Task-Dependence of Texture-Segregation-Specific VEP Components

Recently, texture-segregation-specific components have been isolated in the human visual evoked potential (tsVEPs). As tsVEPs are characterised by a negative peak near 200 ms they occur between luminance-contrast responses (P100) and cognitive responses (P300). The aim of this study was to estimate the temporal overlap of tsVEPs and cognitive VEP components by directing a task to either visual or auditory stimuli. Eight visually normal subjects participated in the experiment. Horizontal and vertical line segments were arranged to yield either an ‘orientation chequerboard’ stimulus or two fields with homogeneous orientations. As auditory stimuli, two tones with different pitches were presented through headphones. Auditory and visual stimuli were temporally uncorrelated, which allowed off-line isolation of VEPs and AEPs by appropriate averaging from the same raw data. VEPs and AEPs were recorded from an array of 13 electrodes ranging from frontal to occipital positions. tsVEPs were isolated under two conditions, where the subjects detected the presence of (a) the orientation chequerboard, or (b) the higher pitch by pressing a button. It was found that (1) tsVEPs could be isolated under both tasks; (2) tsVEPs were strongly modulated by the task; (3) the task-specific modulation occurred in the same time domain as the tsVEP itself, but showed a different topography; (4) AEPs were less modulated by the task. The data suggest that an additional task concerning the gradient content of texture stimuli may modulate the resulting tsVEPs. This may partially account for the interindividual variability in recent tsVEP data, as a comparable task may be introduced tacitly by the subjects.