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 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 Areas of secondary motor cortex Outputs to secondary motor cortex, primary motor cortex, and frontal eye field Ventrolateral prefrontal association cortex
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 1
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.
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 Primary Motor Cortex
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.
M1 controls the execution of voluntary movements. Coding of Movement in M1 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)
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 Mirror Neurons