The Body In Space

The Body In Space

(a “constructed” modality)

How are our limbs, trunk, and head positioned?

Where is our body with respect to the world?

Where are objects with respect to our bodies?

Where are objects with respect to each other?

X-centric

• Egocentric – with respect to the perceiver

• Allocentric – with respect to some external

reference (“the world”)

• Retinocentric – eye centered

• Head centered, etc…

“frames of reference” or “coordinate frames”

sense of space

proprioception: the perception by an animal of stimuli

relating to its own position, posture, equilibrium, or

internal condition.

proprioceptor: sensory receptor, found chiefly in

muscles, tendons, joints, and the inner ear, that

detects the motion or position of the body or a limb by

responding to stimuli arising within the organism.

Definitions

disorder:

sensory ataxia

disorder:

cerebellar ataxia

Two systems that use position sense

• Dorsal Column /

Medial Lemniscus

• position sense

(and locomotion)

• “conscious”

• Cerebellum

• motor coordination

• “unconscious”

Ataxia: unsteady and clumsy movement of the limbs or trunk

due to a failure of the gross coordination of muscle movements

DC/ML System

Dorsal Column/Medial Lemniscus

Responses of VPL neuron to

knee angleTuning of VP thalamic neurons

to joint angle of the knee

From where do these signals come?

Joint Position Sense

• Two observations:

– Constant tug on tendons seemed to have no perceptual

effect

– Anesthetizing finger joints interrupted position sense

• Led to incorrect conclusion that joint receptors must

be responsible for conscious proprioception

Synovial joint (freely moveable)

Joint Receptors Are Not the Whole Story

• Joint receptors primarily discharge at extremes

• When joints are totally replaced by prostheses, joint

position sense is still available

• Muscle spindle inputs do contribute to our position

sense, but they must be appropriately stimulated

• Vibration of the tendon activates Ia afferents, which

leads to a sensation of limb position change

• Large diameter afferents project (via the DC/ML path)

to area 3a of sensory cortex

Muscle Spindles & Golgi Tendon Organs

monitor muscle tensionmonitor muscle length

Joint angle perception

Ferrell et al.

Responses of VPL neuron to

knee angleTuning of VP thalamic neurons

to joint angle of the knee

Sustained response component

Phasic response component

X

•Dorsal root section (dorsal

rhizotomy) eliminates

incoming sensory information

from the limb

Efference Copy

•How can a deafferented limb

be used if it cannot

be directly sensed?

Coordinate System Transformations

• Points in space can be referenced to many

different coordinate systems

– eye (eye-centered/oculocentric, retina/retinocentric)

– head (craniocentric; audition & vestibular)

– body (egocentric)

– world (allocentric)

• As we plan movements, information coded in

various coordinate systems must be unified

Coordinate System

Transformations

Parietal cortex

Eye position

Neck proprioception

Vestibular information

Visual information

Auditory information

Eye-centered coordinates

Head-centered coordinates

Body-centered coordinates

World-centered coordinates

Evidence?

Anderson experiment

Stimulus Stimulus

40 im

p/s

ec

Retina

Gaze

Position

Andersen & Mountcastle, 1983

Nature Neuroscience - 9, 1337 - 1343 (2006)

A human parietal face area contains aligned

head-centered visual and tactile maps

Martin I Sereno & Ruey-Song Huang

Aligned somatosensory and visual maps for a single subject

Somatosensory

(air puff)

Visual

Sensing “Space”

stereognosis:

recognition of objects through tactile exploration

(proprioception, somatic sensation, temperature)

astereognosis:

loss of the ability to recognize objects by touch

Anterior

Parietal

Cortex

Primary somatosensory deficits

• Decreased somatosensory sensitivity

• Sensory ataxia (“afferent paresis”)

• Other perceptual disorders– astereognosis

– asomatognosia – loss of a sense of one’s body

– finger agnosia – inability to point/identify fingers

Posterior

Parietal

Cortex

Anterior

Parietal

Cortex

Posterior Parietal Cortex

• receives projections from primary somatosensory cortex (S1)

• also receives projections from visual and auditory cortices, thalamus, and hippocampus: association cortex

More complex parietal deficits

• Balint’s syndrome (areas 5/7)– “sticky fixation”

– simultagnosia – can only attend to one item at a time

– optic ataxia – difficulty reaching for objects using vision

• Contralateral neglect (right parietal)

• Apraxias – loss of skilled movements (in absence of impaired primary motor function)– Ideomotor apraxia – inability to copy movements or gestures

– Constructional apraxia – impairment in assembling, building, drawing objects, copy figures

Hemi-neglect

Impairments in “constructional ability” (constructional praxis):

Ask patient to draw or copy line figures

Rey-Osterrieth Complex Figure Test

Ideomotor/Ideational apraxia:

Perform simple/complex series of movements

comb your hair/unlock a door, open it, walk through

“Dressing apraxia” – difficulty performing the spatial

arrangement of clothing with respect to the body axis

R

Blanke et al., Nature, 2002