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Neuroscience Research 204 (2024) 1–13
Available online 24 January 2024
0168-0102/© 2024 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (
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).
Review
Article
Primary
somatosensory
cortex
organization
for engineering
artificial
somatosensation
Krista
Lamorie-Foote
a
, Daniel
R. Kramer
a
,
b
, Shivani
Sundaram
a
,
*
, Jonathon
Cavaleri
a
, Zachary
D. Gilbert
a
, Austin
M. Tang
a
,
c
, Luke
Bashford
d
,
e
, Charles
Y. Liu
a
,
f
, Spencer
Kellis
a
,
f
,
Brian
Lee
a
,
f
a
Department
of Neurological
Surgery,
Keck School
of Medicine
of USC, University
of Southern
California,
Los Angeles,
CA, United
States
b
Department
of Neurological
Surgery,
University
of Colorado
School
of Medicine,
Denver,
CO, United
States
c
Department
of Neurological
Surgery,
University
of Texas at Houston,
Houston,
TX, United
States
d
Department
of Biology
and Biological
Engineering,
T
&
C Chen Institute
for Neuroscience,
California
Institute
of Technology,
Pasadena,
CA, United
States
e
Department
of Neuroscience,
Newcastle
University,
Newcastle
upon Tyne, United
Kingdom
f
USC Neurorestoration
Center,
Keck School
of Medicine
of USC, Los Angeles,
CA, United
States
ARTICLE
INFO
Keywords:
Brain
Computer
Interface
(BCI)
Cortical
Stimulation
Topography
ABSTRACT
Somatosensory
deficits
from
stroke,
spinal
cord
injury,
or other
neurologic
damage
can lead
to a significant
degree
of functional
impairment.
The
primary
(SI) and secondary
(SII)
somatosensory
cortices
encode
infor
-
mation
in a medial
to lateral
organization.
SI is generally
organized
topographically,
with
more
discrete
cortical
representations
of specific
body
regions.
SII regions
corresponding
to anatomical
areas
are less discrete
and may
represent
a more
functional
rather
than
topographic
organization.
Human
somatosensory
research
continues
to
map
cortical
areas
of sensory
processing
with
efforts
primarily
focused
on hand
and upper
extremity
information
in SI. However,
research
into SII and other
body
regions
is lacking.
In this review,
we synthesize
the current
state
of knowledge
regarding
the cortical
organization
of human
somatosensation
and discuss
potential
applications
for brain
computer
interface.
In addition
to accurate
individualized
mapping
of cortical
somatosensation,
further
research
is required
to uncover
the neurophysiological
mechanisms
of how
somatosensory
information
is
encoded
in the cortex.
1. Introduction
Patients
with
somatosensory
impairments,
such
as those
afflicted
with
stroke
or paralysis,
suffer
from
a degraded
ability
to manipulate
objects,
control
motor
function,
and
carry
out complex,
multijoint
movements
(Flesher
et al., 2016;
Sainburg
et al., 1995;
Suminski
et al.,
2010;
Lubin,
Strebe
and Kuo,
2017;
Tabot
et al., 2013
). A significant
component
of their
rehabilitation
is compensating
for this lost function,
but recently
efforts
have
focused
on restoring
key somatosensory
defi
-
cits.
Brain
computer
interface
(BCI)
is a promising
means
of restoring
function
for both
motor
and somatosensory
systems.
While
efforts
have
primarily
been
directed
towards
the motor
component
of BCI systems,
somatosensory
BCI can restore
continence,
provide
feedback
on pressure
ulcers,
improve
motor
BCI (Akselrod
et al., 2017;
Lubin,
Strebe
and Kuo,
2017;
Suminski
et al., 2010;
O
Doherty
et al., 2009
), or, ultimately,
return
naturalistic
sensations.
