Sunday, 8 July 2012

Alain Ptito Neural Mechanisms of Blindsight after Hemispherectomy: Tapping into the Unconscious


      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.

      Neural substrates of blindsight after hemispherectomy http://unfweb.criugm.qc.ca/jdoyon/cours_6032/Neuroscientist%202007.pdf
      Unconscious vision: new insights into the neuronal correlate of blindsight using diffusion tractography http://brain.oxfordjournals.org/content/129/7/1822.full
      Neural Substrates of Blindsight in Hemispherectomized Subjects. http://www.bic.mni.mcgill.ca/~sandra/pdfs/Review_2007.pdf
      The nature of consciousness in the visually deprived brain http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3111253/

Comments invited

12 comments:

  1. What do the blindsight patients say it feels like when they successfully detect or localize a stimulus they cannot see. What is the cue?

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    1. I guess what the patients are reporting is the feeling of cells firing in the superior colliculus (and of those in downstream visual areas). Why this particular activity does not enter visual awareness in the typical sense of the term as we normals experience it is a mystery to me. Something special about visual processing in V1/V2 is required for the typical kind of visual awareness, perhaps?

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    2. Didn't a previous speaker say that when they succesfully detect a stimulus these patients feel a little bit like how they feel when they sense that someone is behind them? I'm not sure we could describe the 'cue' in any other way than by comparative descriptions of this kind. I would be especially surprised if they reported that they have a feeling of cells firing since we never feel our own neurons firing. We can never track our neuronal activity through introspection.

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    3. Hi Alexandre, I did not mean literally that the patient was feeling his SC cells firing! I was just merely speculating that the feeling may be made up of his SC cells firing. This just follows from the assumption that we can equate a neural correlate with a certain feeling. I personally buy this assumption but I am pretty neuro-centric!

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    4. Sorry, my bad! I certainly don't want to take issue with that assumption.

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  2. In my opinion, blindsight as presented by Alain Ptito is a good example to illustrate the fact that consciousness is everywhere and that it is irrelevant to question the relevance of consciousness in terms of adaptive function by taking examples as blindsight. By everywhere, I mean it seems to be present in any kind of behavior of our repertoire.

    The example of blindsight has often been used to requestioning consciousness and the fact that some abilities we thought to be completely conscious-related didn't actually necessarily need consciousness to be effective, and therefore consciousness was not really adaptive. However, in the case of blindsight, it seems to me that if the conscious feeling of the a visual stimulus is absent, there's still a feeling that there is something. So even if what matters is not the feeling of having seen something (the feeling of having consciously perceived something), there is something that matters, a kind of unexplained feeling. Even if it seems to be uncousicous, there's always a kind of feeling at some point.
    That being said, can we still say that consciousness has no adaptive function if it has to be present at some point or another for an individual to behave properly? In the case of blindsight, only a tiny proportion of the "normal" consciousness is gone, but the other chunks of consciousness related to all our other acbilities are still there and can compensate for the part of consciousness that can be defective (such as in bilindsight). That's why I think it might be a mistake to questioning the relevance of consciousness in terms of adaptive function based on these kind of cases.
    And what if consciousness was completely gone? it would probably correspond to a comatose state. But as far as I know, a comatose individual can hardly improve his/her fitness as long as he/she remains in that state...

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    1. NEUROSCIENCE, ROBOTICS AND CAUSAL EXPLANATION

      This is why part of the puzzle about the causal role played by feeling comes not just from neuroscience but from robotics: Even if feeling is inextricably (but inexplicably) correlated with doing in organisms, it clearly is not in the case of robots. So why can robots do what they can do without feeling, while in organisms the doing is correlated with feeling?

      The question would of course become even more pointed if a robot could pass the Turing Test (T3) -- but the important thing to remember that the question would remain equally unanswered whether or not the T3 robot would feel: If it did not feel, we would not know what causal role feeling played, and if it did feel, we still would no know. (Of course, as with organisms, only the T3 robot would know whether it felt, if it felt, but that's just the other-minds problem, not the hard problem.)

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  3. Seeing the movie clip of the patient with alleged blindsight navigate the obstacles in the testing room made me think of sleepwalking. To what extent are vision during sleepwalking and blindsight similar/different?

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  4. Seeing the video from the patient, I wonder how the transition between being aware of objects in the visual field and being unaware (of having a different experience to report than the one we have) is felt by the patient. Are they anosognosic?

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  5. Xavier Dery ‏@XavierDery

    Sometimes, in the morning I feel only half my brain is working. According to Ptito, it's normal that I feel great nonetheless! #TuringC

    9:31 AM - 9 Jul 12 via Twicca Twitter app

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  6. I have been talking about this subject a lot lately with my mother so hopefully this will get him to see my point of view.
    WHAT IS CONSCIOUSNESS

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