I’ve recently been re-reading A. David Milner and Melvyn Goodale’s The Visual Brain in Action, as part of a project on the conscious control of behavior (concerning which I’ll hopefully have something to post rather soon). For those who aren’t familiar with the book, it presents a detailed empirical case for Milner and Goodale’s influential interpretation of the “two visual systems hypothesis”, according to which the human brain contains two functionally distinct visual streams, one responsible for providing the contents of visual consciousness and the other responsible for visuomotor control, which is supposed to be an entirely unconscious affair. In the first chapter, Milner and Goodale start to make their case by appeal to evolutionary considerations: they argue that vision must have evolved to control behavior, and that the existence of an unconscious primitive visuomotor module makes sense in this light. In this context, we get remarks like the following (p. 19):
“… it is a truism that all visual processing systems ultimately serve to guide behavior, otherwise they would not have evolved at all.”
And then later on (now in the epilogue to the second edition of the book, at p. 221), they write:
“… unless percepts can somehow be translated into action, they will have no consequences for the individual possessing them. Nor indeed would the brain systems that generate them ever have evolved. In other words, perception per se is not what the ventral stream is there for: perception is not an end in itself, in biological terms, but rather a means to an end.”
Claims like these seem to me to rest on several misunderstandings of the nature of evolutionary explanations, which are worth dwelling on only because they are exceedingly common. In the first place, it simply is not true that everything that “evolved” must “ultimately serve” to do anything at all: this is the whole point of Gould and Lewontin’s idea of a “spandrel”, or a non-adaptive trait that is the byproduct of some other trait that was selected for. Secondly, even if a trait once increased the fitness of an organism or population by guiding behavior in one way, it doesn’t follow that that trait still has this as its end: to give a toy example, suppose that we have taste buds because they enabled our ancestors to distinguish food that was rotten or unripe from food that was good to eat. (I’ve got no idea whether this is true, but that doesn’t matter.) Clearly it doesn’t follow that taste buds still help us do this: these days we’ve got better ways to tell whether food is good for eating, and our sense of taste is mostly good for helping us enjoy our meals, and tell the difference between Shiraz and Cabernet. More generally, even if you believe (as I do) that there’s explanatory value in teleological notions like “means” and “end in itself”, it needn’t be that the “ultimate purpose” of every important human characteristic is one that evolution would have cared about: for example, you might think that the most significant thing that conscious perception does for us is allow us to contemplate the mysteries of the universe, experience beauty, or become morally virtuous, each of which are “consequences” that organisms like us tend to care about even though they’re not especially adaptive.
Or is there something I’m missing?
Of course none of this shows that visual consciousness doesn’t influence human behavior (it does; and in fact I believe it has much more of a role in this than Milner and Goodale think), or that didn’t improve human fitness because of this. And there’s very good reason to believe that the conscious ventral stream was a later adaptation than the unconscious dorsal one, though the real evidence for this is the fact that the latter lies in a phylogenetically older part of the brain than the former. But there’s more lost than gained when we try to argue for these claims with teleologically-laden “Just-So” stories of the sort that I’ve quoted above.
P.S. Hi, this is my first post here. I’m a recent-ish Berkeley graduate who now teaches at a liberal arts college in Maryland. Here is my home page, and thanks to Gualtiero for the invitation to post here, which I’ve taken far too long to get around to doing.