[The neurofunctional foundation of cognitive rigidity in attention deficit hyperactivity disorder: some preliminary findings]. 
It has been proposed that there is an impairment in cognitive flexibility in children with attention deficit hyperactivity disorder (ADHD).  The Wisconsin card sorting test (WCST) is the most widely used neuropsychological test for assessing this process.  Previous studies have reported the presence of a subgroup of children with ADHD which has a low cognitive flexibility.  In addition this subgroup showed a high resistance to the treatment with stimulant medication.  The aim of this study was to examine whether there were different patterns of brain magnetic activity in different subgroups of ADHD, during the performance of a cognitive flexibility task, such as WCST.  We recruited a sample of 18 children, divided into three groups according to DSM IV R diagnostic criteria.  Here we present preliminary data based on a subsample of nine children.  Brain magnetic activity was registered while the children performed the WCST by means of Magnetoencephalography (MEG).  This is a non invasive neuroimaging technique with a high spatio temporal resolution.  Preliminary results showed that ADHD mixed group had a higher rate of perseverative responses.  In addition, a different pattern of brain magnetic activity was noted in this group, showing less activation in anterior cingulate cortex and dorsolateral prefrontal cortex in the left hemisphere during the first 400 ms.  MEG seems to be an useful tool to describe the brain network that subserves cognitive flexibility in different groups of children.  It could have important repercussions in the classification of ADHD, both neuropsychologically and pharmacologically.