29. ERP generators in an omitted-target oddball task: A simultaneous EEG-fMRI study

Visually evoked responses from the blind field of hemianopic patients

Hemianopia is a visual field defect characterized by decreased vision or blindness in the contralesional visual field of both eyes. The presence of well documented above-chance unconscious behavioural responses to visual stimuli presented to the blind hemifield (blindsight) has stimulated a great deal of research on the neural basis of this important phenomenon. The present study is concerned with electrophysiological responses from the blind field. Since previous studies found that transient Visual Evoked Potentials (VEPs) are not entirely suitable for this purpose here we propose to use Steady-State VEPs (SSVEPs). A positive result would have important implications for the understanding of the neural bases of conscious vision. We carried out a passive SSVEP stimulation with healthy participants and hemianopic patients. Stimuli consisted of four black-and-white sinusoidal Gabor gratings presented one in each visual field quadrant and flickering one at a time at a 12Hz rate. To assess response reliability a Signal-to-Noise Ratio analysis was conducted together with further analyses in time and frequency domains to make comparisons between groups (healthy participants and patients), side of brain lesion (left and right) and visual fields (sighted and blind). The important overall result was that stimulus presentation to the blind hemifield yielded highly reliable responses with time and frequency features broadly similar to those found for cortical extrastriate areas in healthy controls. Moreover, in the intact hemifield of hemianopics and in healthy controls there was evidence of a role of prefrontal structures in perceptual awareness. Finally, the presence of different patterns of brain reorganization depended upon the side of lesion.

Brain waves from an "isolated" cortex: contribution of the anterior insula to cognitive functions

Using two independent electrical neuroimaging techniques (BESA and sLORETA), we tested a fMRI-seeded source modeling indicating that in visual discriminative tasks the anterior insula (aIns) participates in the generation of three prefrontal ERP components: the pN1 (at 115 ms), the pP1 (at 170 ms), and the pP2 (at 300 ms). This latter component represented the focus of the present study. Results showed that the pP2 had different activation profiles across hemispheres. The left aIns activity peaked at 420 ms (30 ms before the response) for both Go and No-go trials, that is independently from the ultimate choice (response or inhibition). The right aIns activity started at about 250 ms and progressively increased for a time interval extending after the motor response; its amplitude was larger in case of Go than No-go stimuli. We suggest that the activation of the left aIns reflected the timing of the decision, and the right aIns the categorization and the performance monitoring processes. A control experiment requiring simple (not discriminative) motor response revealed that the pP2 and the aIns activity were nearly absent after the 250 ms; this result confirmed that the aIns activity at this stage is associated with the decisional processes, and not with the motor response per se. The present investigation shed new lights on the insular contribution to perceptual decision-making, and opens to the possibility of assessing the aIns activity via ERP analysis.