THE FUNCTIONAL ROLE OF THE INFERIOR PARIETAL LOBE

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

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 2

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. 2. Maintaining attentive control on current task goals Responding to salient new information or alerting stimuli in the environment. Singh-Curry & Husain’s proposal of IPL function 3

DORSAL–VENTRAL VISUAL STREAM DICHOTOMY MILNER &GOODALE (1995) “Visio n for a ction” ” ption e c r e p r o f n io “Vis 4

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? 5

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 Hemi-neglect Right IPL - spatial perception & action Left IPL - action recognition, grasping & manipulation Limb apraxia Direct homologies between all parts of macaque and human parietal cortex? hemispheric asymmetry, expanded IPS and IPL… üAddresses apraxia and neglect x non-spatial components of neglect 6

CORBETTA AND SHULMAN’S GOAL-DIRECTED AND STIMULUS-DRIVEN STREAMS (2002) Dorsal frontoparietal network Top-Down, goal-directed Includes SPL, IPS, Dorsal frontal cortex Ventral frontoparietal network Bottom-Up, stimulusdriven Includes TPJ, IPL, ventral frontal cortex lateralised to the RH But – the IPL is implicated in sustained attention!! 7

ROLE OF RIGHT IPL IN VIGILANCE & SUSTAINING ATTENTION Vigilance Intensity continuum Sustained Attention Both require Top-down processes! Converging evidence comes from both functional imaging studies in healthy subjects as well lesion studies with RH patients. 8

VIGILANCE & SUSTAINED ATTENTION: META-ANALYSIS OF PET & FUNCTIONALMRI STUDIES 11 studies Healthy subjects Detection of rare events at single locations/ CPT Various sensory modalities 9

VIGILANCE & SUSTAINED ATTENTION: LESION STUDY 44 RH stroke patients, with or without neglect A non-spatial task of auditory sustained attention. 10 Robertson, I. H. et al. , Neuropsychologia, 1997

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) 11

TARGET RELATED SALIENCE: META-ANALYSIS 10 studies, oddball paradigm at single location, various sensory modalities Activation foci: RH IPL, IPS, TPJ & frontal regions Reduced P 3 in Neglect patients Detection of target-related salience requires both ‘top-down’ (discriminating targets from non-targets) as well as ‘bottom-up’ processes! 12

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! 13

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 14

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? 15

RECONFIGURATION: META-ANALYSIS In conjunction analysis only the RH IPL & ventral frontal cortex were activated. Buchsbaum, B. R. et al. , Human Brain Mapping, 2005 16

RECONFIGURATION &LC NA SYSTEM P 300 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 17

IPL & LC PPC receives sensory, motor, goal-related and reward information. Different types of information may compete LC Signals biasing the outcome 18

SINGH-CURRY &HUSAIN’ PROPOSAL: ROLE OF RIGHT IPL IN CONTROLLING BEHAVIOR Right IPL: Detection of salient stimuli in the environment Sustaining attention on task goals Flexibly 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. 19 Back to main