Electrical
stimulation
of the primary
somatosensory
cortex
can produce
percepts
in specific
somatotopic
lo
-
cations,
and recent
work
with
this approach
has demonstrated
reliable,
safe operation
and robust
percepts
(Flesher
et al., 2016;
Armenta
Salas
et al., 2018
). These
percepts
can exhibit
naturalistic
characteristics
(Armenta
Salas
et al., 2018;
Flesher
et al., 2016
), and altering
stimula
-
tion
parameters
can lead
to changes
in percept
intensity
(Lee et al.,
2018
), frequency
(Kramer
et al., 2019c;
Kramer
et al., 2020a
), and
modality
(Armenta
Salas
et al., 2018
). While
some
growing
evidence
suggests
that
microstimulation
may
offer
a promising
alternative
to
large
electrode
stimulation
in improving
naturalistic
sensation
(Flesher
et al., 2016;
Armenta
Salas
et al., 2018
), artificially
mimicking
natural
sensation
remains
challenging
and requires
a detailed
understanding
of
how
the somatosensory
cortex
encodes
and communicates
information.
To achieve
this,
this review
discusses
the functional
and topographic
organization
of the primary
and secondary
somatosensory
cortices
as
possible
targets
for engineering
artificial
sensation
using
somatosensory
* Correspondence
to: Keck
School
of Medicine
of USC,
1510
San Pablo
St, Room
423A,
Los Angeles,
CA 90033,
USA.
E-mail
address:
ss83483@usc.edu
(S. Sundaram).
Contents
lists available
at ScienceDirect
Neuroscience
Research
journal
homepage:
www.scien
cedirect.co
m/journal/ne
uroscience
-research
https://doi.org/10.1016/j.neures.2024.01.005
Received
30 August
2023;
Received
in revised
form
12 January
2024;
Accepted
17 January
2024
Neuroscience Research 204 (2024) 1–13
2
BCI systems
in the future.
This
work
will
primarily
focus
on human
studies
given
that
human
subjects
can most
reliably
articulate
infor
-
mation
about
sensation,
but we include
information
from
non-human
primate
studies
when
pertinent.
2. Somatosensory
cortical
structure
The
somatosensory
system
receives
cutaneous
and
proprioceptive
input
from
superficial
skin receptors
and deep
receptors
within
muscles
and Golgi
tendon
organs
(Nelson,
Blake
and Chen,
2009;
Kaas,
2015;
Kim
et al., 2015).
As a system,
it includes
the primary
somatosensory
cortex
(SI),
secondary
somatosensory
cortex
(SII),
parietal
ventral
so
-
matosensory
area
(PV),
thalamus
parietal
association
areas,
and hypo
-
thalamus
(for visceral
sensation).
Regarding
cutaneous
processing,
this
system
depends
on mechanoreceptors
to relay
information
about
forces
applied
on the skin
surface.
There
are four
types
of peripheral
afferents
important
for tactile
perception:
slowly
adapting
type
1 (SA1)
fibers
that
innervate
Merkel
cell receptors
in the epidermis,
slowly
adapting
type
2
(SA2)
fibers
that
innervate
Ruffini
corpuscles
in the dermis,
rapidly
adapting
(QA1)
fibers
that innervate
Meissner
corpuscles
in the dermal
papillae,
and
Pacinian
corpuscle
(PC)
fibers
that
innervate
Pacinian
corpuscles
in the deep
dermis
(Yau
et al., 2016;
Abraira
and
Ginty,
2013).
SA1
fibers
respond
to maintained
touch
or hair
displacement,
while
SA2
fibers
react
to pressure
on the teeth
and skin
deformations.
QA1
afferents
respond
to low-frequency
skin
vibration,
while
PC fibers
are receptive
to high-frequency
vibration
(Abraira
and
Ginty,
2013;
Kaas,
2015;
Yau et al., 2016;
Romo
et al., 2000).
The rapidly
adapting
afferents,
QA1
and PC, respond
transiently
at the onset
and removal
of
skin deformation,
while
the slowly
adapting
afferents,
SA1 and SA2,
fire
continuously
throughout
indentation
(Yau
et al., 2016;
Hsiao
et al.,
2009).
SA2 and PC have
larger
receptive
fields
than
SA1 and QA1
(Yau
et al., 2016).
