From: rowe@lepomis.psych.upenn.edu (Mickey Rowe)
Newsgroups: rec.pets.herp
Subject: Re: Color vision in snakes?
Date: 11 Jan 93 20:26:51 GMT
Organization: University of Pennsylvania

In article <1993Jan6.184846.17128@midway.uchicago.edu>
     pmb2@midway.uchicago.edu writes:

>In article <1993Jan6.165436.5466@sci.ccny.cuny.edu>
     christ@sci.ccny.cuny.edu (Chris Thompson) writes:

>>A while back, during the thread discussing various snakes' gustatory
>>preferences, several people asserted that snakes posessed color
>>vision, as evidenced by the fact that they would only eat certain
>>colored prey.
>
>given that the discrimination in question was between black and white, the 
>color vision issue was separate.  

Issues involving color vision are rarely black and white (ouch!).  The
confusion typically goes the other way, of courese--it's difficult to
determine if animals have color vision independent of brightness cues.
Distinguishing mice that look black (or brown) to us from mice that
look white to us is something that probably any animal with eyes could
do.  In that sense, yes, the color vision issue is separate.

>Curiously enough, in one group of those  snakes that have been studied 
>the map of space perceived by the heat organs (defined by recording central 
>electrical activity while presenting an infrared stimulus) is precisely 
>aligned with the map of space perceived by the eyes (defined again by 
>making electrical recordings in the visual portions of the central nervous 
>system while presenting visual stimuli).  

"Precisely" aligned is a bit of a stretch.  From the abstract of a
paper by Hartline, Kass and Loop (full citation upon request):

     The infrared and visual spatiotopic tectal maps have similar but
     not identical axes; the infrared magnification is greater than that
     for vision.

The reason for this is that the pits don't sample as large a field as
the eyes, but the IR field of view and the visual field of view both
fill almost the entire tectum.

>The fascinating physiology here is 
>subtended by an anatomical system in which the cranial nerve bringing in infor-
>mation from the heat organ brings its information into register with the 
>visual map starting at the level of the optic tectum and converging again at 
>the level of visual cortex (if I remember correctly, the retinogeniculate
>projection, in parallel with the retinotectal projetion, is not paralleled
>by input from the heat organs, which catches up again through the 
>tectal projections to the cortex.  Thus, the geniculocortical path
>is exclusively visual while the tectal path is mixed).

Is this from the source that you cited below?  I'm somewhat surprised
because I've never seen any mention of visual cortex (or lateral
geniculate nucleus) in snakes--certainly not in the Hartline papers
where the tectal maps for IR were worked out.  

In other news, somebody just sent me a reference to more anatomical
work showing the convergence to the tectum in a boid.  It was
published just last year.  I'll pass it on to you if you're interested.

>Peter
>bell@minerva.cis.yale.edu (posting off another account for now)
>
>reference:  Dorsal Ventricular Ridge, Philip S. Ulinski.  Wiley Interscience,
>1986 or thereabouts.  An excellent review textbook of central sensory 
>processing in reptiles and in birds, with extensive direction to the primary
>literature.

Please let me know if this book contains reference to the cortical
maps you describe above.  If so, then I will look for it.
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