PS263 – Chapter 8 The Sensorimotor System
Three Principles of Sensorimotor Function
1. Hierarchical Organization: Sensorimotor is directed by commands that
cascade down through the levels of a hierarchy – from the association
(highest) to the muscles (lowest) Information flows down
a. Advantage: Higher levels of the hierarchy are left free to perform
more complex functions (parallel – signals flow between & over
b. Functional Segregation: Each level of the system & company
hierarchies tends to be composed of different units, each of which
performs a different function.
2. Motor Output guided by Sensory Input: Eyes, organs of balance, and
receptors in skin monitor body’s responses & feed their information back
into the sensorimotor circuit.
a. Sensory Feedback plays an important role in directing the
continuation of the responses that produced it.
b. Ballistic Responses: not normally influenced by sensory feedback –
brief, all-or-none, high-speed movements (I.e., swatting a fly)
3. Learning Changes the Nature & Locus: During the initial stages of motor
learning, each response is performed under conscious control – after much
practice the indicidual responses become organized into continuous
integrated sequences of action that flow smoothly & are adjusted by sensory
feedback without conscious regulation.
Sensorimotor Association Cortex
At the top of the sensorimotor hierarchy, divided into two major areas:
1. Posterior Parietal Association Cortex: Posterior to the primary
somatosensory cortex, important role in integrating: directing behaviour by
providing spatial information and in directing attention; receives substantial
information, input from more than one system but from 3 systems (visual,
audio & somatosensory)
a. Output goes to areas of the motor cortex in the frontal cortex
(dorsolateral prefrontal association cortex to secondary motor cortex
and frontal eye field: small area of prefrontal, controls eye movement)
b. Damage deficits in perception & memory of special relationships,
accurate reaching and grasping, controlling eye movement & attention
c. Apraxia: Disorder of voluntary movement that is not attributable to a
simple motor deficit or to any deficit in comprehension or motivation.
d. Contralateral Neglect: Disturbance of a patients ability to response to
stimuli on the side of the body opposite to the brain lesion in the
absence of simple sensory or motor deficits – left side of world gone:
i. Egocentric Left: when patients tilt their heads, their field of
neglect is not normally tilted with it - Lesions on the right
posterior parietal lobe
2. Dorsolateral Prefrontal Association Cortex: Receives projections from the
posterior parietal cortex and sends them to areas of the secondary & primary
motor cortex as well as the frontal eye field. Plays a role in the evaluation of
external stimuli & initiation of voluntary reactions to them. PS263 – Chapter 8 The Sensorimotor System
a. Activity of some neurons depends on the characteristics of objects –
activity depends on the locations of objects & combo of both
(response rather than the object) Neurons fire until complete.
Secondary Motor Cortex
Areas that receive much of their input from the association cortex & send
their output to the primary motor cortex -- two areas visible on frontal lobe:
1. Supplementary Motor Area: Wraps over the top of the frontal love & extends
down its medial surface into the longitudinal fissure (SMA, preSMA, eye field)
2. Premotor Cortex: Runs in a strip from the supplementary motor area to the
lateral fissure (dorsal & ventral)
3. Cingulate Motor Area: In the cortex of the cingulate gyrus
These areas are thought to be involved in the programming of specific patterns
of movement after taking general instruction from dorso-prefrontal-cortex.
Mirror Neurons: Neurons that fire when an individual performs a particular
goal-directed hang movement of when she or he observes the same goal-
directed movement performed by another (early 1990s by Rizzolati)
Primary Motor Cortex
Located in the precentral gyrus of the frontal lobe – major point of
convergence of cortical sensorimotor signals and the major point of
departure of sensorimotor signals from cerebral cortex.
Somatotopic layout of the human primary motor cortex is referred to as the
motor homunculus (little man) Penfield, 1937
Stereognosis: process of identifying objects by touch (close eyes & touching)
Cerebellum & Basal Ganglia
Interact with different levels of the hierarchy and coordinate & modulate its
activities – the interconnections are thought to be the reason why damage to
cortical connections between visual & frontal motor does not abolish visually
Cerebellum: Receives information from primary & secondary cortex,
information about descending motor signals from brain stem motor nuclei
and feedback from motor responses via the somatosensory & vestibular.
Major role in motor learning – sequences of movements.
o Damage Patient loses the ability to control direction, force, velocity
and amplitude of movements & the ability to adapt patterns of motor
output to changing conditions.