Abstract: Hemispherectomy subjects (Hs) have offered a unique opportunity to study the role that subcortical structures play in blindsight because the hemisphere contralateral to the blind field is absent or non-functional. We first showed Hs could detect and localize simple targets and moving gratings, discriminate grating velocity and differentiate forms in their blind field. We suggested a role of subcortical pathways i.e. the superior colliculi (SC), with the participation of the remaining hemisphere. We reported the existence of residual vision with awareness in the blind field and showed that Hs were insensitive to motion-in-depth in their hemianopic field and that some possess blindsight as shown by a spatial summation effect i.e. subjects only react to the stimulus presented in their intact field, without being aware that the simultaneous presentation of another stimulus in their blind field lowers their reaction time. We hypothesized that this indirect method to evaluate blindsight could involve subcortical mechanisms without requiring cortical processing, and without the subject's awareness. We then reported that the cellular integrity and metabolism of the ipsilateral SC in the vervet monkey are much less affected than those of the dorsal lateral geniculate nucleus (dLGN) after neonatal hemispherectomy. We underlined the importance of controlling intraocular light scatter and published the first fMRI study on residual vision. We concluded that the SC are likely implicated in blindsight in Hs, and we recently utilized the color vision properties of collicular cells to demonstrate its involvement in the residual visual abilities of Hs. Since the primate SC does not receive retinal input from shortwave-sensitive (S-) cones involved in colour vision, consequently rendering them colour blind to blue yellow stimuli, we tested 3 Hs who had reliably shown blindsight. They demonstrated a spatial summation effect only to achromatic stimuli suggesting that their blindsight is colour-blind to blue/yellow stimuli and is not receiving input from retinal S-cones. We concluded that blindsight is likely mediated by the SC in Hs. We were the first to use Diffusion Tensor Imaging (DTI) Tractography to investigate pulvinar connectivity in humans and SC connectivity in Hs with and without blindsight. We demonstrated the presence of projections from the ipsi- and contralesional SC to primary visual areas, visual association areas, precentral areas/FEF and the internal capsule of the remaining hemisphere in Hs with 'Type I' or 'attention-blindsight' and an absence of these connections in Hs without it. In another study using fMRI, we demonstrated in Hs that achromatic stimuli but not S-cone-isolating stimuli in the blind field of a subject with blindsight activated visual areas FEF/ V5 and that the cortical activation pattern was enhanced by achromatic stimuli only. We concluded that the human SC is blind to S-cone-isolating stimuli, and that blindsight is mediated by an S-cone-independent collicular pathway, at least in Hs.
The SC is the main recipient of retinal projections in lower mammals with a phylogenetically older and more primitive visual system than humans. Similar but weaker retinocollicular projections also exist in humans. Although existing SC connections to the remaining cortical areas seem to play a pivotal role in unconscious vision, blindsight subjects remain unaware of the information processed in their blind visual field. One possibility for the lack of awareness may lie in the lack of synchronicity in cerebral activation. The human visual pathways process information simultaneously and yet are able to work independently of each other (as is the case following a circumscribed lesion in a visual cortical area). For conscious perception, however, a specific synchronized activation pattern of different cortical areas involving ventral, parietal and frontal visual areas is believed to be crucial. Our results indicate that Hs with 'Type I' or'attention blindsight' are able to enhance visual performance in their blind field, but remain unaware of visual processing presumably because they are unable to access a more complex synchronous cortical activation pattern involving higher top-down mechanisms necessary for conscious vision.
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