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28 September 2015 | Dr Craig French (University of Cambridge)
Is all vision colour vision? What about in type 2 blindsight where there is limited consciousness of movement and form without colour?
Does seeing an object require seeing it as spatially located and seeing some of the space it occupies? What about in Bálint’s Syndrome when at any given time a single object and nothing else is seen? How can such pathologies inform our theorising about mental phenomena such as vision?
Pathology based philosophy of mind can take negative and positive forms. On the negative side, reflection upon pathological conditions can give us empirical counterexamples.
We start by highlighting a core claim involved in a philosophical theory of some mental phenomenon, for instance, a claim about the nature of thought, or consciousness, or perception, or whatever. But then we look to a pathological case, where we find the phenomenon in question, but in such a form that the initial claim cannot be sustained.
However, such reflection doesn’t have to take this negative form. It may force us to reconsider the starting claim. But this might instead prompt an attempt to articulate a more subtle version of it. Looked at in this way, reflection upon the pathology can play a positive role, it can help us to develop and finesse philosophical theories.
Thus, pathology based philosophy of mind can be fruitful in different ways. And the significance of such work is not confined to philosophy. For it might also yield interesting perspectives on pathologies themselves.
An excellent example of pathology based philosophy of mind is to be found in Fiona Macpherson’s “The Structure of Experience, the Nature of the Visual, and Type 2 Blindsight”. Macpherson considers a traditional claim in the philosophy of perception: the claim that all vision is colour vision. Call this the Colour Claim.
This claim is as old as Aristotle and his claim that colour is the proper object of sight. And it does seem plausible.
Although various forms of colour blindness are possible, these are typically cases in which colour vision is severely degraded, not cases of visual experience without any experience of colour (including achromatic colours). And for many of us it will be very difficult to imagine a visual experience which is not a colour experience.
Moreover, the Colour Claim earns its theoretical keep. Combined with the idea that only visual experiences are colour experiences, we can use it to explain what distinguishes vision from other senses.
What does Macpherson have to say about the Colour Claim, and what has it got to do with pathology based philosophy of mind? Though she doesn’t frame her discussion in quite this way, Macpherson effectively argues that the Colour Claim comes under pressure from a certain pathological condition: type 2 blindsight.
When we look to the form that visual experience takes in a certain sort of pathological case – type 2 blindsight – we see that the Colour Claim is unsustainable (if, that is, we accept the conception of type 2 blindsight, which Macpherson argues is defensible, and on which it involves visual experience). Let me briefly spell out some of the details of Macpherson’s discussion.
The phenomenon of blindsight involves a puzzling combination of conditions. On the one hand, a subject with blindsight seems to lack conscious experience of stimuli presented in a certain portion of their visual field. Such subjects report that they are blind in that area of the visual field – the area thus gets called the blind field.
On the other hand, such subjects are remarkably accurate at determining the presence of stimuli and certain features presented in the blind field, when forced to guess in experimental conditions (Weiskrantz (2009)).
Now it turns out that some individuals with blindsight have limited consciousness in their blind fields. Thus, Weiskrantz (1998) introduced a distinction between type 1 and type 2 blindsight. The former type is as described above, the latter encompasses cases where there is limited consciousness in the blind field.
In what way is the consciousness involved in type 2 blindsight limited? It is limited in involving consciousness of just movement and form. (More carefully: a plausible understanding of the consciousness of one type 2 blindsight patient – GY – is that he has consciousness of just movement and form (Macpherson, p. 115). Other cases may differ, but we can take this as a model).
The question Macpherson considers is how are we to understand such consciousness. Should we understand it as visual experience? As conscious thought? As conscious feeling? Or somehow else?
Suppose we understand it as visual experience. Well, then we have a pathology based counterexample to the Colour Claim. Since the consciousness involved in type 2 blindsight is consciousness of just movement and form, such visual experience would have to be visual experience of just movement and form, and so visual experience without colour experience. It is, as Macpherson brings out, a matter of hot debate whether the consciousness involved in type 2 blindsight should be understood as visual experience.
