The temporal pattern of motion in depth perception derived from ERPs in humans.

Former studies have demonstrated the cortical regions being involved in visual motion processing. The strength of neuronal activation was found to depend on the direction of motion. In particular the detection of optic flow towards the observer seems of particular importance due to its obvious biological relevance. We used event related potentials (ERPs) to add data of the temporal dynamics of this neuronal processing. Using current density reconstruction, source maxima of differential activation in motion in depth versus planar motion in the time range from 50 to 400 ms after stimulus onset were localized, and the time courses of activation were elaborated. Source reconstruction revealed six regions contributing significant source activity related to the perception of motion in depth: occipital pole, bilateral fusiform gyrus, right lateral superior occipital cortex and bilateral superior parietal cortex. Our data provide evidence for an early involvement of visual occipital cortex in the perception of motion in depth stimuli, followed by activation within parietal cortex, presumably associated with attention information processing. Sub-dividing the effects of the direction of the stimuli in motion in depth perception, optic flow directed towards the observer-induced stronger activation, but this differential activation excluded the parietal cortex. Thus the temporal deconvolution of the electrophysiological data suggests that the differential processing of approaching stimuli is initiated at an early stage of visual perception within the visual association area.

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.

Timing of visuo-spatial information processing: electrical source imaging related to line bisection judgements.

This study deconvolves the temporal dynamics of the neural processes underlying line bisection judgements (i.e., the landmark task). Event-related potentials (ERPs) were recorded from 96 scalp electrodes in 10 healthy right-handed male subjects while they were judging whether horizontal lines were correctly prebisected. In the control task, subjects judged whether or not the horizontal line was transected by a vertical line, irrespective of its position. Using a current density reconstruction approach, source maxima in the time range from 50 to 400ms after stimulus onset were localized and the time courses of activation were elaborated. Five regions, corresponding to those revealed by our previous fMRI studies (e.g., [Fink, G. R., Marshall, J. C., Shah, N. J., Weiss, P. H., Halligan, P. W., Grosse-Ruyken, et al. (2000). Line bisection judgments implicate right parietal cortex and cerebellum as assessed by fMRI. Neurology, 54, 1324-1331]), were identified as contributing significant source activity related to line bisection judgements: right middle occipital gyrus (Brodmann area; BA18); bilateral inferior occipital gyrus (BA19); right superior posterior parietal cortex (BA7) and right inferior posterior parietal cortex (BA40). Temporal deconvolution indicated sequential activation of these regions starting at BA18 as early as 90ms post-stimulus onset, followed by the successive activation of the right superior posterior parietal (BA7), bilateral inferior occipital (BA19) and right inferior posterior parietal cortex (BA40). Three of these areas (BA18, BA17 and BA19) became reactivated within 250ms of stimulus onset. The data provide evidence for an early involvement of the right hemispheric parietal network in visuo-spatial information processing. Furthermore, the temporal deconvolution of the electrophysiological data suggest that iterative processes between and within parietal (dorsal path) and occipital areas (ventral path) mediate bisection judgements.