How blindsight answers the hard problem of consciousness

By | October 4, 2022

Nicholas Humphrey writes:

The cover of New Scientist magazine 50 years ago showed a picture of a rhesus monkey, with the headline ‘A Blind Monkey That Sees Everything’. The monkey, named Helen, was part of a study into the neuropsychology of vision, led by Lawrence (Larry) Weiskrantz in the psychology laboratory at the University of Cambridge. In 1965, he had surgically removed the primary visual cortex at the back of Helen’s brain. Following the operation, Helen appeared to be quite blind. When, as a PhD student, I met her a year later, it seemed nothing had changed.

But something puzzled me. In mammals, there are two main pathways from the eye to the brain: an evolutionarily ancient one – the descendant of the visual system used by fish, frogs and reptiles – that goes to the optic tectum in the mid-brain, and a newer one that goes up to the cortex. In Helen, the older visual system was still intact. If a frog can see using the optic tectum, why not Helen?

While Weiskrantz was away at a conference, I took the chance to investigate further. I sat with Helen and played with her, offering her treats for any attempt to engage with me by sight. To my delight, she began to respond. Within a few hours, I had her reaching out to take pieces of apple from my hand; within a week, she was reaching out to touch a small flashing light… Seven years later (as shown in the video below), she was running round a complex arena, deftly avoiding obstacles, picking up peanuts from the floor.

To anyone who’d observed Helen in 1972 – and didn’t know the history – it would have seemed that her eyesight was now quite normal. Yet, could she really ‘see everything’, as the New Scientist’s cover implied? I didn’t think so. I found it hard to put my finger on what was missing. But my hunch was that Helen herself still doubted she could see. She seemed strangely unsure of herself. If she was upset or frightened, her confidence would desert her, and she would stumble about as if in the dark again. The title I gave to my article inside the covers of the magazine was ‘Seeing and Nothingness’.

We were on the brink of a remarkable discovery. Following on from the findings with Helen, Weiskrantz took a new approach with a human patient, known by the initials DB, who, after surgery to remove a growth affecting the visual cortex on the left side of his brain, was blind across the right-half field of vision. In the blind area, DB himself maintained that he had no visual awareness. Nonetheless, Weiskrantz asked him to guess the location and shape of an object that lay in this area. To everyone’s surprise, he consistently guessed correctly. To DB himself, his success in guessing seemed quite unreasonable. So far as he was concerned, he wasn’t the source of his perceptual judgments, his sight had nothing to do with him. Weiskrantz named this capacity ‘blindsight’: visual perception in the absence of any felt visual sensations.

Blindsight is now a well-established clinical phenomenon. When first discovered, it seemed theoretically shocking. No one had expected there could possibly be any such dissociation between perception and sensation. Yet, as I ruminated on the implications of it for understanding consciousness, I found myself doing a double-take. Perhaps the real puzzle is not so much the absence of sensation in blindsight as its presence in normal sight? If blindsight is seeing and nothingness, normal sight is seeing and somethingness. And surely it’s this something that stands in need of explanation.

Why do visual sensations, as experienced in normal vision, have the mysterious feel they do? Why is there any such thing as what philosophers call ‘phenomenal experience’ or qualia – our subjective, personal sense of interacting with stimuli arriving via our sense organs? Not only in the case of vision, but across all sense modalities: the redness of red; the saltiness of salt; the paininess of pain – what does this extra dimension of experience amount to? What’s it for? And, crucially, which animals besides ourselves experience it, which are sentient? [Continue reading…]

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