PSYB51 Perception and Cognition:
Lecture 11 Somatosensation:
Touch: Mechanical displacements and other physical impacts on the skin.
Proprioception: Muscle sense
Somatosensation: A collective term for sensory signals from the body. More than one sense.
Touch is part of it. It can be on other places not just skin, but we will mainly focus on the skin.
We will focus on tactile sensitivity.
Haptics: Actions play a special role. Haptic perception is something very new – an important
motor perception to touch.
Touch perception has something to do with actively moving our hands for instance. We have
very detailed sensation in fingers. It is involving motor action in order to have a more acute
perception of touch. This concept is haptics.
- Pathways to the brain
- How we perceive pain
When you are holding a pen:
Use pen to poke on something with eyes closed and guess what it is. When we do that we can
tell some details about it. But if we actually touch it even with our eyes closed, we can tell. But
even when we were holding the pen, we were only holding the pen. But our brain was able to
put facts together, and give us a very good idea of the texture, consistency and shape. Even
indirectly exploring things gives us a lot of information from haptic perception.
Close eyes, and tap a pen in rapid succession along arm upwards. Even if you poke at certain
points going up, it feels like you tapped all the way up. This is called cutaneous rabbit illusion.
The study shows the use of fMRI to show that this is causing effects in the somatosensory
cortex (S1). It is a somatotopic map (can be mapped out to contain parts specialized in different
functions). Illusion is immediately evident in levels like S1 which is pretty early for
somatosensation. This tells us that somatosensation is not that great.
The reason is that the spatial resolution is not very great for the arm. What happens is that the
brain concludes that is unlikely that tapping happens only in one place, and tends to happen
along slowly all the way up the arm. This is an example for Bayesian statistics again, we assume
movement is slow, and thus bias the result. Touch receptors: Embedded in outer layer (epidermis) and underlying layer (dermis).
- Multiple types of touch receptors
- Touch receptors (& other somatosensory receptors have 3 attributes:
o Type of stimulation
o Size of the receptive field
o Rate of adaptation
Tactile receptors (four): Mechanoreceptors respond to mechanical stimulation or pressure.
1. Meissner corpuscles
2. Merkel cell neurite complexes
3. Pacinian corpuscles
4. Ruffini endings
The first two are higher up ones. The last two are deeper ones. Know the diagram. These four
specialize in different things depending on receptive field size and adaptation rates.
FA I (fast adaptive 1), FA II, SA I (slow adaptive I) and SA II are just different more efficient ways
to name the previous receptors.
Brail reading you need SA I.
SA II help with sensing how much our skin id folded (e.g. when we form a fist with our hand)
There are other types of mechanoreceptors within muscles, tendons and joints:
- Proprioceptive Receptors: Necessary to sense
o where limbs are/posture
o what kinds of movements are made
- They are 3 types:
1. Spindles: convey the rate at which the muscle fibers are changing in length (i.e. they
determine stretch of muscles.) the spindles themselves do not contribute much to
contraction. Type 1 and type 2 An fibers are wrapped around the spindles. As these
spindles get stretched, that’s what it drives the proprioceptive receptor. This keeps
the system in tune. As muscle fibers contract and become shorter,
2. Golgi receptors in tendons provide signals about tension in muscles attached to
tendons. (Jack-knives reflex: meant as a protective mechanism to not put too much
force on your tendon). They usually run parallel to the muscles.
3. Receptors in joints react when joint is bent to an extreme angle.
Proprioceptive Illusion: 1:
Device that creates mechanical vibration. As we impose vibration of a certain frequency, it
stimulates the muscle spindles by creating the illusion that the muscle is longer than it actually
is. When one arm has the vibrator attached, and you tell them to match the angle with the
other arm, you keep your hand longer. This creates an illusion for posture.
Proprioceptive Illusion II:
If you do this while you are holding your nose – brain knows we are touching the nose. The
conclusion that happens is that the brain knows the body is still a whole, but touching nose is contradictory to the illusion. What people perceive is that their nose/hand is growing. This is
called the Pinocchio illusion – because you feel as if your nose is growing.
Keep in mind it depends whether it is on biceps or triceps. So look at the picture carefully.
So you try to separate a girl’s arm form the top. Then when you relax and close your eyes it
feels like your arm is growing longer. This is an aftereffect for proprioreception. Sometimes
your arms may even start going up.
- Sensory receptors that signal information about changes in skin temperature.
- Warmth fibers, cold fibers
- Body is consistently regulating internal temperature
- Thermoreceptors kick into gear when you make contact with object warmer or colder
than your skin.
The main purpose of this system is to make sure that you maintain your body temperature. It is
not exactly accurate at telling us the temperature but it is efficient in identifying relative
Another important function for this is to stay away from objects that are too hot – but then we
are edging close to nociceptors (pain receptors).
This system can be tricked and is used by plants, for example mint and other plants which
triggers the cold fibers using a chemical substance. So these fibers can respond to chemistry as
well. So these receptors can respond to more than one modality (studied in a pre-final lecture).
Some molecules like capsacin induces the feeling of hot in mammals, but interestingly not in
- Sensory receptors that transmit information about noxious stimulation that causes
damage or potential damage to the skin.
- A-delta fibers: strong pressure, heat; myelinated → Fast
- C fibers: pressure, heat, cold, chemicals; unmyelinated → slow
- There are 2 phases of pain.
This is something we have experienced when we hurt ourselves – the immediate one with
sharp onset, and the throbbing later due to the slow C fibers.
Pain is one of the most important senses we have because it helps us avoid things that are not
good for us. Hansen’s disease makes one lose the perception of pain. This is very scary because
people do not avoiud dangerous objects because they cannot perceive its danger since they
cant perceive pain.
There are two main pathways, about 2 meters long:
- Information must pass through the spinal cord.
- Axons of various tactile receptors combine into single nerve trunks. - Several nerve trunks from different areas of the body
- Once in spinal cord (via dorsal root ganglion): two major pathways: Spinothalamic
(slower) and dorsal column-medial lemniscal (faster).
Spinothalamic: slow because of the multiple synapses. And it conveys nociceptor and
thermoreceptors. Why is pain perception such a slow process when we need to respond to it so
quickly? This is not a problem anyway because we have reflexes which operate on the level of
the spinal cord.
So we don’t need to perceive it constantly. Our reaction may sometimes even be faster than
Dorsal Column-Medial Lemniscal (DCML) Pathways: not that many synapses, so faster.
We can still feel that the hand is moving even if this pathway is gone, because the other
pathway is still good. And we can sense pain and temperature using spinothalamic pathway. As
we move our hand, the hand feels cooler, and we can perceive it via that mechanism as well. So
we can replace information by something else, not very well, but it is still possible.
Every sense goes to the thalamus except for olfaction.
Somatotopy: organizing the brain along the line of the body.
The Sensory Homuculus:
The brain contains several sensory maps of the body, several homunculi, i.e. different subareas
of S1, secondary areas as well.
Brain contains several sensory maps of the body, different subareas of S1, secondary areas as
- Perceived sensation from a physically amputated limb of the body
- 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.
- This is why it is called Phantom pain.
For example, you may have a cramp in your hand, but if this is an illusion, you can’t do
anything, like stretch your hand, to make yourself feel better. This is the phantom pain. A
treatment that works is to put a mirror in a way that the limb is till there in the position where
the missing limb would be, it sometimes