Sensorimotor systems Chapters 8
Three principles of sensorimotor function • hierarchical organization • Two other organizing characteristics? • motor output is guided by sensory input • The case of G.O. – darts champion • The exception? • learning changes the nature and locus of sensorimotor control
Posterior Parietal Association Cortex Function: Integrates information on the position of parts of the body and external objects to direct voluntary movement and attention. Sensory system inputs: visual, auditory and somatosensory. Outputs: dorsolateral PFC, secondary motor cortex and frontal eye fields.
Dorsolateral PFC Frontal eye field Auditory cortex Visual cortex Inputs to Posterior Parietal Association Cortex
Dorsolateral PFC Frontal eye field Auditory cortex Visual cortex Outputs to Posterior Parietal Association Cortex
Damage to the Posterior Parietal Association Cortex Can produce a variety of deficits • Perception and memory of spatial relationships • Reaching and grasping • Control of eye movements • Attention
Damage to the Posterior Parietal Association Cortex Apraxia – a disorder of voluntary movement not attributable to a simple motor deficit (weakness or paralysis) or to a deficit in comprehension or motivation. Results from unilateral damage to the left posterior parietal cortex.
Damage to the Posterior Parietal Association Cortex Contralateral neglect – a disturbance in a patient’s ability to respond to stimuli on the side of the body contralateral to a brain lesion (not a simple sensory or motor deficit). Often associated with large lesions of the right posterior parietal lobe.
Dorsolateral Prefrontal Cortex Function: plays a role in the evaluation of external stimuli and initiation of voluntary responses to those stimuli. Main input: posterior parietal cortex Outputs: secondary motor cortex primary motor cortex frontal eye fields
Dorsolateral Prefrontal cortex Neurons in this area respond to the characteristics of objects (e.g., color/shape), the location of objects or to both. The activity of other neurons is related to the response itself.
Secondary motor cortex Input: most from association cortex Output: primary motor cortex Two classic areas: • SMA • Premotor cortex
Secondary Motor Cortex Current classifications suggest • At least 8 different areas • 3 supplementary motor areas SMA, preSMA & supplementary eye field • 2 premotor areas PMd and PMv • 3 cingulate motor areas CMAr, CMAv&CMAd
Mirror neurons • Discovered in the ventral premotor cortex of the macaque (Rizzolatti et al., 2006) • Social cognition – knowledge of the perceptions, ideas and intentions of others http://video.pbs.org/video/1615173073/
Secondary Motor Cortex • Subject of ongoing research • May be involved in programming movements in response to input from dorsolateral prefrontal cortex • Many premotor neurons are bimodal – responding to 2 different types of stimuli (most common - somatosensory and visual)
Primary Motor Cortex • Precentral gyrus of the frontal lobe • Major point of convergence of cortical sensorimotor signals • Major point of departure of signals from cortex • Somatotopic – more cortex devoted to body parts which make many movements
Primary Motor Cortex • Monkeys have two hand areas in each hemisphere, one receives feedback from receptors in skin. • Stereognosis – recognizing by touch – requires interplay of sensory and motor systems • Damage to primary motor cortex • Movement of independent body parts (e.g., 1 finger) • Astereognosia • Speed. accuracy and force of movement
Other sensorimotor structures outside of the hierarchy (sometimes called extrapyramidal systems) • Cerebellum • Basal ganglia both modulate and coordinate the activity of the pyramidal systems by interacting with different levels of the hierarchy.
Cerebellum • 10% of brain mass, > 50% of its neurons • Converging signals from • primary and secondary motor cortex • brain stem motor nuclei (descending motor signals) • Somatosensory and vestibular systems (motor feedback) • Involved in motor learning, particularly sequences of movement • Damage to cerebellum – disrupts direction, force, velocity and amplitude of movements; causes tremor and disturbances of balance, gait, speech, eye movement and motor sequence learning .
Basal Ganglia • A collection of nuclei • Part of neural loops that receive cortical input and send output back via the thalamus (cortical-basal ganglia-thalamo-cortical loops) • Modulate motor output and cognitive functions • Cognitive functions of the basal ganglia
Descending Motor Pathways • Two dorsolateral • Corticospinal • Corticorubrospinal • Two ventromedial • Corticospinal • Cortico-brainstem-spinal tract • The corticospinal tracts are direct pathways
Ventromedial one direct tract, one that synapses in the brain stem More diffuse Bilateral innervation Proximal muscles Posture and whole body movement Dorsolateral Vs Ventromedial Motor Pathways Dorsolateral • one direct tract, one that synapses in the brain stem • Terminate in one contralateral spinal segment • Distal muscles • Limb movements
Experiments by Lawrence and Kuypers (1968) Experiment 1: bilateral transection of the Dorsolateral (DL) corticospinal tract Results: • monkeys could stand, walk and climb • difficulty reaching improved over time • could not move fingers independently of each other or release objects from their grasp.
Experiments by Lawrence and Kuypers (1968) Experiment 2: The same monkeys with DL corticospinal tract lesions received 1 of 2 additional lesions: • The other indirect DL tract was transected • Both ventromedial (VM) tracts were transected
Experiments by Lawrence and Kuypers (1968) Experiment 2 Results: • The DL group could stand, walk and climb but limbs could only be used to ‘rake’ small objects of interest along the floor • VM group had severe postural abnormalities: great difficulty walking or sitting. Although they had some use of the arms they could not control their shoulders.
Experiments by Lawrence and Kuypers (1968) Conclusions: • the VM tracts are involved in the control of posture and whole-body movements • the DL tracts control limb movements (only the direct tract controls independent movements of the digits.