PSYC 304 Motor Systems

Ingle’s frog study

• unilateral tectal lesions

  • tectum controls prey catching and predator avoidance

• lesioned frogs had no response to prey or threats in that visual field

  • after a few weeks, frog would tongue snap on the opposite side of field

  • this is because severed RGC axons, originally connected to the lesioned tectum, crossed the midline and connected to the intact tectum

conclusions of Ingle’s frog study

• retinotopic mapping was preserved after rewiring, but opposite field

• visuomotor behaivours are modular

  • different behaviours, eg: tongue-snapping vs locomotion use different brain circuits

  • behaviour is guided by specific visuomotor pathways

“what” and “where” model

• ventral stream: for object identity (“what”)

• dorsal stream: for object location "(“where”)

Goodale-Milner 2 systems model - “what/how”

• ventral stream: vision-for-perception

  • eg: object recognition and memory, “what”

• dorsal stream: vision-for-action

  • eg: guiding movements, “how”

evidence for Goodale-Milner “what/how” model

• Patient DF:

  • ventral stream damage: cannot identify objects (what)

  • could still use objects, so dorsal stream (how) was intact

• optic ataxia patients:

  • dorsal stream damage: leads to poor grasping

  • better performance with a delay, because ventral stream compensates for it

• Goodale et al

  • mid-flight hand adjustments were created to reach moving targets

  • readjustments happen without conscious awareness, showing automatic dorsal processing

visual perception vs visually-guided action - difference

• visual perception goals:

  • to build a lasting, conscious understanding of the world

• visually-guided action goals:

  • to guide fast, accurate motor responses in the moment

spatial reference frames for visual perception

• use allocentric frames, centering perception around an object or scene

• describes objects in relation to each other or envionrment

• supports object constancy, eg: recognizing a cup from any angle

spatial reference frames for visually-guided action

• use egocentric frames centered around one’s body

• describes objects in relation to the eyes, head, and hands

• crucial for accurate reaching, grasping, and navigation

real time vs enduring visual representation - visual perception

• builds durable, viewpoint-invariant representations

• integrates context and memory

  • eg: hippocampal connections in the ventral stream

• useful for recognition, planning, object naming

real time vs enduring visual representation - visually guided action

• requires real-time, continuously updating visual input

• tracks the object’s position, size, orientation - relative to the body

• enables fast, unconscious corrections (eg: mid-flight grasp adjustments)

M1

sends voluntary motor commands

upper motor neurons

originate in cortex, travels thru brainstem/spinal cord

lower motor neurons

originate in spinal cord/brainstem → synapse onto muscles

brainstem nuclei

control cranial nerves

spinal cord

control skeletal muscles via spinal nerve

corticospinal tract

pathway for voluntary movement, crosses at medulla

EMG

• measures electrical activity in muscles

• detects timing, strength, and coordination of conrtractions

reflex/movement observation

• test reflex arcs, eg: patellar reflex

• assess voluntary movement quality - based on speed, accuracy, coordination

ACh

• released at the NMJ

• causes muscle fibre contraction

central pattern generators (CPGs)

• spinal circuits produce rhythmic patterns without brain input

• seen in cats: spinal cord severed from brain, still able to walk on treadmill

• indicates a spinal “memory” for some motor movements

motor neuron divergence

• 1 motor neuron can diverge and innervate multiple fibres within a muscle

• allows for coordinated contraction a large, single muscle

neural control divergence

• one motor neuron activates multiple motor units/muscles

• allows for widespread, efficient activation of muscle groups

flaccid paralysis cause

lower motor neuron injury

flaccid paralysis symptoms

• lim muscles, no reflexes, muscle wastage

• reflex circuits cannot function

spastic paralysis cause

upper motor neuron injury

spastic paralysis symptoms

• stiff, overactive muscles, exaggerated reflexes

• brain cannot inhibit reflexes, leading to too much uncontrolled activity

pathway of pyramidal motor system from cortex → spinal cord

1. M1

   • upper motor neurons project thru the:

2. corona radiata

3. internal capsule

4. midbrain

5. pons

6. medulla (decussation of pyramids)

7. then, descend thru spinal cord

8. synapse onto lower motor neurons, leading to muscle

primary motor cortex

• executes voluntary movement

• codes for direction of movement and muscle activation

• somatotopically organized

• shows plasticity

non-primary motor cortex: regions

• supplementary motor area

• premotor cortex

supplementary motor area

• internally guided actions

• involve complex sequences, motor planning

  • eg: tying shoelaces

premotor cortex

• responds to visual and auditory cues

• plans externally-triggered actions

  • eg: catching a ball thrown at you

clinical applications of studying M1

• motor decoding via electrodes, and use algorithms to interpret neuron patterns

  • can control robotic limbs

• helps paralyzed individuals with prosthetic control

• stroke and rehab therapy thru neural plasticity mapping

mirror neuron

activates when you perform an action and observe someone else performing the same action

• found in premotor cortex, F5

mirror neurons - function

• support action understanding, and imitation

• provide neural basis for empathy

mirror neurons and autism

• individuals iwth autism have reduced mirror neuron activation

• difficulty mimicking facial expressions

• higher autism scores = less mirror neuron activity

• don’t know which causes which, ASD or mirror system