lecture 12: action Flashcards
Hierarchical Representation of Action Sequences
-many possible response systems (parts of your body),
you must choose one
-the motor system
implements the chosen action plan by activating “effectors” (parts of your
body that move)
- Primary Motor Cortex (M1)
- Premotor and supplementary
motor cortical regions - Parietal lobe contributions
- Basal Ganglia
Somatotopic Organization of Primary
Motor Cortex (M1)
What do M1 neurons code for?
-stimulating neurons in motor cortex triggers movement
-But what exactly are these neurons coding for? Force of
muscle contraction? Direction of motor movement? The
target location of motor movement?
Tuning Curve for
Broadly Tuned Movement Neuron
Population Vector
*Looked at primary motor cortex activity during
spiral tracing.
*Monkeys kept finger on a circular target on a touch
screen as target traced over a spiral
*Average finger trajectories are shown in A, B
can we recreate neural trajectory through finger trajectory
*You can recreate the drawing trajectory.
*Examples of finger movement shown at left (A & B)
*On right, population trajectories for both an outside in (A) and inside
out (B) spiral movement. The trajectories were constructed by adding
the individual vectors tip to tail.
*This shows that you can decode a pretty good representation of
instantaneous movement velocity from the activity of single neurons
in primary motor cortex
- Primary Motor Cortex-
- Voluntary actions require the primary motor cortex
- Neurons code direction of movement and
populations of neurons determine movement
Abstract Representation of Action: Motor Program or Plan
Many cortical areas involved with action! name some
sma,substantia nigra, primary motor cortex
What is a Motor Plan?
Complete sequence of movements
-Location plan for simple, voluntary reaching movements
-Hierarchical plan for more complex movements
(goal, effector selection, effector movement)
Tapping Task
Motor Sequence Learning
to learn about primary motor cortex and sma
hemiplegia
loss of voluntary
movements on contralateral
side of body
-hyperactive reflexes (for maintaining posture)
-reflexes are normally
inhibited by cortex so that controlled action is possible
alien hand syndrome
patient reaches out and grasps objects
with affected arm, even when they try not to do so
-Patients often aware it is their hand, but just can’t seem to control it.
-Problem with the
goal/planning part of
action.
Functional Role of Different Motor Areas: Two cortical regions
-Supplemental Motor Area (SMA), conceptual level,
higher-order goals of action
-Primary Motor Cortex (M1), motor implementation
level, motor execution
Two types of converging evidence on different motor areas
Two Networks for Motor Areas
Externally-Guided Movement
Internally-Guided Movement
Internally-Guided Movement
-Internal loop = SMA,
prefrontal cortex, basal ganglia, motor cortex
-critical for internally
guided movement (e.g., when you decide to execute a particular
action plan)
-relatively important for expert performing
learned action sequences
Externally-Guided Movement
-External loop = Premotor cortex (PMC), parietal
cortex, cerebellum, motor cortex
-critical for externally guided movement (e.g., guiding action with visual feedback)
-relatively important for novice learning a new action sequence
Premotor cortex (PMC) cell responds more for
externally-driven tapping
Supplementary Motor Area (SMA) cell responds more for
Internally-driven tapping
Basal ganglia also showed greater activity for
internally-driven movement.
– Basal ganglia motor circuit
regulates the excitability of
frontal motor circuits (SMA) and biases likelihood of
movement and nature of movement.
Cerebeller route coordinates timing of and trajectory of
movements using sensory and motor information.
Parkinson’s patients are relatively more impaired on
the internal loop (more trouble with self-initiated
actions than externally driven actions)
Also perform poorly on purely cognitive tasks
requiring executive control (prefrontal cortex).
Santa Clara Medical Center
San Jose, California - 1983
-patient in the ER
-42 year old man
-completely immobilized, bent over, frozen, couldn’t talk, drooling
-resembled someone who had Parkinson’s for 10 Years but Parkinson’s is rare among those under 50
-no structural abnormalities on CT or MRI scans
-then his girlfriend turns up with similar symptoms
-then two more cases, men in their 20s, in Watsonville, CA
Heroin Addicts In the Area Were Turning Into Immobile Statues
Synthetic Heroin and the chemical MPTP
-MPTP in blood, converts to MPP+, which attacks the substantia nigra
like a guided missile
-previously known that the substantia nigra generates most of the brain’s dopamine, and that this region deteriorates with Parkinson’s
-that dopamine system appears to be critical for generating
voluntary, coordinated movements by disinhibiting motor plans
(which are normally inhibited by the basil ganglia)
-without dopamine, basal ganglia sends excessive inhibition, leading to inhibition of voluntary movement.
- Treated with L-Dopa (to increase dopamine) or deep-brain stimulation (to reduce this inhibition).
Important Brain Areas: Action
Substantia nigra:
Primary motor cortex (M1):
supplementary motor area (part of premotor cortex)
Substantia nigra:
Subcortical nuclei that is part of the basal
ganglia, produces most of the brain’s dopamine, plays an
important role in motor control
Substantia nigra is the brain’s primary source of dopamine, a
neurotransmitter that is very important for motor control
Evidence from synthetic heroin abusers and patients with
Parkinson’s disease, both characterized by immobilization
Synthetic heroin (MPTP) converts to MPP+ in the blood,
which selectively attacks substantia nigra
Parkinson’s leads to death of cells in substantia nigra
– Drug treatment (L-dopa) can result in dyskinesia
(trouble in stopping movement). This is the opposite of
what is usually seen in parkinson’s disease, which is
rigidity/akinesia (trouble in initializing movement).
