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

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