Top-down control of MEG alpha-band activity in children performing Categorical N-Back Task

Abstract

Top-down cognitive control has been associated in adults with the prefrontal-parietal network. In children the brain mechanisms of top-down control have rarely been studied. We examined developmental differences in top-down cognitive control by monitoring event-related desynchronization (ERD) and event-related synchronization (ERS) of alpha-band oscillatory activity (8-13 Hz) during anticipation, target detection and post-response stages of a visual working memory task. Magnetoencephalography (MEG) was used to record brain oscillatory activity from healthy 10-year-old children and young adults performing the Categorical N-Back Task (CNBT). Neuropsychological measures assessing frontal lobe networks were also acquired. Whereas adults showed a modulation of the ERD at the anticipatory stages of CNBT and ERS at the post-response stage, children displayed only some anticipatory modulation of ERD but no ERS at the post-response stage, with alpha-band remaining at a desynchronized state. Since neuropsychological and prior neuroimaging findings indicate that the prefrontal-parietal networks are not fully developed in 10-year olds, and since the children performed as well as the adults on CNBT and yet displayed different patterns of ERD/ERS, we suggest that children may be using different top-down cognitive strategies and, hence, different, developmentally apt neuronal networks.

Figure 1

Experimental paradigm: The Categorical N-Back Task (CNBT). Images of animals and non-animated objects were presented sequentially. Each consecutive image was presented for 500 ms, except for the blank image (plain screen) that lasted 1000 ms. The subjects were asked to press one of two buttons whenever an image of a raccoon (Target, T) appeared: right button if the two images preceding the target belonged to the category of animals, left button otherwise. The total time available for the subjects to respond to the target was 2000 ms. 2BT: two animals before the Target, 1BT: one animal before the Target.

Figure 2

Frequency content of the raw MEG data for individual subjects. The spectra were averaged over 24 frontal and 24 parietal-occipital sensors.

Figure 3

Event-related Temporal-Spectral Evolution (TSE) waveforms for a Child (red) and an Adult (blue), displayed for 204 planar gradiometer MEG sensors. The 24 parietal-occipital sensors, that were pooled into subsequent analysis, are marked by the shading. Vertical lines indicate the onset time of the target-raccoon.

Figure 4

Normalized parietal-occipital TSE waveforms, group-averaged across Children and Adults. Vertical dashed lines indicate the onset times of visual stimuli. Shaded rectangles show the 300-ms time-windows used to obtain the ERD/ERS measures: 2BT- two before target, 1BT - one before target, T - target, PR - post-response.

Figure 5

ERD/ERS measures in Children and Adults for each of the four time-windows: 2BT-two before target, 1BT - one before target, T - target, PR - post-response.