THE FUNCTIONAL ROLE OF THE INFERIOR PARIETAL LOBE IN THE DORSAL AND VENTRAL STREAM DICHOTOMYVictoria Singh-Curry & Masud Husain, 2009

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POSTERIOR PARIETAL CORTEX

Bisley J.W. & Goldberg M.E. (2010). Attention, Intention, and Priority in the Parietal Lobe. Annual Review of Neuroscience. 33:1–21

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OVERVIEW

Critical appraisal of the proposed functions of the (right) IPL in existing models of dorsal-ventral stream dichotomy.

Review evidence that shows the right IPL plays an important role in two different, but broadly complementary, aspects of attention: 1. Maintaining attentive control on current task

goals2. Responding to salient new information or

alerting stimuli in the environment. Singh-Curry & Husain’s proposal of IPL

function

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DORSAL–VENTRAL VISUAL STREAM DICHOTOMY MILNER & GOODALE (1995)

“Vision for action”

“Vision for

perception”

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THE POSITION OF THE IPL IN MILNER & GOODALE’S DORSAL–VENTRAL

DICHOTOMY

In humans, the D-S terminates in the SPL & IPS. The IPL isn’t part of the dorsal stream!

High-level spatial representational system that subserves perceptual awareness?

Explains object-centered neglect (rare!), but not impairments of egocentric spatial representation, directing attention or planning movements.

Non-spatial cognitive process in neglect?

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RIZZOLATTI & MATELLI’S TWO DORSAL STREAM MODEL (2003)

D-S is formed by 2 sub-systems: o Dorso-Dorsal stream

(≈ Milner & Goodale D-S, includes SPL)o Ventro-Dorsal stream

Right IPL - spatial perception & action

Left IPL - action recognition, grasping & manipulation

Hemi-neglect

Limb apraxia

Direct homologies between all parts of macaque and human parietal cortex? hemispheric asymmetry, expanded IPS and IPL…

Addresses apraxia and neglectx non-spatial components of neglect

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CORBETTA AND SHULMAN’S GOAL-DIRECTED AND STIMULUS-DRIVEN STREAMS (2002)

Dorsal fronto-parietal network

Top-Down, goal-directed Includes SPL, IPS,

Dorsal frontal cortex

Ventral fronto-parietal network

Bottom-Up, stimulus-driven

Includes TPJ, IPL, ventral frontal cortex

lateralised to the RH

But – the IPL is implicated in sustained attention!!

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ROLE OF RIGHT IPL IN VIGILANCE &SUSTAINING ATTENTION

Vigilance

Sustained

Attention

Intensity continuum

Both require Top-down processes!

Converging evidence comes from both functional imaging studies in healthy subjects as well lesion studies with RH patients.

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11 studies Healthy

subjects Detection of

rare events at single locations/ CPT

Various sensory modalities

VIGILANCE & SUSTAINED ATTENTION: META-ANALYSIS OF PET & FUNCTIONAL

MRI STUDIES

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VIGILANCE & SUSTAINED ATTENTION: LESION STUDY

44 RH stroke patients, with or without neglect A non-spatial task of auditory sustained

attention.

Robertson, I. H. et al., Neuropsychologia,1997

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SALIENCE DETECTION & PHASIC ALERTING

Salience: stimulus properties which make it stand out from the background. Novelty Behavioral or Target-related salience Phasic alerting – a warning of an event of a

behavioral significance

Not only bottom up! (as Corbetta & Shuman claims)

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TARGET RELATED SALIENCE: META-ANALYSIS

10 studies, oddball paradigm at single location, various sensory modalities

Activation foci: RH IPL, IPS, TPJ & frontal regions

Reduced P3 in Neglect patientsDetection of target-related salience requires both ‘top-down’ (discriminating targets from non-targets) as well as ‘bottom-up’ processes!

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NOVELTY: META-ANALYSIS 5 studies, oddball

paradigm Healthy subjects various sensory

modalities More posterior

activation; involves IPL, TPJ, ventral frontal lobe

Detection of novel events requires both ‘top-down’ (memory of target/previously seen stimuli is necessary) as well as ‘bottom-up’ processes!

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PHASIC ALERTING: META ANALYSIS Readiness to detect/ respond to

environmental changes occurring as a result of an exogenous warning stimulus

2 studies RH IPL, TPJ

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THE PROCESS OF RECONFIGURATION Task switching (rule shift, motor response) Study paradigms vary in processes required as

well as working memory load. A variety of bilateral frontal & parietal foci of

activation are found in such studies. What are the critical regions underlying

reconfiguration?

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RECONFIGURATION: META-ANALYSIS

In conjunction analysis only the RH IPL & ventral frontal cortex were activated.

Buchsbaum, B. R. et al., Human Brain Mapping, 2005

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RECONFIGURATION & LC NA SYSTEM

P300 is associated with task switching & with phasic activity of LC NA system

LC phasic activity: facilitates focused selective responding, goal-focused state

LC tonic activity: distractibility, exploration, responses to novel environment stimuli

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IPL & LC

PPC receives sensory, motor, goal-related and reward information.

Different types of information may compete LC Signals biasing the outcome

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SINGH-CURRY & HUSAIN’ PROPOSAL: ROLE OF RIGHT IPL IN CONTROLLING BEHAVIOR

Right IPL: Detection of salient stimuli in the

environment Sustaining attention on task goalsFlexibly switching between these modes via NA system

Lesions may exacerbate spatial biases

Left IPL: Functions underpinning limb praxis

IPL is a crucial node in the fronto-parietal system of allocating resources optimally.

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