Ch. 14 : The Parietal Lobes & Networks

Functional Regions of the Parietal Lobe

parietal lobe can be subdivided into multiple functional regions: post central gyrus (primary somatosensory cortex), posterior parietal cortex, angular gyrus, precuneus, & supramarginal gyrus

• interior parietal lobe consists of the supra marginal & angular gyrus

• parietal cortex, particularly the inferior portion, has expanded during human evolution + has a homunculus

  • Broadmann & von Economo both mapped the parietal cortex based on cytoarchitecture, but disagree on boundaries

• Precuneus = inside region of the parietal lobe, next to the postcentral gyrus

3 sections:

1. Anterior : sensorimotor (close to post central gyrus)

2. central : cognitive

3. Posterior : visual

Connections of the Parietal Cortex (parietal lobe)

→ somatosensory areas of the postcentral gyrus (in parietal lobe) project to secondary somatosensory areas in the parietal lobes & motor planning and motor control areas in the frontal lobe

Area PE = secondary somatosensory area that projects to motor areas to guide movement by providing info abt limb position (kinaesthetic info)

Area PF = receives input from somatosensory areas VIA PE → projects to motor areas

• receives info from primary & PE (backup for PE)

Area PG = integrates info from visual, somatosensory, auditory, vestibular, & oculomotor systems WITH cognitive input from the cingulate to control spatially guided behaviour

• gets info directly from PRIMARY somatosensory (not PE or PF) & integrates, thus more important

  • related to V5 of occipital

• combines everything to make a whole picture

• NOT a secondary area, its an assoc. areas bc its integrating

• more complex - sends to frontal, temporal & motor

Anatomy of the Dorsal Stream in the Parietal Lobe

dorsal visual stream : contains info abt how/where for action

Parietal Lobe DORSAL stream: composed of 3 proposed pathways

1. Parieto-Premotor pathway = primary “how” path for motor control

• planing how we will move

2. Parieto-Prefrontal pathway = involved in working memory for visuospatial objects

• remembering what movements to do, in order to plan - “creating muscle memory”

3. Parieto-Medial-Temporal pathway = protects to the hippocampus & parahippocampal regions in temporal lobe - suggested importance in spatial recognition & navigation

• bc temporal is part of the “what” ventral pathway for vision, thus helps with navigation

→ posterior parietal cortex is important for visuospatial behaviours

→ more ventral regions of parietal cortex are involved in perceptual fxns

Theory of Parietal Lobe’s Function

Theory: Parietal lobe’s main goal is controlling movement

• anterior region (front part) : processes somatosensory info (info from body senses)

• posterior region (back part) : integrates somatosensory info & visual info to control movement

→ parietal lobe is involved in creating a multisensory map of the world around us to enable effortless interactions with the world

• consists of multiple maps - each serving a specific behavioural need, thus diff ppl can have diff types of maps

Parietal Lobe’s Functions: Sensorimotor Transformation

sensorimotor transformation = integration of movement intention with sensory feedback abt how the intended movement compares to the actual movement to perform smooth movements twds the target

• similar to the cerebellum, but cerebellum is more gross motor whereas this is more fine movements

Area PRR = involved in motor planning by encoding the desired outcome of the movement

• damage to this area can alter how we move

• recordings (electrical activity) from PRR can be used to control a prosthetic device/limbs

ex. moving with the mind study:

• rewarded if the monkey’s brain activity indicates they’re PLANNING to reach for the green triangle cue, don’t have to physically reach

  • thus, rewarding for a certain thought process rather than physical behaviour

  • idea that them thinking abt the movement can control a prosthetic

    

Parietal Lobe’s Functions: Spatial Navigation

→ to navigate the world we need an internal map of the world & a list of what to do at each decision point in the route

• info abt navigation is likely to be delocalized - thus info abt a single map is prob NOT stored together so that damage to one area wont destroy everything

• research suggests the medial-parietal region is important for route knowledge in humans

  • cells are active when a specific movement is made at a particular location (ex. diff cells activating based on a decision made along the route)

  • cells control body movements to specific locations (ex. using your leg at the gym vs. walking)

→ if the medial-parietal region is inactivated in a monkey: animal gets lost & CANT navigate correctly

