Lecture Six; Brain Machine Interface Flashcards
What was the first study of neural movement?
Fritsch
First idea that;
- Motor cortex was responsible for motor control
- Areas of the cortex are connected to specific muscles
What did fritsch do?
- Very first electrical recording generating a MAP
- Used observation
What did penfield study do?
Drew the motor homonculus
Note; functions are not perfectly delineated i.e there are areas of overlap that encode for movement
What is the evarts study?
Relation of pyramidal tract activity to force exerted during voluntary movement
Describe the setup for the evarts study;
- AWAKE Monkey with stimulating electrode in the pyramidal tract
- Recording electrode in wrist area of right motor cortex
- Wrist flexion / extension, with and without load atnagonist
What was recorded in the evarts study as the monkey performed the wrist movements?
- Recording SINGLE neurons in the motor cortex.
Confirm these are PTNs by stimulating the spinal cord
In the evarts study what movement did the monkeys perform?
Extension to flexion movement
Describe the neural recordings in the evarts study, no load, extension -> flexion ;
1/3
- Extensors tonically active, holding it in extension (when move to flexion, little activity present)
- Move to flexion, burst of activity then tonic
- CTN active with agonist muscle
look at page 20
Describe the neural recordings in the evarts study, FLEXOR load, extension -> flexion ;
2/3
- Burst of flexor activity following movement, then tonic
- No extensor activity
- CTN (corticopsinal tract neurons) activity increases with increased load
Describe the neural recordings in the evarts study, EXTENSOR load, extension -> flexion ;
3/3
- Extensor burst prior to movement
- no CTN activity, flexion movement results from relaxation of agonist
What was the conclusion of the evarts study;
- Single cell firing encodes for contraction
- “neurons behave like muscles”
i. e direction + muscle contraction all in same direction therefore if neurons encoded direction then activity would be the same for all movements (doesnt happen in this study)
THis is old and not true in reality
Describe the setup for the georgopoulus study 1982;
- The activity of SINGLE CELLS in the motor cortex were recorded while monkeys made movements in 8 directions (45 degree intervals) in a 2d apparatus.
- Same start point
- Same amplitude of movement
In the georgopololus study, how many neurons were able to be examined?
- The frequency discharge of 241/323 cells (74.6%) varied in an orderly fashion with the direction of movement
In the georgopolus study when was neuron discharge rate the highest?
Discharge was most intense with movements in a preferred direction
and was reduced gradually when the movement was made farther and farther away from the preferred direction
What graph did the georgeopolus researchers create?
Resulted in a bell shaped directional tuning curve
Describe the relationship of the directionally tuned cells;
75% of the 241 directionally tuned cells, the frequency of discharge, D, was a sinusoidal function of the direction of movement.
Firing rate = baseline + m (modulation) cos (movement direction (angle) - Preferred direction (angle))
What did they find with tuning curves and the neuron population?
Preferred directions differed for different cells so that the tuning curves partially overlapped
What was the georgeopolus conclusion / hypothesis?
Movement trajectory in a desired direction might be generated by the cooperation of cells with overlapping tuning curves
‘Population coding’
Describe the schwartz 1988 study;
- Awake monkeys, single neuron recording, 8 3D directions of movements
- Equal amplitude of movement
- Same starting position
What was schwartz 1988 describing?
The relationship between the neuronal activity in primate motor cortex and the direction of arm movement in 3D space.
What was found in the schwartz study?
- Discharge rate of cells varied in an orderly fashion with the direction of movement i.e neurons have a preferred 3D direction of movement (highest discharge)
What was the conclusion of the schwartz 1988 study?
Preferred directions are uniformly distributed in space i.e all directions are equally represented (populations of neurons)
What formula did the schwartz 1988 study create?
Firing rate = baseline + Bx Dx + By Dy + Bz Dz
Bx = Preferred direction, x plane Dx = Direction, x plane
(vector representation)
What did the georgopolus + schwartz 1988 study describe?
A code by which a population of motor cortical neurons could determine uniquely the direction of reaching movements in 3D space.
What did the georgopolus + schwartz 1988 study find?
The neuronal population code (cosine tuning) assumes that for a particular movement direction each cell makes a vectorial contribution (votes) which direction in the cells preferred direction and magnitude proportional to the change in cells discharge rate associated with the particular direction of movement
What do these vectors contribute to?
The vector sum of these contributions is the outcome of the population code (population vector algorithm) and points in the direction of movement in space well before the movement begins
What was the conclusions of the georgopolus + schwartz 1988 study about population size and confidence?
Confidence cone,
Describes the confidence for the number of neurons in the population vector as to how accurate it will be
<50 = best
Describe the kakei study;
The activity of motor neurons (m1) was recorded in monkeys trained to perform a task that dissociates three major variables of wrist movement; muscle activity, direction of movement at the wrist joint, and direction of movement in space.
i.e trying to determine what exactly is represented in M1, muscle action of direction
What were the findings of the kakei study?
- 28/88 neurons displayed changes in activity that were muscle like
- 44/88 displayed changes in activity that were related to the direction of wrist movement in space independant of the pattern of muscle activity that generated the movement.
Thus both muscles and movement appear strongly in M1
Describe the kakei results;
- Muscles will shift their direction of movement preference based on hand orientation (pronation vs mid vs supination)
- Neurons dont change their direction preference regardless of hand orientation
- Neurons increase their rate of firing with changes in hand orientation (gain modulation, muscles more or less activated to perform the movement based on ahdn orientation)
Neurons behave like muscles
What was the schwartz 7.5 study set up?
Illusion where monkeys saw they were drawing ellipses but over time the computer software altered the algorithm so that they were actually drawing circles but they saw ellipses.
Illusion
What did schwartz 7.5 study find?
Pre-motor cortex encoded ellipses,
M1 had smaller signals (visumotor transformation??)
Describe the churchland study set up;
Monkeys performed tasks at varying speeds while neuron recordings were taken
What did the churchland study find?
Neurons may encode velocity
Tuning for direction was prominent but the preferred direction was inconsistent with respect to time, instructed speed, and or reach distance
What two theories was the churchland study deliberating?
Dynamic system vs representative
Describe churchlands dynamic system;
- Individual neurons may not represent anything
- Instead their activity is whatever is needed to generate the correct movement
- Neural networks show complexity
What is a dynamical system?
A dynamic system whose state evolves with time according to a fixed rule
Where you start determines where you end up
What was the shenoy study?
Reviews premotor cortex function in the dynamical systems view
What did the shenoy study conclude?
Where in the neural space you start the trial after the prepatory period determines the path you take and where you end up
i.e where you start in the prepatory period determines everything, dynamic system
