PSYC 427 Lecture Notes - Lecture 8: Vector Calculus, Kinematics, Polar Coordinate System
14 May 2018
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PSYC 427 – LECTURE 8
ACTIVITY OF MOTOR CORTEX CELL POPULATIONS
Georgopoulos proposed that movement in a particular direction was determined not by the action of single neurons but by the net action of a broad population of
cells. It is the aggregated net response of multiple cells that underlies properties of movement. This neural code was able to be better studied following advances in
technology, such as robot arms.
The oke’s task is to ake oeets i the hoizotal plae hile holdig a oot a
Above is a population vector analysis applied to eight movement directions in two-dimensional space.
• Each cluster represents the same population of cells
• The movement directions are shown at the center of the diagram.
• Population vector (dashed): points in or near direction of movement
Any neuron showing direction-related activity has a tuning curve with firing rates corresponding to movements in different directions
• Turning curves are pretty broad: not much firing at extremes but still a wide range within which cell activity can be seen
For this particular cell in the primary motor cortex, 135 degrees is when there is peak firing.
• The cell fires progressively less in adjacent directions away from the preferred direction of movement
Above is the schematic for how the population vector is constructed, based on firing rates of four different cells
• Dashed line: tuning curve
o The activities of motor cortical neurons tend to be broadly tuned to the direction of movement
find more resources at oneclass.com
find more resources at oneclass.com
• Solid line: preferred direction (PD)
• Length of line: firing rate
o For each movement direction, the activity of each cell scales the length of a vector aligned with its PD
• Bold line with arrowhead: population vector
o The vector sum of all cells defines a population vector
o Most contribution to this sum comes from cells with a PD that is congruent with actual movement direction
Dishage deeases as dietio of oeet is futhe aa fo the pefeed dietio of oeet. Fo eaple, the ed cell has a PD of 90 degrees.
• Thus, he the dietio of atual oeet is upads, thee legth of the ed line is longest (maximum firing rate).
• O the ota, the ed lie is ael isile he atual oeet is doads opposite to PD.
IN MULTI-JOINT MOVEMENTS, DOES MOVEMENT-RELATED ACTIVITY OF CELLS REFLECT FORCE REQUIREMENTS OF THE TASK, AS HAS BEEN SHOWN IN SINGLE-JOINT
MOVEMENTS?
The monkey was trained to reach in eight directions while external loads pulled the arm in one of these directions.
Left: activity in no-load condition
• Red lie: ell’s pefeed dietio of oeet
Right: effect of load direction on the discharge rate of a single cell in the motor cortex during movements in eight different directions
Dashed line: magnitude of discharge plotted as length of vector extending in the direction of executed movement
• Radius of circle: magnitude of cell activity while holding the arm at the central starting position before movement
• Brown area: area in which the cell shows activity
Findings:
The position of each polar plot (B) corresponds to the direction in which the load pulled the arm
• The ell’s fiig ate ieases i all dietios he the a is pulled ight.
• This rightward direction is the load axis of the cell, which is approximately opposite to the ell’s pefeed dietio
Conclusions:
• The directional tuning of the cell (PD) was relatively constant under different load conditions
• Cell is not coding direction of movement
o Movements in the same direction have different firing rates, depending on direction of load
• Cell is not coding direction of force applied to compensate for load
o Firing rate would always be greatest in direction opposite to the load; this is not the case
• Firig rate is the etor su of the ell’s diretioal tuig i oiatio ith fore to offset load
o The ell’s fiig ate is elated to the amount of force required to maintain an arm trajectory in a given direction, not just to the direction itself.
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Georgopoulos proposed that movement in a particular direction was determined not by the action of single neurons but by the net action of a broad population of cells. It is the aggregated net response of multiple cells that underlies properties of movement. This neural code was able to be better studied following advances in technology, such as robot arms. The (cid:373)o(cid:374)ke(cid:455)"s task is to (cid:373)ake (cid:373)o(cid:448)e(cid:373)e(cid:374)ts i(cid:374) the ho(cid:396)izo(cid:374)tal pla(cid:374)e (cid:449)hile holdi(cid:374)g a (cid:396)o(cid:271)ot a(cid:396)(cid:373) Above is a population vector analysis applied to eight movement directions in two-dimensional space. Each cluster represents the same population of cells. The movement directions are shown at the center of the diagram. Population vector (dashed): points in or near direction of movement. Any neuron showing direction-related activity has a tuning curve with firing rates corresponding to movements in different directions. Turning curves are pretty broad: not much firing at extremes but still a wide range within which cell activity can be seen.
