Wednesday, October 3, 2007

Magnetic Birds

BoingBoing covered a story today about a recent study done by researchers at the University of Oldenburg which reveals that birds--long suspected to be in tune with the Earth's magnetic field (that's how they navigate on long distance migrations)--may actually see the Earth's magnetic field. Aside from the obvious question, which is "how much would a bird trip out if you put it in an MRI machine," this discovery raises some interesting questions about the nature of visual perception and sensory modalities generally. First, a little background.

The scientists injected the birds with a tracer dye, which can be tracked as it travels throughout the body. They put one dose in the eyes of the birds (where the magnetically sensitive cells are found) and another in the part of the brain they call "Cluster N," which apparently is most active when the birds are navigating. They found that both doses of the dye traveled to the thalamus which is (in part and among many other things) responsible for processing visual information. The finding "strongly supports the hypothesis" that birds navigate using magnetic information conveyed through the visual system.

Many philosophers want to say that senses are defined by the modality in which they express themselves--in other words, what makes vision vision
(as opposed to, say, hearing) is nothing more than the way it feels to see something. Everyone knows the difference between what it is like to hear a car and see a car--that's all that separates vision and audition. This study, I think, shows this position for the nonsense that it is. Most people would freely admit that this "magneto-vision" of birds is not a veridical case of vision--instead, it seems that it's a new sense that is very closely related (and expressed in the same modality).

Similarly, other philosophers (*cough*Eripsa*cough*) want to argue that all that defines a sense is the neuronal mechanisms it uses to instantiate it; that is, what we mean by "vision" is "originates in the eye and is processed in structure x in the brain." I'm probably over simplifying, and I'm sure I'll get a comment about it, but it seems to me that this study also proves this second view false. Unless you want to say that vision is only vision when it works just the way it does for you and I (which leaves out all sorts of liminal cases like color blindness or blindsight), linking a sense and its neuronal correlate seems to lock you into the view that a bird's "magneto-vision" is just a special case of vision.

Instead, I want to give a (surprise) naturalistic account of a senses, in which each sense is defined by the peculiar way the biological features interact with a certain aspect of the environment. I've talked at length about this before, so I'm not going to repeat myself here--suffice it to say that we need to consider both the subjective aspect (i.e. modality) and the objective aspect (i.e. the environmental cause of the sense and the biological way that sense is processed) in order to fully account for any sense (or perception in general).

What do you think? Is this a new sense, or a different kind of vision?


Adam Harris said...

I think that this differentiation of the senses is a bit odd. Each sense is a different mode of gaining information about the universe. Vision is the absorption of photons by the optic nerve. Hearing is the detection of density changes in the medium ambient in the ear. Touch is nothing more than the detection of the atomic coulomb repulsion force. Smell and taste are chemical detectors (and as such are linked). I think the important point here is that while the birds detect these magnetic fields and the signal travels to a similar portion of the brain, they aren't detecting photons, and as a result this shouldn't be considered vision. What's really going on here, in my opinion, would be closer to touch than vision because what they are feeling is a force acting on a part of their body instead of absorbing energy from an external source, detecting changes in ambient density, or uncovering the chemical composition of something.

Nick said...

Hmm. I beg to differ, Adam. Most of all, I don't agree with your definitions. They seem to be entirely based on the data involved, with no consideration of modality: according to your definition of sight, you could stick an optic nerve on a rock, and the optic nerve would "see".

I think we have to consider, just as the sapient blogowner said, both the (subjective) modality, and the (biological) data.

Adam Harris said...

I actually agree with your assessment. If you attached the optic nerve to a rock, the nerve would see. The rock wouldn't have any faculty to understand the signal it was receiving from the nerve, but the signal would probably still be sent (especially if the rock is neutral and the absorption of the photon excites an electron in the nerve as I assume it would and the electron would have to travel through the nerve into the rock to satisfy charge balance). The main distinction in senses comes from whether we are to define them as detection of the universe, or signal interpretation by the mind, because these two things are completely separate. I would argue that for the most part we should define senses as detectors because for the most part those who lack specific senses lack the ability to make detection and do not lack the ability in the brain to understand the signal if it were sent (exceptions to this are true as the brain attempts to compensate for lacking senses, but are generally not true when one comes into being or first loses the sense). As a result I believe we can define senses as mere detections and not interpretations (such is why I don't believe in the subjectivity of color). Once the distinction is made that senses lie detection and not analysis, it is then obvious that detection of the magnetic force by these birds is closer to touch, or the detection of coulomb force, than it is to sight which is detection of frequency and energy.

Jon said...

Hmm, I don't like that either, Adam. Think about the difference between you and I and, say, a video camera. Both of us have light sensitive "organs" that change states based on the ambient optic array, and both our organs are sensitive to approximately the same spectrum of light. A video camera rig can even be set up to react to certain visual stimuli (e.g. by sending you an email that there is someone walking around your house), but there still seems to be a crucial difference between what I'm doing when I look at (say) the Mona Lisa and what a video camera is doing when it "looks" at the Mona Lisa.

I admire (and agree with) your appeal to naturalism here, but I just think that there's more we have to take into account--the subjective nature of any sensory experience. I get that you want to deny the overall subjectivity of things like color (I share that goal), but it's equally a mistake, it seems to me, to go too far in the other direction.