Download Motor Systems I Cortex

Motor Systems I
Cortex
Reading:
BCP Chapter 14
Principles of Sensorimotor Function
Hierarchical Organization
• association cortex at the
highest level, muscles at the
lowest
• signals flow between levels
over multiple paths
Motor output is guided by
sensory input
Learning changes the nature
and locus of sensorimotor
control (e.g., conscious to
automatic)
Sensorimotor Association Cortex
Association cortex is at the top
of the sensorimotor hierarchy.
There are two major areas of
sensorimotor association cortex:
• posterior parietal
• dorsolateral prefrontal
Each is composed of several
different areas with different
functions.
Posterior Parietal Association Cortex 1
Before a movement can be
initiated, need to know:
• current position of body parts;
and
• location of external objects of
interest
The PPAC receives input from
the dorsal streams of the
somatosensory, auditory and
visual systems and thus plays
an important role in integrating
these two types of information.
Posterior Parietal Association Cortex 2
Electrical stimulation of the PPAC
causes patients to experience
the intent to perform a particular
action
Damage to PPAC:
• apraxia (left): inability to make
a requested movement (i.e.,
cannot form intent)
• contralateral neglect (right):
inability to respond to stimuli
contralateral to the lesion
Outputs to dorsolateral prefrontal
association cortex, secondary
motor cortex, and frontal eye field
Dorsolateral Prefrontal Association Cortex
The DLPFAC receives inputs
from, and projects to, the PPAC
Areas of secondary
motor cortex
Given an intent to move, the
DLPFAC, with input from other
frontal lobe areas (in particular
the ventrolateral PFC; the endpoint of the ventral streams),
anticipates the consequences of
various movements and forms a
plan of action
Outputs to secondary motor
cortex, primary motor cortex, and
frontal eye field
Ventrolateral
prefrontal
association
cortex
Secondary Motor Cortex 1
Inputs from association cortex
(mainly DLPFAC)
Three major areas (each
subdivided): premotor,
supplementary and cingulate
The secondary motor cortex
converts general plans of
action into a specific set of
instructions
• active during imagining or
planning of movements
Outputs to primary motor
cortex
Secondary Motor Cortex 2
• Nearly all premotor area (PMA) cells
(94%) show extrinsic (i.e.
movement-related) activity:
– Electrical stimulation of PMA
generates complex movement
patterns, such as hand-shaping
or reaching.
• For cued movements
– Proportion of cells active during
the “instruction” period prior to
movement is higher in PMA than
in primary motor cortex (M1).
– M1 > PMA during movement.
• Suggests PMA devising specific
movement strategies prior to
execution.
Primary Motor Cortex
Located in the precentral gyrus
of the frontal lobe (in front of the
central fissure)
Major point of convergence of
cortical sensorimotor signals.
Major, but not only, point of
departure of signals from
cortex.
Controls the execution of
movement
Functional Organization of M1
The motor cortex is organized in a
somatotopic manner; that is,
according to a body map
Most of primary motor cortex is
dedicated to controlling body parts
that are capable of intricate
movements, such as the hands and
face.
Each site receives feedback (from
primary somatosensory cortex) from
receptors in the muscles and the
joints that the site influences.
Coding of Movement in M1
M1 controls the execution of
voluntary movements.
Intrinsic Space Hypothesis:
M1 controls muscles, i.e. lowlevel movement dynamics, by
controlling parameters such as
movement force.
Extrinsic Space Hypothesis:
M1 controls movements, i.e.
higher-level, more abstract
kinematic aspects of movement,
such as direction, range, and
speed of movement.
Intrinsic Space Hypothesis
• Electrical “microstimulation” of motor
cortex can elicit twitch in individual
muscles, as long as stimulation is
weak (i.e., current spread is minimal).
• M1 modulates discharge activity
related to movement dynamics:
– Muscle force
– Movement velocity
– Joint position.
Extrinsic Space Hypothesis
Activation of the homunculus
at a given site with natural
duration and amplitude
stimulation elicits complex,
species-typical movements
involving that body part.
• M1 neurons prefer a
particular direction of
movement.
• However, directional tuning
is broad, esp. compared to
precision of actual
movement.
Activity of M1 neuron in
highly-trained monkey
during goal-directed
reaching (Georgopoulos
1983)
Movement Direction Coding in M1
• Neural representation of
movement direction is best
expressed by a population
(“ensemble”) code:
– Each M1 neuron “votes” for
movement direction according
to its firing rate for that
direction.
– Directional vector sum of the
population (red arrows) closely
matches movement direction.
Georgopoulos (1983)
Mirror Neurons
Many neurons in motor cortex (up to
50% in some areas) are active not
only when performing a specific action,
but are also active when a person
imagines or watches the same action.
The body does not move when mirror
neurons fire because the overall level
of activity is lower than needed.
A possible neural basis of learning by
imitation, mental rehearsal