The neural basis of residual vision and attention in the blind field of hemianopic patients: behavioural, electrophysiological and neuroimaging evidence.

Hemianopia is a visual field defect characterized by blindness in the hemifield contralateral to the side of a lesion of the central visual pathway. Despite this loss of vision, it has been shown that some unconscious visual abilities (“blindsight”) might be present in the blind field; the probability of finding this phenomenon can be increased by presenting moving stimuli in the blind field which activate the motion visual area (hMT), bypassing the damaged primary visual area (V1). As a consequence, visually guided behaviour is made possible but perceptual awareness is lacking. The present research project consists of three experimental sessions carried out with six hemianopic patients and healthy participants, in order to explore the neural basis of blindsight or residual vision, to assess whether unseen visual stimuli presented to the blind field can evoke neural responses in the lesioned or intact hemisphere and to evaluate whether shifts of spatial attention to the blind field can enhance these responses as well as the behavioral performance. In the first session we assessed the presence of blindsight or conscious residual vision by testing for the presence of unconscious above chance performance in motion and orientation discrimination tasks with stimuli presented to the blind area. We found evidence of unconscious above chance performance in one patient (L.F.) in the Motion Discrimination Task. In this case the above chance performance was associated with a feeling of something occurring on the screen, reported by the patient that resembles the so-called Blindsight Type II. In the second session we used a neuroimaging technique with the purpose of: i) assess the presence of abnormalities in the cortical representation of the blind visual field in the visual cortex, ii) evaluate position and activation of area hMT and iii) assess the structural connectivity and the integrity of white matter fibers in the same patient. To do that, by using a 3 Tesla Scanner, we carried out a fMRI session with Retinotopic Mapping, hMT Localizer and Diffusion Tensor Imaging procedures (DTI). In patient A.G. we found a retinotopic organization of low-level visual areas in the blind as well as in the intact hemisphere, despite the lesion involving mainly the dorsal portion of the left primary visual cortex. Importantly, we documented an activation of area hMT in the damaged hemisphere and the integrity of the entire visual pathways except for the optic radiations in the area of the lesion. In the third session we used an electrophysiological approach to study the neural basis of attention in the blind field of hemianopics. In order to obtain a reliable response with stimuli presented to the blind field, we used the Steady-State Evoked-Potentials (SSVEP) technique that is likely to be more informative than transient Visual Evoked Potentials in these kind of patients. This session included a passive stimulation and an attentional task. The former was performed to assess the response to stimuli flickering at a specific frequency in four visual field quadrants, two in the left and two in the right hemifield. In this session, we found in all hemianopic patients that visual stimuli presented to the blind hemifield produced a modulation of the neural response involving the damaged as well as the intact hemisphere. In the attentional task we found that orienting attention toward the blind field yielded an enhanced evoked response with respect to the non-orienting condition, even toward the blind field despite lack of perceptual awareness. Thus, SSVEP confirmed to be a useful means to assess a neural response following stimulus presentation in a blind field. In a broader perspective these results represent novel interesting evidence on the neural bases of unconscious vision in that they show that despite being unseen visual stimuli presented to the blind field elicit reliable neural activity in various cortical areas.