Somatosensory Cortex

Somatosensory Cortex
Primärer Somatosensorischer Kortex (SI: Brodmann Area 1,2,3a,3b)
Sulcus
Centralis
Posteriorer Parietaler Kortex BA: (5, 7)
Sekundärer Somatosensorischer
Kortex (SII)
Postzentraler Gyrus
Zentraler
Sulcus
1
7
3b
2
3a
5
« Take Home Message » Somatosensory Cortex
• The primary somatosensory cortex (S1) is in the post-central
gyrus, corresponding to the Brodmann’s areas 1, 2 and 3, and
contains four subfields: Area 1, 2, 3a and 3b.
• Associative somatosensory areas 5 and 7 are located more
posteriorly in the parietal lobe
• S1 is organized somatotopically: there is a relation between
the location of a single neuron in S1 and the position of its
receptive field on the surface of the skin
• Receptive fields of the leg are represented near the midline in
S1 whereas receptive fields of the forelimb and face are
represented more laterally in S1
Functional Organization
Thalamus
Ventroposterior
lateral nucleus
(VPL)
Functional Organization
Cortex
« Take Home Message » Functional Organization
• The primary somatosensory cortex (S1) is organized in
columns (following the same principles as previously seen for
V1)
• Somatotopy corresponds to a first principle of column
• Adaptive properties (slow versus rapid adaptation)
corresponds to a second principle of column
• Before reaching S1, the information is transferred first
through the thalamus (mainly ventroposterial lateral nucleus
VPL), where somatotopy is also present.
Homunculus
« Take Home Message » Homunculus
• The body representation in the primary somatosensory cortex
(S1) is distorted (homunculus), in which some body parts are
over-represented (hand, face) whereas other body parts are
under-represented (trunk, leg).
• The distortion is related to discriminative properties which
are more prominent in some territories (hand, face) than
others.
Somatosensory receptive fields
Somatosensory training effects
Somatosensory Area 3b
Learning frequency discrimination modifies Somatosensory Area 3b
« Take Home Message » Receptive Fields
• Receptive field size increases from the fingers to the palm of
the hand to the arm. This increase is reflected in a decrease in
acuity.
• Training results in changes in Somatosensory Cortical
receptive fields (for example SII area 3b). For a frequency
discrimination task, the representation of the trained digit
expands at the cost of adjacent digit representations. When
fingers are surgically joined, receptive fields cover both
fingers.
Large scale reorganization of Area 3b following partial peripheral Lesion
HAND before lesion
« Take Home Message » Large scale plasticity
• Following the loss of a limb or a spinal chord lesion
depriving the cortex of sensory input from a limb, there is
large scale reorganization of somatosensory cortex.
• The cortical representation of the lost limb is taken over by
adjacent cortical represenations.
• This reorganization is not immediate, but can takes weeks or
months.
Phantom Limb sensations / pain
« Take Home Message » Phantom Limb
• Patients who have lost an arm or leg often experience
phantom pain. This is pain that they feel in the lost limb
despite the fact that it is absent. The pain is cortical in origin.
• The mirror box can be used to tread phantom pain.
After visual deafferentation, inputs
from other sensory processing
areas reach the occipital cortex
Unmasking and strengthening of the
connections lead to enhanced
multisensory processing at the level
of the occipital cortex
Crossmodal plasticity I
Pascual-Leone et al., 2005
Ann. Rev. Neurosci. 28:377-401
Braille activation in the visual cortex of an early blind subject
Crossmodal plasticity II
« Take Home Message » Crossmodal Plasticity
• When visual cortex is deprived of visual input, it can be
recruited for other purposes.
• An example of this is the involvement of occipital cortex in
Braille reading in congenitally blind subjects.
• Transcranial Magnetic Stimulation can be used to locally
disrupt cortical processing.
Auditory Cortex
Primärer Auditorischer Cortex
Sekundärer Auditorischer Cortex
« Take Home Message » Auditory Cortex
• The primary auditory cortex (A1) is located in the temporal
lobe (most of it is not visible, in the lower bank of the lateral
sulcus)
• The primary auditory cortex (A1) is tonotopically organized
• The position of a single neuron in A1 is related to the
characteristic frequency of this neuron (the tip of its tuning
curve or the frequency of the tone to which it is most
sensitive)
A1 Functional organization
EE neurons: excited by ipsi- and contralateral input
EI neurons: stimulated by unilateral input, inhibited by contralateral input
« Take Home Message » (Fig. 152)
• The primary auditory cortex (A1) is organized in columns (as
seen previously for V1 and S1)
• Tonotopy is a first principle of column
• Binaural interaction is the second principle of column
Auditory Neurons code behavioral significance
4800 Hz - right key touch
4800 Hz - left key touch
Spikes/s
50
50
0
0
-2
-1
0
1
IS
2
3
4
F3
Right target
F4
-1
0
1
IS
Time (s)
- First epoch: tonal stimuli F3 and F4
Left target
-2
2
3
4
Time (s)
- Second epoch: tonal stimuli F4 and F5
Left target
F4
the same tone F4 instructs two different movements
Right target
F5
Spikes/s
100
100
Auditory Imagery
PAC (red) and Brodmann's area 22 (green) during gaps in familiar songs
with lyrics (FL), familiar instrumentals (FI), unknown songs with lyrics
(UL) and unknown instrumentals (UI)
Language
3:05
1:50