THUS, the parietal lobe if more about goal/fxn of movement, rather than body area homunculus

Parietal Lobe’s Functions: Complexity of spatial information

→ posterior-parietal damage impairs the ability to tell left from right + the mental manipulation of objects

background:

• control of some movements (limbs, eyes) is fairly simple

• defining left and right is more complex + depends on frame of reference

• manipulation of mental images can be considered application or extension of manipulating objects with ur hands - THUS, same cells are active in brain as tho you were physically manipulating them

non-notes to support this: the posterior region (back part) of the parietal lobe integrates somatosensory info & visual info to control movement, thus when damaged it cannot do this

Other Parietal-Lobe Functions

1. Arithmetic

• some pts w parietal-lobe dysfunction have a condition known as Acalculia ( inability to perform calculations )

  • may be bc math can be interpreted as having a spatial component that would require the parietal lobe (ex. having to borrow in a subtraction problem)

2. Language

• pts w posterior-parietal damage may have language difficulties bc the spatial organization of letters in words & words in a sentence matters

  • can result in symptoms of dyslexia

3. Movement Sequences

• posterior-parietal damage makes it difficult to copy observed movements

  • bc mirror neurons are involved

Parietal Lesions: Somatosensory Symptoms

→ damage to the postcentral gyrus (somatosensory cortex) & adjacent posterior-parietal cortex results in somatosensory symptoms

ex. not feeling when you touch something

postcentral gyrus damageresults in:

• high sensory thresholds (need increased stimulation to perceive)

• impaired ability to sense position

• impaired stereognosis (struggle to identify objects by touch)

w afferent (twd CNS) paresis, postcentral gyrus damage results in:

• loss of feedback abt limb positions → causing clumsy movements

→ bc we’re NOT receiving feedback due to afferent paresis (not sent to CNS)

Parietal Lesions: Somatoperceptual Disorders

Astereognosis = loss of the ability to identify an object by touch

• with damage to secondary somatosensory cortex

ex. present pt with 2 objects at same time and remove them at same time (simultaneous extinction) → ask how many objects they held

• pts w damage to secondary somatosensory cortex notice & only report holding ONE object (didn’t identify the other by touch)

Numb Touch = reports loss of sensation from a region, BUT can still accurately report where they were touched in that region

• consciously CANT feel, but physically can

  • somatosensory equivalent of blind sight

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tests of tactical perception of objects:

• test A : blindfolded & have to identify what pattern of block was in their hand using only touch

• test B : have to identify which 2 objects are the same using only touch

Parietal Lesions: Somatosensory Agnosias

Asomatognosia = pt loses knowledge abt their own body or condition (what their body currently feels)

types:

• Anosognosia = unawareness of illness (dont realize they’re sick)

• Anosodiasphoria = indifference to illness (know they’re sick, but DODNT care)

ex. wont take medicine bc they dont care abt their health

• Autopasgnosia = inability to locate & name body parts on themselves and/or others

  • Fingeragnosia = more common in kids w spinibifida or if they struggle w #’s

• Asymbolia for pain = lack of typical avoidance rxns to pain

  • can feel & react to pain, but don’t learn to avoid it after

Parietal Lesions: Balint Syndrome

Balint syndrome = rare condition requiring pt to have all 4:

1. normal vision + can recognize and use objects, pictures, and colours

2. can move their eyes, but CANT fixate on a target

3. has simulatagnosia

4. has optic ataxia & is unable to reach a specified target using visual guidance

- lack dorsal stream, thus CANT incorporate vision & movement

→ results from BILATERAL parietal lesions (both sides)

Parietal Lesions: Contralateral neglect

Contralateral neglect = pts neglect the LEFT side of their own body & visual field (bc its controlled by R-hemis)

• symptoms:

  • only copy right side of a picture

  • only dress right side of body

  • only read the right half of compounded words (ex. read cream in the word “ice cream” or read ball in “football”)

  • ignore tactile stimualtion to L-body (ex. wont react to being tickled on the L-side of body)

• usually dont know there is anything wrong

• can recover by looking around & starting to respond to stimuli on neglected side

  • start to neglect both sides is untreated or treated incorrectly

causes:

• results from RIGHT parietal stroke (right parietal lesion)

  • can also occur w damage to frontal lobes, cingulate cortex or subcortical regions - most common w damage to parietal tho

• can be induced in healthy ppl by applying TMS to the Right intraparietal sulcus & angular gyrus

why?

