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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
  • 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 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
vstm and brain
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
question
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?
method
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)
experiment 1
Experiment 1
  • Behavioral:
    • Simple object: hole? v.s. Complex object: outline?
experiment 11
Experiment 1
  • fMRI:
    • IPS:
    • LOC:
experiment 12
Experiment 1

Simple shape feature:

Complex shape feature:

experiment 13
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?
experiment 2
Experiment 2
  • Behavioral:Uniquely different simple and complex objects
experiment 21
Experiment 2
  • fMRI:
    • IPS:
    • LOC:
experiment 22
Experiment 2

Simple shape feature:

Complex shape feature:

experiment 23
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
experiment 3
Experiment 3
  • Whether brain activation observed reflect VSTM encoding, maintenance, or retrieval and comparison?
  • Set size 1, 2 or 4
experiment 32
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?
experiment 4
Experiment 4
  • Behavioral:
experiment 41
Experiment 4
  • fMRI:
    • IPS:
    • LOC:
experiment 42
Experiment 4

simultaneous off-centre:

sequential off-centre:

sequential centred:

experiment 43
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
conclusion
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.
conclusion1
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.
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