Temporal activation patterns of lateralized cognitive and task control processes in the human brain.

In a recent fMRI study with identical word stimuli we demonstrated task-dependent lateralization of brain activity during visual processing, with left-hemispheric activations for letter decisions and right-hemispheric activations for visuospatial decisions (Stephan, K.E., Marshall, J.C., Friston, K.J., Rowe, J.B., Ritzl, A., Zilles, K., Fink, G.R., 2003. Lateralized Cognitive Processes and Lateralized Task Control in the Human Brain. Science 301, 384-386). In order to explore the temporal dynamics of these lateralized processes we here recorded multichannel event-related potentials (ERPs) using the same stimuli. ERP data were analysed with current source density reconstruction (CDR). Contrasting the ERP results elicited by the two tasks, source deconvolution showed enhanced activity during letter decisions in Broca's area from 200-250 ms during letter decisions and during visuospatial decisions in the right posterior parietal cortex (PPC) from 175-200 ms and 250-275 ms. Prior to these activations ERP data revealed an initiation of activity within the anterior cingulate cortex (ACC) from 125-150 ms followed by a late activation of this region from 400-425 ms. Consistent with our previous fMRI study the current electrophysiological data support the notion that lateralized cognitive processes may depend on task requirements rather than stimulus properties. The current results extend our previous findings as they allow insights into the temporal dynamics of these lateralized processes and their relations to task control processes. The temporal deconvolution of ERPs suggests an early differential involvement of Broca's area in letter-processing and of PPC during visuospatial processing. In addition, activation of ACC prior and after this differential activation is consistent with previous findings suggesting that this area may be involved in cognitive control.