SA1 fibers
carry
spatial
resolution
higher
than
QA1
fibers
and encode
for object
orientation
and curvature
(Khalsa
et al., 1998;
Yau
et al., 2016;
Phillips,
Johnson
and Hsiao,
1988).
Both
slowly
and rapidly
adapting
afferents
ascend
through
the dorsal
column-medial
lemniscus
pathway
of the spinal
cord.
The dorsal
col
-
umns
have
a somatotopic
architecture
with
more
medial
fibers
carrying
afferents
from
the sacral
nerves
and more
lateral
fibers
carrying
affer
-
ents
from
the cervical
nerve
roots
(Smith
and Deacon,
1984).
These
fi
-
bers
then
synapse
in the cuneate
(lateral
fibers
for upper
extremity
afferents)
and gracile
(for lower
extremity
medial
fibers
for lower
ex
-
tremity
afferents)
nuclei
in the medulla,
and
project
to the ventral
posterolateral
(VPL)
nucleus
of the contralateral
thalamus.
Afferents
from
the head,
face,
and mouth
ascend
to the principal
trigeminal
nu
-
cleus
and project
to the ventral
posteromedial
(VPM)
nucleus
of the
contralateral
thalamus
(Kaas,
2015).
Electrical
microstimulation
of VPL
nucleus
evokes
primarily
natural
sensation
in multiple
areas,
including
across
the hand
and in the digits.
Perceived
intensity
correlates
with
stimulation
amplitude;
as intensity
increases,
the response
is perceived
as less natural
(Swan
et al., 2018).
From
the thalamus,
sensory
infor
-
mation
projects
to SI. Mountcastle
(1957)
first demonstrated
a columnar
structure
to neuronal
architecture
in the somatosensory
cortex
of cats,
in
which
neurons
within
narrow,
vertical
columns
were
activated
by the
same
peripheral
receptors
(e.g.,
Meissner
s
corpuscles
or Golgi
tendon
organs)
(Mountcastle,
1957;
Horton
and Adams,
2005).
In rats,
the septa
between
cortical
columns
are connected
to the thalamus,
underscoring
the close
integration
between
SI and the thalamus
(Kaas,
2015;
Kim and
Ebner,
1999).
The primary
somatosensory
cortex
consists
of Brodmann
areas
1, 2,
3a, and 3b on the postcentral
gyrus.
From
anterior
to posterior,
area
3a
lies deepest
within
the central
sulcus;
area
3b follows,
extending
onto
the postcentral
gyrus;
area
1 lies on the crown
of the postcentral
gyrus;
and area
2 lies on the posterior
bank
of the postcentral
gyrus,
extending
into
the postcentral
sulcus
(Kaas,
2015;
Roux,
Djidjeli
and
Durand,
2018).
Area
3b receives
thalamic
inputs
from
VPM
and the core
regions
of VPL
(Yau
et al., 2016;
Kaas,
2015;
Cerkevich,
Qi and Kaas,
2013).
Neurons
in this subregion
respond
best to cutaneous
stimuli
(Yau
et al.,
2016).
Area
3a receives
proprioceptive
information
from
the shell
re
-
gion
surrounding
the VPL
(Yau
et al., 2016)
and projects
to the motor
cortex,
premotor
neurons
in the brain
stem
and spinal
cord
(Kaas,
2015),
and to areas
1 and 2 (Jones,
1986).
Areas
1 and 2 are thought
to be
responsible
for multimodal
integration
of tactile
inputs
(Kim
et al.,
2015).
Area
1 receives
input
from
areas
3a and 3b and is presumably
important
for texture
(Kaas,
2015;
Jiang,
Tremblay
and Chapman,
1997;
Jones,
1986).
Neurons
in areas
1 and 3b respond
to bars
and edges
in
their
receptive
fields
at preferred
orientations
and weakly
otherwise,
suggesting
that these
areas
play
a role in the tactile
perception
of edges
(Yau
et al., 2016).
Area
2 receives
cutaneous
and proprioceptive
input
from
areas
3b and 1, and the thalamus
(Kaas,
2015;
Yau et al., 2016),
and integrates
this information
to discriminate
object
size and shape
(Kaas,
2015).
