Timing of visuo-spatial information processing: Electrical source imaging related to line bisection judgements
Introduction
A variety of functional imaging techniques have been used to study the cerebral structures implicated in visuo-spatial attention. In particular, the involvement of right posterior parietal and temporo-occipital cortex in the performance of line bisection judgements, a task which is widely used to assess visuo-spatial neglect, has been repeatedly demonstrated using fMRI and PET (e.g. Fink et al., 2000; Fink, Marshall, Weiss, & Zilles, 2001; Fink et al., 2003; Weiss, Marshall, Zilles, & Fink, 2003). Data from vascular lesion studies hints to an impact of right temporal regions in spatial awareness (Karnath, Ferber, & Himmelbach, 2001).
A recent electrophysiological study used waveform analysis and topographic mapping to further explore the neural processes involved in line bisection judgements. The results confirmed the role of posterior parietal cortex in line bisection judgements but also suggested the involvement of the right temporo-parietal junction at an earlier stage of processing (Foxe, McCourt, & Javitt, 2003). This former study performed an additional multiple dipole source reconstruction constricted by a priori source location information derived from fMRI-data. However, concerning long latency event-related potentials we are dealing with here, dipole source localization seems to be crucial, since the cortical activation is expected to be distributed and multifocal. Thus, in this field, this approach is highly depending on a priori knowledge in terms of the localization of the suspected generators.
Meanwhile improvements in source reconstruction techniques have helped to refine the spatial accuracy of electrophysiological recordings. In this context, current density reconstruction (CDR) techniques provide a tool for modeling distributed and spatially distinct source activity independent of prior knowledge concerning the generators and therefore allow evaluation of the temporal dynamics of electrical cortical activation involved in a given task. Accordingly, we here aim at deconvolving the temporal dynamics of the neural processes involved in performing line bisection judgements (i.e., the landmark task) by means of a current density approach based on multichannel recording of event-related potentials (ERPs). We additionally attempt to give a contribution to the discussion to which extent parietal or temporal cortex are involved in the processing of visuo-spatial perception.
Section snippets
Subjects
Ten healthy right-handed male subjects (age: 25–38 years, mean = 29.6 years) participated in the study. None had a history of neurological or psychiatric disease. All subjects had normal or corrected to normal vision and were right-handed, as assessed by the Edinburgh laterality inventory (Oldfield, 1971). All subjects gave written informed consent in accordance with the Declaration of Helsinki. The study protocol was approved by the local ethics committee.
Experimental conditions
The subjects sat comfortably on a bed in
Psychometric results
Table 1 presents the psychometric results of the line bisection judgements and the control task. As expected, the reaction time as given by the latencies from stimulus onset to the button press that registered the subject's decision was longer in the line bisection judgements (mean 667 ms) than in the control task (mean 543 ms). Mean accuracy for the line bisection judgements was 83% correct decisions versus 94% correct decisions in the control task. Both differences (reaction time and error
Discussion
Five regions were identified as contributing significant source activity related to the line bisection judgement task: S1: right middle occipital gyrus; S2: right superior posterior parietal cortex; S3 + 4: bilateral inferior occipital gyrus; and S5: right inferior posterior parietal cortex. Temporal deconvolution revealed a sequential activation of these areas starting at the extrastriate visual cortex, followed by the right superior posterior parietal cortex, bilateral inferior occipital cortex
Acknowledgment
GRF is supported by the Deutsche Forschungsgemeinschaft (DFG-KFO112).
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