• neglect may be due to impaired sensation & perception (parietal lobe damage) OR impaired attention (frontal lobe/cingulate damage)

Parietal Lesions: Effect on Object Recognition

→ pts w POSTERIOR parietal damage : may have difficulty recognizing objects in unfamiliar views/orientations

• bc: difficulty mentally rotating the image to match the traditional depiction w this damage

Parietal lesions: LEFT Parietal Syndromes & symptoms

Gerstmann syndrome = pt experiences finger agnosia, left-right confusion, agraphia (CANT write), & acalculia

• results from damage to LEFT parietal lobe near area PG

→ other LEFT parietal lesions are associated with these symptoms:

1. difficulties writing, reading & grammar

2. apraxia

3. dyscalculia

4. decreased digit span in verbal working memory (now 5± 2 instead of 7 ± 2)

5. difficulty with left-right discrimination

Parietal lesions: Apraxia

Apraxia = loss of skilled movement

types:

1. Ideomotor apraxia = unable to copy movements made by other ppl

• results from LEFT parietal lesions

ex. can do the dance, but cant copy others doing it

2. Construction apraxia = issues w spatial organization + unable to assemble puzzles, draw pictures, or copy facial movements

• struggle to construct a whole image, but can see individual pieces

• results from right or left POSTERIOR parietal damage

Testing for Apraxia

• serial arm-movement copying test = asked to copy movements - degree they’re able to copy shows degree of ideomotor apraxia

  • assesses ideomotor (copying movements)

• serial facial-movement copying test = asked to copy facial movements

  • assesses ideomotor (copying) & construction (copy facial movements)

  • after this you do another test to see if its ideomotor or construction apraxia

Parietal Lesions: Effect on Drawing

→ damage to either hemisphere of parietal love causes impairments in drawing ability

• more severe impairment w damage to RIGHT parietal

findings:

• Left parietal damage : make fewer recognizable drawings + use fewer lines

  • bc:

    • damage to L-hemisphere damages lang & can make a mental list of what to draw

    • damaged feedback to correct/refine fine motor for drawing

ex. drawing a stick figure, while intact brain would draw more detail

• Right parietal damage : tend to neglect the LEFT side of the image

note: drawing assessments tell us a lot abt brain fxn (damage in any lobe)

Parietal Lesions: Disorders of Spatial Cognition

→ damage to the Left & right parietal lobes results in deficits in spatial cognition (ex. poor mental rotation)

• bc: tasks like mental rotation require the formation of mental images AND the manipulation of that image

ex. w parietal damage u struggle to picture route from CCIS to SUB

findings:

• Left parietal damage: may impair formation of the mental image

  • suggests L-parietal is involved in formation

• Right parietal damage: may impair the manipulation of mental images

  • suggests R-parietal is involved in manipulation

Other Functions of Parietal Lobe: Spatial attention

→ research suggests the parietal cortex many fxn to shift attention b/w stimuli - the ACT of shifting, NOT deciding when to switch

• bc: shifting attention involves resetting the visuomotor guidance system associated with the parietal lobe b/w targets

→ explains why contralateral neglect occurs w parietal damage - cant shift attention to other side

Comparing the Left & Right Parietal Lobes

→ there are differences in the symptoms experienced by pts w Left vs Right parietal lesions

THUS, some asymmetries are predictable

• overlapping of some symptoms suggests functions of the parietal hemispheres aren’t completely lateralized (integration theory)

  • these overlapping symptoms may be due to the preferred cogntive mode of the pt

findings:

• RIGHT-hemis of parietal :

  • dominance in cube counting & paper cutting

  • more r-parietal lesion pts w unilateral neglect, dressing disabilities, & topographical loss

• LEFT-hemis of parietal:

  • dominance in right-left discrimination & wiggles sorting test

Major Parietal Lesions symptoms & their Assessment

→ lesions localized to a single brain area are rare, so symptoms dont usually present clearly bc multiple areas are damaged

findings:

