace Phase:

   • Object approaches the target surface.

   • Forces begin to reduce.

6. Unload Phase:

   • Digits release the object.

   • FA afferents fire again during contact loss.

Key Concepts of graph

• Grip force is modulated based on predicted weight.

  • If prediction is wrong (e.g., object heavier than expected), the object slips, triggering tactile feedback to correct grip.

• People with anesthetized skin (no tactile input) struggle with grasping despite intact proprioception—shows how critical tactile feedback is.

• Predicted tactile events are compared to actual tactile feedback.

  • Mismatch (e.g., slipping) activates corrections.

• Tactile signals encode:

  • Timing and location of contact,

  • Direction and strength of forces,

  • Friction, object weight, and contact breaking.