Click to jump to Larger Figure of Time-Frequency panels

Oscillatory Activity in Visual Short-Term Memory (VSTM)

Title: Oscillatory Activity in Parietal and Dorsolateral Prefrontal Cortex During Retention in Visual Short-Term Memory: Additive Effects of Spatial Attention and Memory Load
Authors: Grimault, S., Robitaille, N., Grova, C., Lina, J.-M., Dubarry, A.-S., & Jolicoeur, P. (2009).
Journal: Human Brain Mapping, 30, 3378-3392.

A layman's summary of this work appears in mauve boxes; see also [PDF]. The abstract, main figures, and conclusion are included below.

Click to jump to Figure of Source Localization
Figure: Time-frequency Results
Figure: Mean phase synchrony maps
Figure: Group analysis projected onto a brain template
Conclusions             Glossary

This work used time-frequency methods (also known as wavelet analysis, see graph below) to show that alpha oscillations (8-15 hertz) increased in amplitude as memory load increased during VSTM. A novel method was devised to localize the sources of brain activity, with the parietal cortex as the most important locus.

Fig.1

The figure above shows a sample trial sequence from the experiment. The task was to indicate if the disk (in the second to last "probe" presentation) on the side indicated by the initial arrows was in the same color as the disk presented initially at that location. The trial conditions were left/right, load (2 or 4 disks per side) and same/different.

In the four panels below, the horizontal scale is in Seconds, the vertical scale is in hertz (frequency of oscillation). The power at the particular frequency is indicated by the colour. A clear time-frequency pattern emerges.

Fig.3
Time-frequency Results

Unexpectedly, the analyses 'revealed synchronized activity at distant sensor locations... Synchrony maps were calculated for two seed sensors (MLP33, MRP33) ..., indicating the phase-coherence between these sensors and all other sensors.' In layman's terms, this is evidence for communication between brain regions.

Fig.5
Mean phase synchrony maps

The figure at the left shows those areas where the effect of load (2 versus 4 disks) was statistically significant.

Fig.6AB

Group Analysis
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Small Fig5

Conclusions

In the present study we combined the methods used by Vogel and Machizawa [2004] with time-frequency analysis techniques. We found activation during the retention period, in the 8–15 Hz band in parietal regions (inferior IPS, IOS) that increased as VSTM load increased. Activations in IPS and IOS provided converging evidence for a special role of this region for retention in VSTM [Todd and Marois, 2004]. The load effect on a-band power was not modulated by other factors, however (hemifield of stimulus encoding, hemisphere of the sensors). Importantly, we also found oscillatory activity in DLPFC that varied in amplitude with VSTM load. This activity was synchronized with the activity in inferior IPS/IOS, suggesting that DLPFC and inferior IPS/IOS may be important nodes in a network implementing VSTM functions in the human brain. We also found a decrease in power during the retention period, in the 8–15 Hz band in parietal regions, contralateral to the hemifield from which stimuli were encoded that was statistically independent from load-related effects, suggesting a possible dissociation between attentional control and VSTM.


The data for this work was also the subject of the paper by Robitaille et al, 2009.

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Last update: August 31, 2010.