But Macpherson defends the idea that the consciousness involved in type 2 blindsight can be understood as visual experience (without pretending to offer a decisive case for this). Thus what we effectively get are (defeasible) grounds for the claim that type 2 blindsight is a pathology based counterexample to the Colour Claim.
Macpherson’s work is a particularly excellent example of pathology based philosophy of mind. Not only does it advance a philosophical issue (the status of the Colour Claim), it furthers our understanding of a pathological condition itself (type 2 blindsight).
Some of my own work – “Bálint’s Syndrome and the Structure of Experience” – falls into the mould of pathology based philosophy of mind. One traditional philosophical claim about seeing an object is that it must involve (a) seeing the object as spatially located, and (b) seeing some of the space occupied by the object. I call this the spatial perception requirement (SPR). (There is instructive discussion of some such requirement and how it finds support in Kant, Husserl, and Wittgenstein in Schwenkler (2012)).
(SPR) comes under pressure from Bálint’s Syndrome – a pathological spatial perceptual disorder defined in terms of three main deficits: simultanagnosia, optic ataxia, and optic apraxia. Simultanagnosia is the inability to see more than one object simultaneously, optic ataxia is an inability to reach accurately for seen objects, and optic apraxia is a condition whereby gaze remains fixated despite a lack of a problem with eye movement. A striking illustration of the first two deficits can be found here:
The pressure to (SPR) comes from the dominant interpretation of Bálint’s Syndrome on which subjects can see objects (though only one at a time) yet cannot see either space or spatial location. The dominant interpretation has it that Bálint’s Syndrome is a radical form of spatial blindness.
Lynn Robertson and colleagues have extensively tested a patient with Bálint’s Syndrome, RM, and their findings support this interpretation. Thus, they describe Bálint’s Syndrome and its manifestation in RM as follows:
These patients perceive a single object… yet they have no idea where it is located. It is not mislocated. Instead it seems to have no position at all (2004, p. 108)… During early testing of his [RM’s] extrapersonal spatial abilities he often made statements like, “See, that’s my problem. I can’t see where it is.”… objects that popped into his view were not mislocated per se. Rather, they simply had no location in his perceptual experience (pp. 158–159)… RM had… completely lost his ability explicitly to represent space (1997, p. 302).
A defender of (SPR) might be tempted to claim that RM just doesn’t see objects. But this is empirically implausible, as argued convincingly by Schwenkler. Not only can RM see (and identify) objects, he can see them as coherent wholes, as having shape, size, and colour (though his colour perception is often erroneous, as emphasized in Campbell (2007)).
What I argue, however, is that Bálint’s Syndrome doesn’t give a counterexample to (SPR) (see §4 of my paper). Instead, it helps us to see how sometimes seeing objects involves only very limited space and spatial location awareness.
There are two aspects to my discussion on this point. First, I argue that the evidence doesn’t rule out that individuals with Bálint’s Syndrome are capable of a severely limited form of location awareness.
The empirical evidence points to various problems with egocentric and allocentric location awareness – seeing objects as located relative to oneself and other things. But it doesn’t rule out purely object-centric location awareness; seeing an object as there where ‘there’ picks out just the space occupied by and defined in terms of the boundaries of whatever object is seen.
Second, I argue that the idea that individuals with Bálint’s Syndrome can see space is empirically consistent. Again, such space perception would have to be severely limited: confined to just object-space.
Bálint’s patients may not be able to see the object-space as a space in a larger region of space, nor will their perception of such space be independent of and dissociable from their perception of the object (as it might be for us when we see a figure as upside down, where its top points to the bottom of the space in which it’s seen), but, for all that, they can still clap their eyes upon regions of space delimited by the objects they see.
I argue further that we have positive reason to suppose that Bálint’s patients do see object-spaces. RM, for instance, can see objects as shaped and extended. He can thus see objects as taking form in and extending into space – as occupying space. This suggests that he can see the object-spaces of the objects he sees.