• lesions resulting in symptoms that involve multiple senses tend to occur at PG

• areas for movement (reaching, eye movements, tactile agnosia) tend to occur BEFORE PG, thus at PE and/or PF

  

Assessment: Testing for Areas of the Parietal lobe damaged

Test 1 = identifies if the parietal is damaged

• tests somatosensory threshold & tactile form recognition

Test 2 = identifies if damage is to R-hemis parietal

• tests for contralateral neglect & visual perception

Test 3 = identifies if damage is to L-hemis parietal

• tests spatial relations, language, & apraxia

Assessment: Test 1

assessment tests 1 = identifies is parietal lobe is damaged

• increased somatosensory threshold & poor tactile form recognition indicate parietal lobe damage

→ Somatosensory threshold: blindfolded subject reports whether they felt 1 or 2 touches

• 2 points are initially set 3cm apart & distance is reduced until they only detect 1 touch

→ Tactile form recognition: blindfolded subject manipulates blocks & has to place them into a matching hole shape + shapes are removed → asked to draw shapes from memory

• 2nd part goes beyond somatosensory, thus requires integration of other areas info

  • shape manipulation likely involves areas PE & PF

  • drawing task likely involves area PG

→ increased somatosensory threshold (requires increased stimulation to perceive) or poor tactile form recognition indicates parietal lobes is damaged

Assessment: Test 2

assessment test 2 = indicates if damage is on R-hemis parietal (can also be on left, but its rare)

• contralateral neglect & poor visual perception indicate R-parietal damage

→ Contralateral neglect: subjects need to draw a vertical line in the middle of a horizontal line where theres lines of L-side and R-side of page (side they can recognize)

• subjects w contralateral neglect: shift the horizontal line more to the right & skip the examples on L-side of page

  • indicates neglect of L-side, thus R-hemis parietal damage

→ Visual perception: pts have to draw the remainders of incomplete faces/objects

• sensitive to damage on R-hemisphere temporoparietal junction (for ability to indemnify object & its manipulation)

  • if unable to : indicates damage to R-hemisphere temporoparietal junction

→ neglect of L-side of body/visual field or inability to draw the remainders of faces/obhects (poor visual perception) indicates RIGHT parietal lobe is damaged

Assessment: Test 3

assessment test 3 = indicates if damage is on L-hemis parietal

• poor spatial relations (left-right distinction), poor language & apraxia (loss of skilled movement) indicate L-parietal damage

→ Spatial relations: have to identify diff orientations of body parts in space as left or right (identify L&R on others) OR ask pt to touch their right ear w left damage (identify L&R on themselves)

• sensitive to L-parietal damage & L-frontal damage

  • thus, if unable to: indicates L-parietal damage & L-frontal damage

→ Language: asked to touch certain colour ( CAN do w L-damage), touch blue circle & red triangle (CANT do w L-damage, bc they mix up the colour & shape info)

• impairments are assoc w L-hemis area PG damage

→ Apraxia: doesn’t have a standardized test

ex. Kimura Box test = asks subject to make a sequence of precise movements

• if they cant = apraxia

→ poor spatial relations (L&R distinction), mixing up language info to identify objects & apraxia (loss of skilled movement) indicates LEFT Parietal damage

Parietal Lobe Networks

→ fMRI studies suggest the parietal lobe is involved in more behaviours than predicted from lesion studies

• see more deficits in humans than using animal models, bc lesioning animals doesn’t show how complex our parietal lobe is

• (-) : fMRI studies DONT show all the areas involved in a task, unlike lesion studies

Temporoparietal junction = involved in attention, lang, memory, social processing & self-perception

research suggests 5 diff networks within this junction:

• R-side: favours attention

  • if damaged: contralateral neglect (displayed by fMRI)

• L-side: favours memory & language

Inferior-Parietal lobe (IPL) = connects & communicates with temporal lobe

• connected to STS pathway

Networks of Parietal lobe

1. Default mode network (always on): inferior parietal lobule is active

2. Parietal Memory network = involved in learning & memory (watching something happen → become able to do that thing)

• includes: precuneus, angular gyrus & midcingulate cortex

3. Dorsolateral parietal network = performs a variety of spatial fxns by connecting prefrontal cortex, temporal cortex & hippocampus

• this network is involved in mental map & navigating