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Re: Homing Pigeons? Try Homing Crocodiles...

Scott <hmwh@together.net> quotes his poem:

> Recognition: human bones have de ja vu
> built in as sure as birds' brains
> minced, condensed, melted down make steel..

Back in the 1980s there were claims that magnetite in the skulls of
humans provides us with a subconscious ability to detect earth
strength magnetic fields and use that information for navigation.  To
say that some people still believe that is like saying some people
still believe birds are not dinosaurs.

And Dann Pigdon <dannj@alphalink.com.au> wrote:

} The latest neurological research by German scientists has shown that
} signals sent by cryptochromes in the eyes of garden warblers are
} processed by the optic lobe in the brain:
} http://www.abc.net.au/news/stories/2007/09/27/2044505.htm

First a minor quibble...  You might want to note that it was you who
introduced the phrase "optic lobe".  When that term is used to
describe vertebrate anatomy it refers to a mid-brain structure
homologous to the human superior colliculus.  Most of the processing
that goes on there in humans is thought to be sub-conscious (it
assists in the planning and execution of eye movements and to some
extent directing attention).  To the extent that that is correct, the
structure is not involved in what we normally think of as vision and
seeing.  If your reporting had been accurate you'd have undercut it
there, but in fact the report to which you directed our attention
indicates that the projection of magnetosensory receptors in bird eyes
goes to the wulst, an analogue of human primary visual cortex.  The
latter is definitely involved in conscious visual perception.  Whether
the former also relates to visual experiences comes back to some
sticky philosophical questions...

Anyways, I looked at the PLoS ONE paper and I'm still not convinced.
They haven't come close to demonstrating that the bird's experience of
magnetic fields is like their experience of vision (even if you go out
on the philosophical limb that you can attribute such experiences to
the animals).  Before I can take their claim seriously they need to
show that it is possible to fool the birds with an optical stimulus.
For instance, they could put the birds into a mu-metal apparatus to
cancel out all external magnetic fields.  Then with a projector like
that used in a planetarium, they should produce a stimulus that would
excite the birds' magnetoreceptors in the same way that an
earth-strength magnetic field would.  If they are right, then with the
experiment repeated in the same room with an applied magnetic field
replacing the optical stimulus the birds' behavior should be
substantially the same.  Keep in mind that this would be a very
expensive experiment to perform.  But extraordinary claims and
extraordinary evidence and all that...

And John Scanlon <riversleigh@outbackatisa.com.au> wrote:

] } not something you'd expect in largely nocturnal crocs
] and I beg to differ: if there's something like a virtual heads-up display of
] lines of force it would be much more accessible (contrasting with the visual
] background) at night, when it would also be more useful (migratory birds do
] a lot of night-flying, do they not?)

The PLoS ONE article claims that the brain area stimulated by magnetic
fields is much more active at night than during the day but only in
migratory song birds.  I didn't look at the references, but they claim
there is evidence that this area is specialized for seeing under very
low levels of illumination.

Back to Dan:

} This is of course assuming that magnetic fields aren't interpreted
} as a fifth visual band (after red, green, blue and UV),

Ouch ouch ouch ouch ouch!  Ouch.  Ouch.  We can*not* currently say
much of anything about the way birds might categorize colors (again
even given that you'll go out on the limb to attribute experiences to
them that are anything remotely like what we understand as
"experience").  The general thinking among color vision scientists is
that "red", "green", and "blue" are peculiarities of human vision.
Whether that's true is a bugger-all hard question to address.  And so
virtually no one has even tried.  Ironically one of the few animals
widely cited as having provided us some evidence is the domestic
pigeon.  Given comparisons of spectral lights (that is lights that
have all of their energy concentrated over a narrow range of the
spectrum), the wavelength at which humans might define the category
distinction between blue and green is not similarly a border between
two categories of pigeon vision.  Of course studies done more recently
than that work (which was carried out in the 1970s) indicates that
there is a huge variation among humans as to where that category
boundary should lie.  There are stimuli that one person will say is
completely blue with no hint of green and another person will say is
completely green with no hint of blue.  We need to figure this out in
people before trying to figure it out in other animals.

Mickey P. Rowe     (mrowe@lifesci.ucsb.edu)