Developmental neural networks in children performing a Categorical N-Back Task.

The prefrontal and temporal networks subserving object working memory tasks in adults have been reported as immature in young children; yet children are adequately capable of performing such tasks. We investigated the basis of this apparent contradiction using a complex object working memory task, a Categorical n-back (CN-BT). We examined whether the neural networks engaged by the CN-BT in children consist of the same brain regions as those in adults, but with a different magnitude of activation, or whether the networks are qualitatively different. Event-related fMRI was used to study differences in brain activation between healthy children ages 6 and 10 years, and young adults (20-28 years). Performance accuracy and RTs in 10-year-olds and adults were comparable, but the performance in 6-year-olds was lower. In adults, the CN-BT was highly effective in engaging the bilateral (L>R) ventral prefrontal cortex, the bilateral fusiform gyrus, posterior cingulate and precuneus, thus suggesting an involvement of the ventral visual stream, with related feature extraction and semantic labeling strategies. In children, the brain networks were distinctly different. They involved the premotor and parietal cortex, anterior insula, caudate/putamen, and the cerebellum, thus suggesting a predominant involvement of the visual dorsal and sensory-motor pathways, with related visual-spatial and action cognitive strategies. The findings indicate engagement of developmental networks in children reflecting task-effective brain activation. The age-related pattern of fMRI activation suggests a working hypothesis of a developmental shift from reliance on the dorsal visual stream and premotor/striatal/cerebellar networks in young children to reliance on the ventral prefrontal and inferior temporal networks in adults.

Increased MEG activation in OCD reflects a compensatory mechanism specific to the phase of a visual working memory task.

We examined spatio-temporal patterns of evoked magnetoencephalographic signals (MEG) in patients with obsessive-compulsive disorder (OCD) during the Encoding, Retention, and Retrieval phases of a Delayed Matching-to-Sample working memory task (DMST). The question was whether the mechanisms of abnormally increased cortical excitability, frequently reported in OCD, relate to a global cortical disinhibition and unselective over-processing of stimuli or, alternatively, to a compensatory mechanism of effortful enhanced inhibitory control. The DMST-related network of activation in OCD was found similar to that of Controls, and to that reported in other neuroimaging studies. The pattern of increased MEG activation in OCD patients was phase specific. During the Encoding phase, the activation was enhanced in the region of anterior insula and reduced in the posterior-inferior parietal cortex. During Retention, the activation was reduced in the occipital, parietal, superior temporal sulcus, and dorsolateral prefrontal cortex (BA 6/8/9). During Retrieval, a significant increase of activation was found in the right anterior insula extending towards the orbital region and right superior temporal sulcus, along with reduced activation in the left parietal cortex. The performance accuracy was high in OCD and comparable to Controls, although the RTs were prolonged. The results are discussed as being consistent with the hypothesis of a compensatory mechanism of effortful inhibitory control. This mechanism may be a major contributor to the increased cortical activation during Encoding and, in particular, Retrieval of the DMST task in patients suffering OCD. The findings do not support the concept of a faulty working memory mechanism per se in OCD.