– Can also be treated with deep brain stimulation
Primary motor cortex (M1):
Involved in executing movement;
damage can result in hemiplegia
– M1 is connected to alpha motor neurons through pyramidal
motor tracts
– Alpha motor neurons connect spinal cord to muscle
– Muscle is also connected back to spinal cord through
sensory neurons (feedback loop)
- SMA (part of premotor cortex)
Represents higher order goals
or action intentions; damage can result in alien hand syndrome
parkinson’s disease
rigidity/akinesia (trouble in initializing movement).
Drug treatment (L-dopa) of synthetic heroin can result in
dyskinesia
(trouble in stopping movement)
Action System
Connecting the brain to the body:
Pyramidal (“corticospinal”) motor tracts connect primary
motor cortex (M1) to alpha motor neurons that originate in
the contralateral (opposite) side of the spinal cord (part of CNS)
* Thus, right motor cortex controls left side of body and vice versa
Muscles have sensory receptors that detect changes in the
muscle, and these receptors send information to the dorsal
root via
Muscles have sensory receptors that detect changes in the
muscle, and these receptors send information to the dorsal
root via
A motor plan
is a complete sequence of movements
* Location plan for simple, voluntary reaching
* May also be hierarchical for more complex movements
Hierarchical Representation of Action
Sequences
Some actions are so fast that it’d be too slow to contemplate the
next movement at each stage (e.g., stacking cups competition)
It might be faster to link all movements together into one
hierarchical action plan consisting of both low-level,
simpler actions, as well as high-level, more conceptual
actions
- This action plan can be triggered by an abstract goal (e.g.,
“do a 3-stack”, “do a 6-stack”) à motor system carries
out the action plan for that goal
For complex action sequences, there are “centrally stored”
motor plans that are independent of the muscle that is used
to carry it out
There must be an abstract action plan that is represented
CENTRALLY and INDEPENDENTLY of the muscle group that is
performing the action
2 major cortical motor areas in frontal lobe:
- Primary motor cortex (M1): Involved in executing
movement - Supplementary motor area (SMA): Represents higher
order goals or action intentions - Evidence from TMS and lesion studies (following stroke)
Primary motor cortex (M1)
Involved in executing movement
* TMS to M1 results in loss of coordination of the corresponding
body part (e.g., losing finger coordination during tapping task)
Supplementary motor area (SMA)
Represents higher
order goals or action intentions
* TMS to SMA results in losing track of the goal or plan of
action (e.g., losing track of the finger tapping sequence)
M1 lesion results in
hemiplegia à loss of voluntary
movements on opposite side of body, and hyperactive
reflexes
- SMA lesion results in
alien (autonomous) hand syndrome à
aware that it’s their own hand, but cannot control it
We know that stimulating neurons in M1 triggers movement–
but what do these neurons code for?
- Neurons code for direction of movement
- Neurons have broad tuning (i.e., anything slightly downward
gives a strong response)
Evidence from single cell recordings in monkeys while they
reach in different directions:
This neuron still responds maximally when moving down from 90
degrees to the center– thus it responds to DOWNWARD MOTION in
general
Precise movements require population coding (e.g., combining
the response from multiple neurons)
Population Vectors
Example: We have 2 cells, one preferring 180 degrees and the
other preferring 90 degrees
You can combine the responses from both of these
neurons to get a more precise direction – called a
“population vector”
The summed activity of multiple neurons determines
the direction of movement!
Summed neuronal activity can tell us where a monkey will
move before the monkey actually moves (clear direction of
population vector appears after 100-200 ms)
Population vectors are the code for movement!
Each neuron has a preferred direction– within a population of
neurons, neurons that prefer one direction will have a stronger
response, and neurons that prefer another direction will have a
weaker response
- The summed activity of ALL active neurons (i.e., population
vector) determines the direction of movement - The summed activity can tell us where a monkey will move
before the monkey actually moves
Premotor cortex responds more to
externally-driven tapping
How are more complex action sequences represented?
– How are they stored?
It might be faster to link all movements together into one
hierarchical action plan consisting of both low-level, simpler
actions, as well as high-level, more conceptual actions
* This action plan can be triggered by an abstract goal (e.g., “do
a 3-stack”, “do a 6-stack”) à motor system carries out the
action plan for that goal
* For complex action sequences, there are “centrally stored”
motor plans that are independent of the muscle that is used
to carry it out
What are the 2 cortical motor areas that are responsible for
action?
– What is each area thought to be responsible for?
– What happens if you TMS these regions?
– What happens if you damage these regions?
Primary motor cortex (M1): Involved in executing
movement
TMS to M1 results in loss of coordination of the corresponding
body part (e.g., losing finger coordination during tapping task)
M1 lesion results in hemiplegia à loss of voluntary
movements on opposite side of body, and hyperactive
reflexes
Supplementary motor area (SMA): Represents higher
order goals or action intentions
- TMS to SMA results in losing track of the goal or plan of
action (e.g., losing track of the finger tapping sequence)
SMA lesion results in alien (autonomous) hand syndrome à
aware that it’s their own hand, but cannot control it
SMA responds more to
internally driven tapping
centrally stored motor plans
motor plans that are independent of the muscle used to carry it out (you can use your hands or feet to write a sentence)
