The hypothesis that human beings have not one, but two distinct visual systems, now seems to be widely accepted. There are certainly convincing stories to be told about it both in neurological and functional terms. Neurologically, it seems that two different streams draw on the information provided by the primary visual area of the brain. A ventral stream goes eventually to the inferior temporal cortex, while a dorsal stream goes to the posterior parietal lobe. The brain being what it is, things are not quite that simple, of course, and the two streams are not completely isolated, but there seems to be good enough reason to think of the them as essentially independent.Functionally, there is evidence that the brain deals separately with conscious visual perception on the one hand and ‘automatic’ control of actions on the other. The former system, for example, is easier to fool than the latter. When presented with certain kinds of geometrical optical illusion, we may be deceived consciously about the apparent size of an object, but when we reach out to take the object, our fingers open to just the right size anyway. There’s also evidence from the effects of injuries: damage to the relevant regions of the brain may damage our conscious awareness while leaving us apparently able to use our eyes for practical purposes. Perhaps the most extreme cases are the famous instances of blindsight; subjects who cannot see at all so far as their conscious minds are concerned, but who can point to an object, or reach for one, with much greater accuracy than chance guesses could provide. Ramachandran, for one, has suggested that the two visual streams provide a satisfying answer to the puzzle of blindsight.
It is therefore an attractive hypothesis that the two streams serve regions of the brain with complementary roles, one delivering conscious visual experience and the other feeding into accurate motor control; one stream telling you what, and the other where, as some describe it. It seems to me, incidentally, that there is an interesting side-implication here about our fellow primates. Since they have the same two-stream system (in fact, I believe it was originally discovered in macaques), the implication is that they must have the same experience of doing some things deliberately, and others unthinkingly. We might have been tempted to think that animals, relying on instinct, operate on a kind of permanent autopilot, always in the same sort of state we are in when walking along without thinking of where we’re going; but that seems to be ruled out at least as far as primates are concerned.
Be that as it may, a further step has been taken by many researchers, who propose that the two visual systems must encode information differently. The system which delivers conscious perception, drawing on the ventral stream, must surely encode the positions of objects allocentrically, ie in relation to the scene before us, rather than egocentrically, ie plotting distance and direction from the observer. Since this system is concerned above all with identifying things, it’s surely most helpful to have things encoded in a way that doesn’t change every time we move around the room, they argue. In the case of the other system, where the top priority is such tasks as deciding how far to extend your arm in order to grab something, an egocentric scheme of coding makes more sense. This view is buttressed by an argument that the allocentric coding of the perception system is what makes it more vulnerable to optical illusions.
However, in a paper for Philosophy and Phenomenological Research, Robert Briscoe now rides to the rescue of egocentricity. He thinks that our conscious perception can perhaps be regarded in this respect as an extension of proprioception, the special sense which tells us where the different parts of our body are; and both, he claims, are plausibly based on an egocentric coding scheme.
He dissociates himself from two positions which appear to have been severely undercut by the two-systems hypothesis. The first, Experience-Based Control (EBC) asserts that our fine motor control draws on the richness of our conscious perception (not all of which necessarily gets our full attention); the other, the Grounding Hypothesis, is even stronger, claiming that it is necessarily grounded in conscious experience. All such views are rejected by Briscoe, which leaves him free to sidestep the evidence which tells against them. Turning to the arguments based on optical illusion, he argues that the undeniably greater vulnerability of conscious experience might not stem from allocentric coding, but from the diversity of sources drawn on by conscious experience, and hence be attributable to difficulties with integration.
And after all, he concludes, moving from defence to attack, the two systems do in fact communicate and co-operate, with conscious experience identifying the targets and the other stream sorting out the details of the movements required. If they use different coding schemes to represent the positions of objects, there are going to be some problems of translation which don’t arise if both systems are egocentric.
This all seems fairly convincing to me, but I wonder whether the dispute will eventually turn out to have been misconceived: one of those either/or disputes where neither hypothesis actually catches the truth properly. Perhaps allocentric and egocentric coding aren’t really the only alternatives and one or both systems operate on some mixed, intermediate,or entirely different principle. In fact, Briscoe’s own views hint at this to some degree. In his view, conscious perception is an extension of proprioception; but proprioception does not, he says relate everything to some arbitrary central point – rather, it relates different body parts to each other. If we extend that approach to the external world, we seem to get a system which relates objects to each other rather than a central observer; that sounds as if it has a tinge of allocentrism about it. I accept that a clear distinction can be drawn in practice between the ability to make allocentric and egocentric judgements, but that difference doesn’t have to be reflected all the way down to the level of the coding schemes involved.
Briscoe goes on to offer a few concluding remarks which perhaps reveal something of his underlying motivation: in essence, he wants to preserve a strong connection between conscious perception and agency. I sympathise with this, even with the strong claim that we need at least a potential for agency before we can have perception. But I remain to be convinced that egocentric coding of conscious visual experience is indispensable to the cause.