Visually evoked responses from the blind field of hemianopic patients

Abstract

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.

Plasticity of the human visual brain after an early cortical lesion

“…Despite the challenges to uncover the mechanisms mediating awareness (for commentary see Berlucchi et al, 2017), it has been proposed that a fronto-parietal network plays a key role in the processes (Sanchez-Lopez et al, 2017;Passingham and Lau, 2017). Interestingly, this fronto-parietal network is also important for visuospatial attention which can modulate decision criteria and consequently awareness (Vernet et al, 2017).…”

Weak proactive cognitive/motor brain control accounts for poor children’s behavioral performance in speeded discrimination tasks

“…In visuo-motor tasks after stimulus onset, in addition to the well documented sensorial (P1 and N1) and endogenous (N2 and the P3) components, three more components have been identified: the prefrontal N1, P1 and P2 (pN1, pP1 and pP2 respectively) peaking between 110 and 300 ms after the stimulus onset and localized in the rostral part of anterior insula Sulpizio et al, 2017). While the pN1 and pP1 are related to top-down perceptual processing associated with stimulus physical salience and awareness and sensory-motor integration respectively Sanchez-Lopez et al, 2017), the pP2 has been associated with evidence accumulation process, that is the efficacy of the stimulus-response mapping Darriba & Waszak, 2018;Di Rollo et al, 2016;Perri, Berchicci, Lucci, Spinelli, & Di Russo, 2015b, 2015a. These anterior insular ERP components were recently characterized by the present research group; however, in the past, several research groups found that anterior ERP components were generically localized in frontal areas (Foxe & Simpson, 2002;Potts, Liotti, Tucker, & Posner, 1996).…”

Feature-based attention potentiates recovery of fine direction discrimination in cortically blind patients

“…Cortically blind (CB) subjects possess multiple residual visual abilities within their blind field (Mazzi, Savazzi, & Silvanto, 2017;Sanchez-Lopez et al, 2017;Weiskrantz, Warrington, Sanders, & Marshall, 1974). Visual perceptual training in CB fields can further improve performance on trained tasks, even if the subject initially had no residual ability to perform the task, recovering some of the vision lost at the trained locations (Bergsma, Elshout, van der Wildt, & van den Berg, 2012;Bergsma & van der Wildt, 2009;Chokron et al, 2008;Das, Tadin, & Huxlin, 2014;Huxlin et al, 2009;Raemaekers, Bergsma, van Wezel, can der Wildt, & van den Berg, 2011;Raninen, Vanni, Hyvarinen, & Nasanen, 2007;Sahraie et al, 2006;Sahraie et al, 2003;Vaina et al, 2014).…”