At the RIN 13 animal navigation conference there was a lot about bird navigation and particularly, the Continental Robin, Erithacus rubecula which it is proposed, navigates through the action of the cryptochromes in their eyes which are disrupted by radical pairs caused by the effect of a magnetic field. This means that in theory they should be able to navigate using this mechanism as a compass. Experiments by the Wiltschkos (see below) show that the light required to drive this reaction are short-wavelength range of the spectrum up to 565 nm green.
I found it difficult to see how this might work as the European Robins migrate at night. I was worried that there is not enough energy at night to drive the reaction at a high enough intensity.
Miriam Liedovegel (one of our heroes) kindly replied:
“One photon is enough to activate the mechanisms, and during nights there is still quite a bit (and certainly more than enough for cryptochromes to work) of light available. if it is pitch black (what never happens in nature but we know from experiments), birds don’t migrate”.
Simon Raggett who is real collaborator of ours adds:
“My main comment at the moment is that perhaps one should get away from the idea of birds displaying information, certainly in any way that is analogous to displays that we see or think we see in the external world. I’m more or less totally ignorant about the brains of birds but it would if they were so different in principle from the more familiar brains of mammals.”
The visual representations that we see, and which bear no resemblance to the external world, are built up gradually in the brain moving from the back of the brain towards the front and tend to emerge fully fledged in the inferior temporal region. Visual images are now seen to correlate to heightened activity in a relatively small minority of neurons. In a famous experiment, the activity of a single neuron jumped from baseline to 50 Hz when the subject was shown a picture of Jennifer Aniston but did not respond to other images. The theory is that some neurons specialise in say actresses, while other might specialise in iconic towers in Europe.
So as far as the image is displayed anywhere it is displayed in single neurons. But of course there is nothing about a neuron that resembles any kind of screen display. A neuron comprises essentially a lipid membrane around a dense protein structure and with DNA in the nucleus of the cell. This leaves us with the hard problem of what is the consciousness that makes you conscious of Ms Aniston’s image. From the point of view of the birds their information or display is very likely to arise in the same way as the Aniston image.
There is another twist to this. The Aniston process lies in what is known as the ventral stream involved with conscious visual images. But there is an alternative dorsal visual stream which is unconscious. This can handle much of our movements, such as those on effectively auto pilot and those in fast reaction sports, where the reaction has to come faster than the rather laggard conscious processing.
There have been interesting studies where people with impaired conscious vision can put a letter into a letter box which they cannot see. On the other people with impairments to the dorsal stream cannot put a letter into a box which is in clear sight. The point of this so far as the birds are concerned is that they might not need to be conscious of the navigation process at all, with the dorsal stream being just conventional algorithms.
Ironically perhaps this is very annoying to the trad ‘brain’s a computer’ people because it shows that there is a part of the brain which obviously needs consciousness as distinct from the conventionally computational bit.
This description of the conscious and subconscious mind fits with our abilities such as being able to drive a car completely on auto pilot. The subconscious does things like this and runs the whole body without our conscious attention.
This piece by Simon Raggett helps us understand how the birds might navigate without thinking about it at all. However, dowsers who follow routes tend to feel something when they are following say an imprinted route made by people for instance passing that way before. I get a strong benign headache when I am on the line and it goes away when I move off the line. Equally some people feel the hair on the back of their necks go up.
So it looks as though there is enough energy to drive the reaction for the robins to navigate even when they fly at night. Here we discuss how the robins might react to the information that is presented to them.
Magnetic orientation of migratory robins, Erithacus rubecula, under long-wavelength light
The avian magnetic compass is an inclination compass that appears to be based on radical pair processes. It requires light from the short-wavelength range of the spectrum up to 565 nm green light; under longer wavelengths, birds are disoriented. When pre-exposed to longer wavelengths for 1 h, however, they show oriented behavior. This orientation is analyzed under 582 nm yellow light and 645 nm red light in the present study: while the birds in spring prefer northerly directions, they do not show southerly tendencies in autumn. Inversion of the vertical component does not have an effect whereas reversal of the horizontal component leads to a corresponding shift, indicating that a polar response to the magnetic field is involved. Oscillating magnetic fields in the MHz range do not affect the behavior but anesthesia of the upper beak causes disorientation. This indicates that the magnetic information is no longer provided by the radical pair mechanism in the eye but by the magnetite-based receptors in the skin of the beak. Exposure to long-wavelength light thus does not expand the spectral range in which the magnetic compass operates but instead causes a different mechanism to take over and control orientation
Wiltschko, R., Denzau, S., Gehring, D., Thalau, P., Wiltschko, W. 2011 The Journal of Experimental Biology. 214, 3096-3101. DOI: 10.1242/jeb