Area
2 is densely
interconnected
across
hemispheres.
In
non-human
primates
(NHPs),
area
2 had
dense
callosal
connections
while
area
1 had less and area
3b had few callosal
connections
(Killackey
et al., 1983;
Iwamura,
2000),
which
permit
the integration
of informa
-
tion
from
both
sides
of the body
and is thought
to be important
for
bilateral
coordination
(Kaas,
2015).
Overall,
proprioceptive
information
from
deep
receptors
generally
diverges
to neurons
in areas
3a and 2,
whereas
cutaneous
information
generally
transmits
to neurons
in areas
3b and 1 (Powell
and Mountcastle,
1959;
Blankenburg
et al., 2003).
However,
a significant
proportion
of neurons
in areas
3a and 3b can
respond
to both
tactile
and proprioceptive
stimuli,
respectively,
each
with
distinct
mechanisms
for sensory
processing
(Kim
et al., 2015;
Trzcinski
et al., 2023).
Although
different
neuronal
populations
in these
areas
show
multimodal
response
properties,
these
responses
are not
distinctly
separate
(Kim
et al., 2015).
Area
2 projects
to association
areas
in the posterior
parietal
cortex
and to SII on the upper
bank
of the lateral
sulcus,
ventral
to SI in the parietal
operculum
(Yau
et al., 2016;
Eickhoff
et al., 2007).
SII receives
the majority
of its inputs
from
SI and
the
thalamus,
and projects
to the posterior
parietal
cortex
and motor
and
premotor
cortices
(Kaas,
2015).
3. Somatotopy
3.1. Primary
somatosensory
cortex
In 1937,
Penfield
was the first to demonstrate
a somatotopic
repre
-
sentation
of the entire
body
in SI, termed
the homunculus,
with
each
location
of the contralateral
body
surface
represented
within
the cortex.
Using
direct
cortical
stimulation
in awake
neurosurgery
patients,
he
demonstrated
that
from
medial
(interhemispheric)
to lateral,
SI is
organized
to represent
the genitalia,
lower
limbs,
upper
limbs,
hands,
and face
(Rasmussen
and Penfield,
1947;
Penfield,
1937;
Kaas,
2015;
Kaas,
1983;
Kaas,
2012).
Structures
such
as the hand,
lip, and tongue
are
represented
in a proportionally
larger
amount
of cortex
than
larger
structures,
such
as the trunk
(Penfield,
1937;
Rasmussen
and Penfield,
1947).
The
homunculus
appears
to be a characteristic
feature
of the
sensory
cortices,
with
SII exhibiting
some
topographic
organization
(Penfield,
1937;
Nguyen
et al., 2004;
Disbrow,
Hinkley
and Roberts,
2003).
Still,
not all somatosensory
regions
exhibit
the same
homuncular
properties.
Three
recent
studies
using
fMRI
found
that relative
overlap
between
digits
was larger
in areas
1 and 2 than
in area
3b (Krause
et al.,
2001;
Nelson,
Blake
and Chen,
2009;
Pfannmoller
et al., 2016).
It has
been
suggested
that
area
3a
s
somatotopy
is similar
to 3b based
on its
deep
receptors,
which
run in parallel
to those
of 3b (Kaas,
2004).
The
somatotopy
of area
1 is a mirror
reversal
of 3b around
the medial-lateral
axis,
flipping
the anterior-posterior
organization
(Kaas,
2015;
Nelson,
Blake
and Chen,
2009;
Blankenburg
et al., 2003).
For example,
in area
1,
the fingertip
is located
posteriorly
to the palm,
whereas
the fingertip
is
located
anteriorly
to the palm
in 3b (Blankenburg
et al., 2003).
Recent
studies
have
validated
the primary
selectivity
and organization
of the
somatosensory
homunculus
but show
that the representation
of various
body
parts
are
distributed
throughout,
overlapping
with
regions
K. Lamorie-Foote
et al.
Neuroscience Research 204 (2024) 1–13
3
selective
to other
areas
(Muret
et al., 2022
).