If I am right, we can resist the idea that in Bálint’s Syndrome visual awareness of space and spatial location goes missing. In the final section of my paper I suggest, instead, that we can characterize the experiences of those with Bálint’s Syndrome in terms of the idea that the visual field goes missing (on the particular conception of the visual field we find in Martin (1992, 1993), Richardson (2010), and Soteriou (2013), see also Mac Cumhaill (2015)).
The visual field involved in ordinary visual experience delimits a cone of physical space, which we are aware of, and in which things are seen as located, in relation to oneself, and other things (including regions space). Such experience is limited, and its limits can be specified in terms of the boundaries of the space delimited by the visual field: the boundaries seem to be boundaries beyond which things can’t now be seen. But they seem to be boundaries beyond which things could be seen, if one alters one’s point of view.
As Richardson and Soteriou emphasise, such limits come not from any object or space one perceives, but are rather manifestations of one’s own sensory limitations. Thus a change in the limitation strikes one as a change in one’s own sensory limitations, and not as a change in any object or space in the world (as presented in experience). Suppose I gradually come to have a narrower field of view.
The way in which my experience is limited thus changes: it becomes more limited. Before the change, the limitation was to a region of space of such-and-such a size, but now it is to a region of space of a smaller size. In reflection upon my experience, this strikes me as a change in my sensory limitations, not as change in the (presented) world, as the shrinking of some object or space.
Now even if the experiences of Bálint’s patients involve awareness of space and spatial location, they are nothing like this.
Suppose RM sees an apple in apple-space. RM can be aware only of what falls within the apple-space he sees. But this limitation is set by the apple he sees, and its spatial structure. If RM goes from being aware of the apple, to being aware of, say, a church, or a banana, the way in which his experience is limited will change accordingly.
The spatial specification of the limitation will now be set by the spatial structure of the church or the banana. The spatial structure of RM’s experience is beholden to whichever object he happens to see and this is quite unlike how things are in experiences which involve a visual field.
References
References
Campbell, John (2007). “What’s the role of spatial awareness in visual perception of objects?” Mind and Language 22.5, pp. 548–562.
French, Craig (2015). “Bálint’s Syndrome and the Structure of Visual Experience”. Unpublished Manuscript. http://craigafrench.github.io/assets/CFBalintsPaper.pdf
Mac Cumhaill, Clare (2015). “Perceiving Immaterial Paths”. In: Philosophy and Phenomenological Research 90.3, pp. 687–715.
Macpherson, Fiona (2015). “The Structure of Experience, the Nature of the Visual, and Type 2 Blindsight”. In: Consciousness and Cognition 32, pp. 104–128.
Martin, Michael G. F. (1992). “Sight and Touch”. In: The Contents of Experience. Ed. by Tim Crane. Cambridge: Cambridge University Press, pp. 196–215.
— (1993). “Sense Modalities and Spatial Properties”. In: Spatial Representation: Problems in Philosophy and Psychology. Ed. by Naomi Eilan, Rosaleen McCarthy, and Bill Brewer. Oxford: Oxford University Press, pp. 206–217.
Richardson, Louise (2010). “Seeing Empty Space”. In: European Journal of Philosophy 18.2, pp. 227–243.
Robertson, Lynn (2004). Space, Objects, Minds and Brains. Hove, East Sussex: Psychology Press.
Robertson, Lynn et al. (1997). “The Interaction of Spatial and Object Pathways: Evidence from Balint’s Syndrome”. In: The Journal of Cognitive Neuroscience 9.3, pp. 295–317.
Schwenkler, John (2012). “Does Visual Spatial Awareness Require the Visual Awareness of Space?” In: Mind and Language 27.3, pp. 308–329.
Soteriou, Matthew (2013). The Mind’s Construction: The Ontology of Mind and Mental Action. Oxford: Oxford University Press.
Weiskrantz, Lawrence (1998). Blindsight: A Case Study and Implications. Paperback Edition. Oxford: Oxford University Press.
— (2009). Blindsight: a case study spanning 35 years and new developments. Oxford: Oxford University Press.
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