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Dissociable neural mechanisms supporting visual short-term memory for objects

Dissociable neural mechanisms supporting visual short-term memory for objects. Xu, Y. & Chun, M. M. (2006) Nature , 440 , 91-95 . Introduction – in behavioral. VSTM capacity is limited: up to 4 objects When complexity increases, the capacity drops.

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Dissociable neural mechanisms supporting visual short-term memory for objects

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  1. Dissociable neural mechanisms supporting visual short-term memory for objects Xu, Y. & Chun, M. M. (2006) Nature, 440, 91-95

  2. Introduction – in behavioral • VSTM capacity is limited: up to 4 objects • When complexity increases, the capacity drops. • Capacity is variable and modulated by the complexity of visual objects encoded.

  3. Introduction – In neuro-network • VSTM: • Frontal/prefrontal • control and maintenance, increase with memory load • Intra-parietal sulcus (IPS) • correlate most strongly with memory load • Other parietal regions • Occipital regions

  4. VSTM and brain • Lateral occipital complex (LOC) : • Higher activation for objects, object recognition • Correlated with object retaining success? • Inferior IPS: • Parietal attention mechanism: visual attention toward objects • Spatial information • The role in maintaining visual objects? • Superior IPS

  5. Question • Whether VSTM capacity is limited to a fixed number of objects or whether it is variable? • What is the relationship between memory behavior and brain?

  6. method • A series of behavioral + fMRI experiment • Capacity • Behavioral: Cowan’s K • fMRI: ROI activity along task • Behavioral : • Visual object recognitionwith different set size (1, 2, 3, 4 or 6)

  7. Experiment 1 • Behavioral: • Simple object: hole? v.s. Complex object: outline?

  8. Experiment 1 • fMRI: • IPS: • LOC:

  9. Experiment 1 Simple shape feature: Complex shape feature:

  10. Experiment 1 • Whereas activations in the inferior IPS tracked a fixed number of objects regardless of object complexity, those in the superior IPS and LOC followed the actual number of objects held in VSTM as object feature complexity changed. • Potential grouping strategy: only encoding and remembering the hole-present shapes without retaining features from the other shapes?

  11. Experiment 2 • Behavioral:Uniquely different simple and complex objects

  12. Experiment 2 • fMRI: • IPS: • LOC:

  13. Experiment 2 Simple shape feature: Complex shape feature:

  14. Experiment 2 • Even after grouping cues were removed, the results of the second experiment mirrored those of the first experiment. • The lower VSTM capacity for the complex objects was due to perceptual processing limitations rather than memory limitations? • Encoding time for 4 objects was 200 or 500 ms • 4 objects were presented simultaneously for 200 ms or simultaneously 2 at a time for 200 ms with 500 ms blank between

  15. Experiment 3 • Whether brain activation observed reflect VSTM encoding, maintenance, or retrieval and comparison? • Set size 1, 2 or 4

  16. Experiment 3

  17. Experiment 3 • Brain activation observed in the first two fMRI experiments mainly reflected activations during VSTM encoding and maintenance. • Whether LOC and IPS activations during VSTM tasks track object identity or simply the locations occupied by objects in the display and in memory?

  18. Experiment 4 • Behavioral:

  19. Experiment 4 • fMRI: • IPS: • LOC:

  20. Experiment 4 simultaneous off-centre: sequential off-centre: sequential centred:

  21. Experiment 4 • LOC: • represents the visual objects held in VSTM and some object location information • Inferior IPS: • More spatial in nature • Indexing a fixed number of objects by means of their location (even when the encoding of spatial location is not required) • Superior IPS: • Both object identity and some location information

  22. Conclusion • Dissociable neural mechanisms in the superior and the inferior IPS and the LOC. • All three parts of the brain work in parallel to support VSTM during encoding and maintenance. • Inferior IPS representations are limited by a fixed number of objects at different spatial locations.

  23. Conclusion • LOC and superior IPS are not limited by a fixed number of objects, but rather by object complexity and the amount of visual information encoded. • LOC and superior IPS: detailed representation of visual objects in VSTM during both encoding and maintenance. • VSTM capacity is determined both by a fixed number of objects and by object complexity.

  24. The End

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