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.

• Capacity is variable and modulated by the complexity of visual objects encoded.

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

• 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

• 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

• A series of behavioral + fMRI experiment• Capacity• Behavioral: Cowan’s K• fMRI: ROI activity along task

• Behavioral : • Visual object recognition

with different set size (1, 2, 3, 4 or 6)

Experiment 1

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

Experiment 1

• fMRI:• IPS:

• LOC:

Experiment 1

Simple shape feature:Complex shape feature:

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

• Behavioral:Uniquely different simple and complex objects

Experiment 2

• fMRI:• IPS:

• LOC:

Experiment 2

Simple shape feature:Complex shape feature:

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

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

Experiment 3

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

• Behavioral:

Experiment 4

• fMRI:• IPS:

• LOC:

Experiment 4

simultaneous off-centre:sequential off-centre:sequential centred:

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

• 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.

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.

The End