3.1.1.
Upper Limb
The fingers
are represented
on the gyrus
with
digit
5 more
medial
than
digit
1 (Kramer
et al., 2020b
), and the arm more
medial
than
the
hand
(Nakagoshi
et al., 2005
) (Fig.
1), consistent
with
Penfield
s
homunculus
(Penfield,
1937
). Individual
digit
somatotopy
in area
3b has
been
suggested,
as the cortical
regions
of adjacent
digits
lie closer
together
than
the representations
of digits
farther
apart
on the hand.
For
example,
the cortical
distance
between
the representations
of digit
3 and
digit
1 is larger
than
the distance
between
digit
1 and digit
2 (Pfann
-
moller,
Schweizer
and
Lotze,
2016;
Sathian
and
Zangaladze,
1996
).
Medial-lateral
and anterior-posterior
somatotopy
of the hand
in area
1
has been
shown
with
little
variability
in a direct
cortical
stimulation
study
on 50 awake
operative
patients
(Roux,
Djidjeli
and Durand,
2018
).
Roux
et al. (2018)
found
that
fifth
digit-first
digit
somatotopy
was
medial-lateral
on the cortex,
as expected,
and symmetric
across
hemi
-
spheres.
The
anterior-posterior
somatotopy
involved
different
finger
regions,
with
the fingertip
posterior
on the postcentral
gyrus
to the
phalanx,
on the anterior
bank
of area
1 (Roux,
Djidjeli
and Durand,
2018
).
To examine
digit
somatotopy
in area
3b, an fMRI
study
delivered
electrical
stimulation
to the caput,
base
of the third
metacarpal
bone,
and
third
digit
distally,
medially,
and
proximally.
The
fingertip
was
located
most
anteriorly
in area
3b, with
the caput
located
most
poste
-
riorly.
In area
1, the caput
was located
most
anteriorly
with
the fingertip
most
posteriorly
(Blankenburg
et al., 2003
), consistent
with
the previous
study
(Roux,
Djidjeli
and
Durand,
2018
). These
results
support
the
finding
that areas
1 and 3b are mirror
reversals
of each
other
around
the
medial-lateral
axis (Fig. 1). Somatotopy
could
not be determined
in area
2, as cortical
representations
overlapped
(Blankenburg
et al., 2003
).
In an analysis
of fMRI
images,
there
was
an enlargement
of the
thumb
representation
in areas
1 and 2, with
a trend
towards
enlarge
-
ment
in area
3b compared
to other
digits.
This
may
reflect
the impor
-
tance
of sensory
integration
in the thumb
(Martuzzi
et al., 2014
).
Schellekens
et al. (2021)
recently
demonstrated
that
the cortical
Fig. 1.
Cortical
SI Representations
of the Upper
Extremity,
Top left,
a colored
upper
extremity
matching
corresponding
cortical
areas
of somatosensation.
Upper
extremity
somatosensation
is separated
into the proximal
limb
(light
violet),
distal
limb
(cyan),
proximal
palm
(dark
violet),
distal
palm
(red),
proximal
digits
1
5
(dark
shades
of pink,
yellow,
green,
orange,
and blue),
and distal
digits
1
5 (light
shades
of pink,
yellow,
green,
orange,
and blue).
Top right,
a representation
of the
organization
of Brodmann
areas
1 and 3b (labeled
and underlined
in black)
on an unfolded
portion
of cortex.
Area
1 somatotopy
is mirrored
to that of 3b around
the
anterior-posterior
axis.
In 3b, representations
of distal
portions
of the digits
are located
anteriorly
on SI within
the sulcus,
whereas
they
are located
posteriorly
in area
1 on the gyrus
of SI. Note
the larger
area
of the cortex
dedicated
to digit
1 somatotopy
compared
with
the other
digits.
In the cortical
representations
below
(bottom
left and right),
only
area
1 is visible
because
it sits on the gyrus
whereas
area
3b is located
within
the sulcus.
Bottom
left and right,
lateral
and superior
cortical
representations
of upper
extremity
somatosensation.
