Developmental Change in Networks Supporting Visual Short-term Memory

In a recent issue of the Journal of Cognitive Neuroscience, authors Sherf, Sweeney, and Luna describe how a simple memory-guided visual saccade task can draw out developmental differences in the recruitment of a variety of frontal and parietal regions. In their task, 30 subjects (9 children with mean age 11.2; 13 teenagers with mean age 16, and 8 adults with mean age 29.5) crawled inside an fMRI scanned to complete a simple memory task, in which they viewed a display containing a target, would have to return their eyes to the center of the screen, and then - after a 5 second delay - to gaze in the location previously occupied by the target. In baseline trials, subjects instead had merely to gaze at a target, for which no visual short-term memory was required.

By subtracting the BOLD signal in the baseline task from that in the memory-guided saccade task, the authors were able to isolate the regions specifically active in visual short term memory. (Note to the casual reader: you may wish to skip the italicized text below).

By focusing their analyses on 22 regions-of-interest (areas that have been implicated in visual short-term memory previously), the authors determined that all age groups recruited right DLPFC, right ACC, bilateral anterior insula, right superior temporal gyrus (STG), right interoccipital sulcus (IOS), and right basal ganglia. However, the amount of activation in each of these regions was related to age, as follows:

• Both right basal ganglia (BG) as well as bilateral anterior insula (AI) activity declined with age, with BG activity decreasing most sharply between childhood and adolescence, and the AI activity decreasing most sharply between adolescence and adulthood. BG activation was also negatively correlated with performance.
  
• Both right ACC and right IOS activity increased with age.
  
• The right and left DLPFC showed a dissociation in their relationship to age, such that left DLPFC increased greatly with age (most sharply between adolescence and adulthood), but right DLPFC was increased only for the adolescents.
• Right STG showed the opposite trend as right DLPFC, such that right STG was least activated in adolescents, relative to children and adults.

Yet other regions were activated only in older age groups. For example, children did not appear to use left DLPFC or left supramarginal gyrus, while the other age groups did (the inferior parietal activity in supramarginal gyrus, but not activity in DLPFC, was associated with better performance). The reverse trend was never found - in other words, children never recruited regions that were not also recruited to some extent by their older counterparts.

The authors describe how their results support a trend from the extensive use of basal ganglia in childhood to increasing reliance on prefrontal cortex with age. They also note their results are consistent with the idea that children rely more on ventral pathways than the dorsal stream - even in primarily spatial tasks like this one - perhaps as a result of the longer developmental timeframe for the dorsal pathway. Parts of the dorsal stream only become recruited by this task in adolescence.

Although DLPFC activity was highest among adolescents, which is confusing given the established role for DLPFC in working memory tasks as well as its relatively long developmental timeline, it appears that DLPFC activity actually became more focal in adults relative to adolescents, perhaps reflecting increased specialization and/or pruning.

Finally, the increasing use of ACC regions with age may reflect the increasing use of self-monitoring and error-checking practices among older subjects (in fact, activity in this region increased by 400% between adolescents and adults).

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