Can monkeys have blindsight? Sean Allen-Hermanson defends the idea in a recent JCS paper. Blindsight is one of those remarkable phenomena that ought to be a key to understanding conscious perception; but somehow we can never quite manage to agree how the key should be turned. Blindsight, to put it very briefly, is where subjects with certain kinds of brain lesions deny seeing something, but can reliably point to it when prompted to try. It’s as though the speaking, self-conscious part of the brain is blind, but some other part, well capable of directing the hand when given a chance, can see as well as ever.
There are a number of ways we might account for blindsight. One of the simplest is to suppose that the visual system is degraded but not destroyed in these cases; the signals from the eye are still getting through, but in some way at reduced power. This reduced power level puts them below the limit required for entry into conscious awareness, but they are still sufficient to bias the subject towards the correct response when they are prompted to guess or have a random try. Another popular theory suggests that the effect arises because there are two separate visual channels, only one of which is knocked out in blindsight. There is a good neurological story which can be told in support of this theory, which weighs strongly in its favour; against it, there have been reports of analogous phenomena in the case of other senses, where it is harder to sustain the idea of physically separate channels. Allen-Hermanson cites claims for touch, smell and hearing (I’ve wondered in the past whether the celebrated deaf percussionist Evelyn Glennie might be an example of “deafhearing”); and even suggestions that the case of alexithymia, in which things not consciously perceived nevertheless cause anxiety or fear, might be similar. It’s possible, of course, that blindsight itself comes in more than one form with more than one kind of cause, and that there is something in both these theories – which unfortunately would make matters all the more difficult to elucidate.
For those of us whose main interest in is consciousness, blindsight holds out the tantalising possibility of an experimental route into the mystery of qualia, of what it is for there to be a way something looks. It’s tempting to suppose that what is missing in blindsight patients is indeed phenomenal experience. Like the much-discussed zombies, they receive the data from their senses and are able to act on it, but have no actual experience. So if we can work out how blindsight works, we’ve naturalised qualia and the hard problem is cracked…
Well, no, of course it isn’t really that easy. The point about qualia, strictly interpreted, is that they don’t cause actions; qualia-free zombies behave just the same as normal people, and that includes speech behaviour. So the absence of qualia could have no effect on what you say; since whatever blindsight patients are missing does affect what they say, it can’t be qualia. Moreover we have no conclusive evidence that blindsight patients have no visual experience; it could be that they have the experience but are simply unable to report it. That might seem a strange state to be in, but patients with brain damage are known to assert or confabulate all sorts of things which are at odds with the evidence of their senses; in fact I believe there are subjects who claim with every sign of sincerity to see perfectly when in fact they are demonstrably blind, which is a nice reversal of the blindsight case.
Still, blindsight is a tantalising glimpse of something important about conscious experience, and has all sorts of implications. To pick out one at random, it casts an interesting light on split-brain patients. In blindsight cases, we can have an apparent disconnect between the knowledge the patient expresses with the voice, and the knowledge expressed with the hand; that’s pretty much what we get in many experiments on split-brain patients (since normally only one hemisphere has use of the vocal apparatus and the other can only express itself by hand movements). Any claims that split-brain patients are therefore shown to be two different people in a single skull are undercut unless we’re willing to take up the unlikely position that blindsight patients are also split people.
One interesting extension of blindsight research is the apparent discovery by Cowey and Stoerig of the same phenomenon in monkeys. There is an obvious difficulty here, since human blindsight experiments typically rely on the subject to report in words what they can see, something monkeys can’t do. Cowey and Stoerig devised two experiments; in the first the monkeys were trained to touch a screen where a stimulus appeared; all were able to do this without problems. In the second experiment, the stimulus did not always appear on cue; when it did not, the monkeys were required to press a separate button. Normal monkeys could do this without difficulty, but monkeys with lesions thought to be analogous to those causing blindsight now went wrong when the stimulus appeared in their blind spot, hitting the ‘no stimulus’ button. Taking the two experiments together, it was concluded that blindsight was effectively demonstrated; the damaged monkeys who could earlier touch the right part of the screen even when the stimulus was in their blind spot, later ‘reported’ the same stimulus as absent.
(Readers may wonder about the ethical propriety of damaging the brains of living primates for these experiments; I haven’t read the original papers, but I suppose we must assume that at any rate the experiments had medical as well as merely philosophical value.)
Of course, these experiments differ significantly from those carried out on human subjects, and as Allen-Hermanson reports, reasonable doubts were subsequently raised in a 2006 paper by Mole and Kelly, who pointed out that relying on two separate experiments, which made differing demands, made the results inconclusive. In particular, the second task was more complex than the first, and it could plausibly be argued that the result of having to deal with this additional complexity was that the monkeys simply failed to notice in the second experiment the stimulus they had picked up successfully in the first.
Allen-Hermanson’s aim is to rescue Cowey and Stoerig’s conclusions, while acknowledging the validity of the criticisms. He proposes a new experiment: first the monkeys are trained to press a green button if there is a stimulus (no need to point to where it is any more), and a red one if there is none. Then we introduce two different stimuli: Xs and Os. Both the green and red buttons are now divided in two, one side labelled for X, the other for O. If there is a stimulus, the monkeys must now press either green X or green O depending on which appeared: if there is no stimulus, they can press either red button. Allen-Hermanson believes the blindsighted monkeys will consistently press red X correctly if the stimulus is X, even though they are effectively asserting that there is no stimulus.
Maybe. I can’t help feeling that all the monkeys will be puzzled by a task which effectively asks them to state whether a stimulus is present, and then, if not present, say whether it was an X or O. The experiment has not been carried out; but Allen-Hermanson goes on to suggest that Mole and Kelly’s alternative hypothesis is actually implausible on other grounds. On their interpretation, for example, the blindsighted monkeys simply fail to notice a stimulus in their blind spot: yet it has been demonstrated that they cannot recognise objects as salient in monkey terms as ripe fruit when they are presented to the blind spot – so it seems unlikely that we’re dealing with something as simple as inattention.
What would it mean if monkeys did have blindsight? It would seem to show, at least, that monkeys are not automata; that they do have something which corresponds to at least one important variety of human consciousness. Allen-Hermanson proposes working further along the mammalian line, and he seems to expect that mammals and even some other vertebrates would yield similar results (he draws the line at toads).
At any rate, we’re left feeling that human consciousness is not as unique as it might have seemed. I can’t help also feeling more strongly than before that the really unique feature of human awareness is the way it is shot through with language; we may not have the only form of consciousness, but we certainly seem to have the talkiest.