The fingers
are represented
on the gyrus
with
digit
5 (dark
blue
and light
blue)
more
medial
than
digit
1 (dark
pink
and light
pink)
on the cortex,
and the cortical
representation
of the arm (cyan
and light
violet)
more
medial
than
the palm
(red
and dark
violet).
Just medial
to
the proximal
upper
extremity
representation
on the cortex
is the head
representation
(colored
green-blue
to match
color
schemes
in later
figures).
The lateral
border
of the hand
representation,
sitting
just lateral
on the cortex
to digit
1, is the beginning
of the face
somatotopy
(colored
baby
blue
to match
later
figures).
K. Lamorie-Foote
et al.
Neuroscience Research 204 (2024) 1–13
4
organization
of fingertip
somatotopy
may
reflect
processing
order
by
measuring
population
receptive
field
size (i.e.,
the population
of neurons
responding
to a stimulus
as measured
by fMRI)
during
vibratory
stim
-
ulation
of the fingertips
(Schellekens
et al., 2021
). The authors
found
that population
receptive
field
size increased
from
area
3 to area
1 and
was even
greater
in area
2 (Schellekens
et al., 2021
), which
indicates
that
spatial
information
integration
increases
anterior
to posterior.
Arbuckle
et al. (2022)
used
fMRI
to measure
cortical
response
to
multi-finger
stimulation
in 10 healthy
participants
(Arbuckle,
Pruszyn
-
ski and Diedrichsen,
2022
). They
discovered
that neuron
activity
in area
3b represented
discrete
single
finger
somatosensation,
whereas
activity
in areas
1, 2, and 4 constituted
sensory
integration
from
multiple
fingers
(Arbuckle,
Pruszynski
and Diedrichsen,
2022
). These
results
indicate
that
integration
of multi-digit
somatosensation
takes
place
in posterior
SI (Arbuckle,
Pruszynski
and Diedrichsen,
2022
).
Within-limb
somatotopy
in SI has also
been
shown
with
fMRI.
Manual
tactile
stimulation
of four
points
on the
upper
limb
demonstrated
a cortical
medial-lateral
organization
when
stimulation
occurred
proximally
to distally
on the limb
(Nakagoshi
et al., 2005
).
Overall,
the upper
limb
is arranged
in a medial-lateral
direction
on the
cortex,
with
recent
studies
proposing
anterior-posterior
somatotopy
of
the digits
(Roux,
Djidjeli
and Durand,
2018;
Blankenburg
et al., 2003
).
It is worth
noting
that
some
recent
studies
have
challenged
the
traditional
view
of somatotopic
organization.
For example,
Trzcinski
et al. (2023)
demonstrated
that
neurons
in 3b can respond
to multiple
fingers,
although
the firing
rate
across
these
cells
was heterogeneous,
indicating
the presence
of a
preferred
digit
(Trzcinski
et al., 2023
). In
addition,
Hirabayashi
et al. (2021)
chemogenetically
silenced
areas
of
the hand-finger
region
of SI, which
expectedly
impaired
grasping
but
also
surprisingly
directly
disinhibited
foot
activity
(Hirabayashi
et al.,
2021
). Their
findings
indicate
that there
may
be foot representation
in
the hand
region
of SI, or more
conservatively,
a node-node
reliance
network
responsive
to inhibition.
These
studies
demonstrate
cross-digit
integration
across
SI and
challenge
the previous
simplistic
view
of
Fig. 2.
Cortical
SI Representations
of the Lower
Extremity,
Top left,
a colored
lower
extremity
matching
corresponding
cortical
areas
of somatosensation.
Lower
extremity
somatosensation
is separated
into
the hip (green),
proximal
limb
(red),
distal
limb
(blue),
heel
(orange),
digit
1 (pink),
and digit
5 (yellow).
The lower
extremity
is organized
with
more
distal
portions
of the limb
represented
more
medially
on SI cortex.
Top right,
medial
view
of cortical
representations
of the lower
extremity.
The most
medial
portion
of the lower
extremity
cortical
representation
is digit
5 (yellow)
with
nipple
somatotopy
representation
just medial
to it (light
brown
to match
color
schemes
in later
figures).
