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

2. P OSTERIOR PARIETAL CORTEX 2 Bisley J.W. & Goldberg M.E. (2010). Attention, Intention, and Priority in the Parietal Lobe. Annual Review of Neuroscience. 33:1–21

3. O VERVIEW 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 goals 2. Responding to salient new information or alerting stimuli in the environment. Singh-Curry & Husain ’ s proposal of IPL function 3

4. D ORSAL –V ENTRAL VISUAL STREAM DICHOTOMY M ILNER & G OODALE (1995) 4 “Vision for action” “Vision for perception”

5. T HE POSITION OF THE IPL IN M ILNER & G OODALE ’ 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

6. R IZZOLATTI & M ATELLI ’ 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 6 Hemi-neglect Limb apraxia Direct homologies between all parts of macaque and human parietal cortex? hemispheric asymmetry, expanded IPS and IPL… Addresses apraxia and neglect x non-spatial components of neglect

7. C ORBETTA AND S HULMAN ’ S GOAL - DIRECTED AND STIMULUS - DRIVEN STREAMS (2002) Dorsal fronto- parietal network Top-Down, goal-directed Includes SPL, IPS, Dorsal frontal cortex 7 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!!

8. R OLE OF R IGHT IPL IN V IGILANCE & S USTAINING A TTENTION 8 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.

9. 11 studies Healthy subjects Detection of rare events at single locations/ CPT Various sensory modalities V IGILANCE & SUSTAINED ATTENTION : M ETA - ANALYSIS OF PET & FUNCTIONAL MRI STUDIES 9

10. V IGILANCE & 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

11. 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

12. T ARGET 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 patients Detection of target-related salience requires both ‘top-down’ (discriminating targets from non- targets) as well as ‘bottom-up’ processes ! 12

13. N OVELTY : 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

14. P HASIC 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

15. T HE 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

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

17. 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 17

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

19. S INGH -C URRY & H USAIN ’ PROPOSAL : R OLE OF R IGHT 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