Dissociable neural mechanisms supporting visual short term memory for objects
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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

Dissociable neural mechanisms supporting visual short-term memory for objects

Xu, Y. & Chun, M. M. (2006) Nature, 440, 91-95


Introduction in behavioral
Introduction – in behavioral memory for objects

  • 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.


Introduction in neuro network
Introduction – In memory for objectsneuro-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


Vstm and brain
VSTM and brain memory for objects

  • 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


Question
Question memory for objects

  • 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?


Method
method memory for objects

  • 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)


Experiment 1
Experiment 1 memory for objects

  • Behavioral:

    • Simple object: hole? v.s. Complex object: outline?


Experiment 11
Experiment 1 memory for objects

  • fMRI:

    • IPS:

    • LOC:


Experiment 12
Experiment 1 memory for objects

Simple shape feature:

Complex shape feature:


Experiment 13
Experiment 1 memory for objects

  • 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?


Experiment 2
Experiment 2 memory for objects

  • Behavioral:Uniquely different simple and complex objects


Experiment 21
Experiment 2 memory for objects

  • fMRI:

    • IPS:

    • LOC:


Experiment 22
Experiment 2 memory for objects

Simple shape feature:

Complex shape feature:


Experiment 23
Experiment 2 memory for objects

  • 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


Experiment 3
Experiment 3 memory for objects

  • Whether brain activation observed reflect VSTM encoding, maintenance, or retrieval and comparison?

  • Set size 1, 2 or 4


Experiment 31
Experiment 3 memory for objects


Experiment 32
Experiment 3 memory for objects

  • 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?


Experiment 4
Experiment 4 memory for objects

  • Behavioral:


Experiment 41
Experiment 4 memory for objects

  • fMRI:

    • IPS:

    • LOC:


Experiment 42
Experiment 4 memory for objects

simultaneous off-centre:

sequential off-centre:

sequential centred:


Experiment 43
Experiment 4 memory for objects

  • 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


Conclusion
Conclusion memory for objects

  • 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.


Conclusion1
Conclusion memory for objects

  • 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.


The end
The End memory for objects


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