The phenomenon of blindsight is astonishing, and seems to have major implications for consciousness. In this post, I’ll base a lot of the discusion on the book Blindsight and the Nature of Consciousness, by the philosopher Jason Holt. It seems to be the most in-depth analysis of blindsight’s implications for consciousness. Let’s start with Holt’s description of what blindsight is:
“Blindsight is the surprising ability of people with a certain kind of brain damage to perceive things visually even though they lack visual experience completely.”
Here is the beginning of the Wikipedia entry for Blindsight:
“Blindsight is the ability of people who are cortically blind due to lesions in their striate cortex, also known as primary visual cortex or V1, to respond to visual stimuli that they do not consciously see.”
In some cases people with blindsight have no visual experience at all, and in other cases they are lacking visual experience in one half of their visual field. Here’s what’s astonishing. Let’s consider someone with full blindsight, so they have no visual experience at all. When prompted to perform a task such as guessing whether cards have X’s or O’s on them, the subjects can perform at much better than chance level. That is, they can still respond to visual stimuli, even though they have no visual experience. They have no phenomenal consciousness of the visual stimuli when performing the task.
What are the implications for consciousness studies? Well, here is one. I recently discussed the illusionist, or eliminativist, view of phenomenal consciousness. At the end of the post, I said I’d soon discuss another challenge to illusionism. In the illusionist view, qualia — our instances of subjective experience — are not real. Yet with blindsight we have a case where subjects are missing something — visual experience — that normally-sighted subjects do have. Holt quotes Searle’s view that people who doubt the existence of subjective experience should “ask themselves what it is that we have that such patients seem to lack.” Colin McGinn has made a similar point, that blindsight forces us to acknowledge the reality of subjective experience, because of these abnormal cases where it is missing.
I highly recommend Holt’s book as an important, well-written, and accessible book on blindsight and consciousness. He states that, while blindsight doesn’t give a knock-down argument in favour of any particular view of consciousness, it does demonstrate the reality of subjective experience, of qualia. Illusionism just isn’t tenable.
Strangely enough, though, illusionists such as Dennett have tried to use blindsight as an argument for illusionism. It’s a strange argument, and Holt dedicates a whole chapter of the book just to critiquing this. Here is my summary of Dennett’s basic argument (from Consciousness Explained):
- Consider someone with full blindsight, where they have no visual experience.
- This person could conceivably be trained to perform better and better at these prompted tasks, to the point where they perform at the same level as normally sighted subjects. Dennett calls this superblindsight.
- A person with superblindsight is functionally equivalent to a normally sighted person.
- But if this person is functionally equivalent, then the qualia make no difference. If something makes no difference — is causally inert — then we should not posit its existence.
- So we have no reason to posit the existence of qualia. Qualia are not real.
This is a kind of conceivablity argument, and Holt is willing to grant Step 2 above. But he points out that Step 3 is wrong. They may be functionally equivalent in performing the task, but they will not be functionally equivalent in their reports of how they performed the task. Holt also states that Step 4 is very strange. Why should we deny the existence of causally inert things? Do we want to deny the existence of numbers (separate from countable things) or the meanings of sentences (separate from their expressions)?
The entire structure of Dennett’s argument strikes me as ridiculous. Let’s make a template:
- We begin with something missing.
- But a person could be trained to make up for that missing thing.
- So now the person is functionally equivalent to someone who is not missing that thing.
- So the thing that was missing makes no difference.
- So the thing that was missing is not real.
I’m finding it hard to believe that this would pass muster as a serious argument amongst philosophers. This is the appeal and danger of Dennett — he is very smart, a good writer, and has a lot of liveliness and confidence in his writings, and those qualities can be distracting from the quality of the arguments being made. Notice that Step 1 is already giving up the game.
Holt argues, contra Dennett and the Churchlands, that consciousness is not something that we decide to discard or keep based on whether it is needed for predicting behaviours. It is something that needs to be explained in its own right. The same point has been made by David Chalmers and, more recently, by Philip Goff. It seems high time to leave illusionism behind and focus on the hard problem.
Holt contrasts the phenomenon of blindsight with the phenomenon of hemispatial neglect, where in the latter case a subject has visual experience but neglects or does not notice part of it. Holt ends his chapter on Dennett’s argument with this:
“Let me put this succinctly. If Dennett is right, we cannot distinguish blindsight from neglect. But we can. Q.E.D.”