Bottom
left and right,
lateral
and superior
views
of lower
extremity
cortical
representations.
Within
the lower
extremity,
the cortical
representation
that is most
lateral
on SI is the hip (green)
with
the torso
representation
(dark
brown
to match
later
figures)
just lateral
to it.
K. Lamorie-Foote
et al.
Neuroscience Research 204 (2024) 1–13
5
human
somatotopy,
although
evidence
from
these
studies
suggests
that
somatotopy
may
be preferably
but not exclusively
coded
to certain
regions.
3.1.2.
Lower
Limb
Penfield
proposed
a medial-lateral
leg somatotopy
with
the toes
inferior
to the leg in the interhemispheric
fissure
(Penfield,
1937
). In his
studies,
only
10 out of 400 patients
described
sensation
localized
to the
toes
with
direct
cortical
stimulation.
Five
reported
sensations
in the
hallux,
four
reported
sensations
in all lower
limb
digits,
and one re
-
ported
sensation
in digit
5 (Hashimoto
et al., 2013;
Rasmussen
and
Penfield,
1947
). Since
then,
fMRI
studies
have
supported
Penfield
s
homunculus,
finding
a lateral-medial
cortical
representation
of the
lower
limb
as tactile
stimulation
moves
proximally-distally;
with
the leg
more
lateral
and draping
over
and into
the interhemispheric
fissure,
ending
with
the toes
more
medial
and
deeper
within
the fissure
(Nakagoshi
et al., 2005;
Bao et al., 2012;
Akselrod
et al., 2017
) (Fig. 2).
Bao
et al. (2012)
expanded
on this
theory,
demonstrating
inferior-superior
somatotopy
when
tactile
stimulation
sites
were
moved
from
medial
to lateral
on the lower
limb
(Bao et al., 2012
). For example,
stimulation
of the medial
leg resulted
in activation
that was inferior
to
stimulation
of the lateral
leg on fMRI
(Bao et al., 2012
). In another
fMRI
study,
the hallux
representation
was larger
than
that
of the fifth
digit,
calf,
or thigh
(Akselrod
et al., 2017
), similar
to the findings
of the thumb
(Martuzzi
et al., 2014
). Tactile
stimulation
of the leg and foot
did not
result
in cross-activation
of their
cortical
representations.
For example,
the hallux
and fifth
digit
s peripheral
stimulation
did not result
in acti
-
vation
of the leg cortical
areas.
The
total
volume
of the lower
limb
representation
in area
2 was decreased
compared
to that
in 3b and 1
(Akselrod
et al., 2017
), similar
to findings
on the digits
of the hand
(Martuzzi
et al., 2014
). While
foot somatotopy
could
not be discerned
in
this study,
in the somatosensory
cortex
of monkeys,
lower
limb
digits
are
organized
lateral-medial
in area
3b and
rostral-caudal
in area
1
(Akselrod
et al.
). Human
lower
limb
digits
may
be organized
Fig. 3.
Cortical
SI Representations
of the Torso,
Head,
and Neck,
Top left,
a colored
torso
(dark
brown),
nipple
(light
brown),
head
(green-blue),
and neck
(red)
matching
corresponding
cortical
areas
of somatosensation.
Top right,
medial
view
of the cortex
shows
the nipple
cortical
representation
(light
brown)
just medial
to
the representation
of digit
5 of the lower
extremity
(yellow).
Bottom
left and right,
lateral
and superior
views
of the torso,
head,
and neck
cortical
representations.
Torso
somatotopy
representation
(dark
brown)
is located
medially
to the neck
representation
(red)
and laterally
to the hip representation
(light
green).
Nipple
somatotopy
representation
(light
brown)
is located
inferiorly
to the lower
extremity
fifth
digit
somatotopy
representation
(yellow).
Head
somatotopy
representation
(green-blue)
is medial
to the proximal
upper
extremity
cortical
representation
(light
violet,
same
as in Fig. 1).
K. Lamorie-Foote
et al.