PZ Myers posted Entry 1451 on September 6, 2005 03:37 PM.
Trackback URL: http://www.pandasthumb.org/cgi-bin/mt/mt-tb.fcgi/1447


How we sense the world has, ultimately, a cellular and molecular basis. We have these big brains that do amazingly sophisticated processing to interpret the flood of sensory information pouring in through our eyes, our skin, our ears, our noses…but when it gets right down to it, the proximate cause is the arrival of some chemical or mechanical or energetic stimulus at a cell, which then transforms the impact of the external world into ionic and electrical and chemical changes. This is a process called sensory signaling, or sensory signal transduction.

While we have multiple sensory modalities, with thousands of different specificities, many of them have a common core. We detect both light and odor (and our cells also sense neurotransmitters) with similar proteins: they use a family of G-protein-linked receptors. What that means is that the sensory stimulus is received by a receptor molecule specific for that stimulus, which then actives a G-protein on the intracellular side of the cell membrane, which in turn activates an effector enzyme that modifies the concentration of second messenger molecules in the cell. Receptors vary—you have a different receptor for each molecule you can smell. The effector enzymes vary—it can be adenylate cyclase, which changes the levels of cyclic AMP, or it can be phospholipase C, which generates other signalling molecules, DAG and IP3. The G-protein that links receptor and effector is the common element that unites a whole battery of senses. The evolutionary roots of our ability to see light and taste sugar are all tied together.

Continue reading Evolution of sensory signaling (on Pharyngula)

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Comment #46785

Posted by Eric on September 6, 2005 5:52 PM (e)

you have a different receptor for each molecule you can smell.

Just to jump in as a layman, but wasn’t there a researcher who suggested that there was a frequency detection mechanism, and not a seperate receptor for every single molecule? What is the status of this research? I haven’t seen anything current on it. Great link.

Comment #46806

Posted by PZ Myers on September 6, 2005 9:12 PM (e)

I should say every class of molecule…we have about a thousand different olfactory receptor genes (3% of our genome), and can discriminate something like 10,000 different odors.

Comment #46815

Posted by SEF on September 7, 2005 5:02 AM (e)

suggested that there was a frequency detection mechanism

I remember that, from some years ago now. It was about detecting vibration of particular bonds in combination with others. One of the demonstrations involved mixing chemicals containing those bonds (in suitable environments) to simulate a larger aromatic compound which had the same significant bonds. Being me, I don’t recall any names of people involved though.

Comment #46855

Posted by Mark on September 7, 2005 11:33 AM (e)

Nice overview. It would be nice if you mentioned that this approach, no matter however much one extends and refines it, is incapable of explaining why we have conscious experience. All these mechanical process don’t imply subjective experience and can go on completely “in the dark.”

The second problem is that this account while good above a certain level of “course” description, is false below that level because it’s based on an outdate (by about 75 years) and false physical theory of matter and energy. This makes it a useful fiction but still a fiction given our modern views of physics. My readings of this make our modern theories of physics crucial in helping understand some issues surrounding the problem of consciousness.

Comment #46901

Posted by Brian on September 7, 2005 3:17 PM (e)

I am not sure what theory others were talking about when they said that there is a “frequency detection,” but this sounds similar to James. J. Gibson’s account of invariant detect.

What Gibson found was that perception was [b]not[/b] on sensation, but detection of information (by the way, Gibson’s idea of information is what I am using in a paper that I argue against Dembski). Information specifies [i]both[/i] the environment and the observer. Information is never an object, but information [i]about[/i] an object. Even though the optical array may change, there is always persistence among the optical array. This persistence is the invariants. When this change and persistence is [i]detected[/i] we have perceived the meaningful environment. This entails that the brain does not represent the environment, but rather is part of the perceptual apparatus that detects information.

Furthermore, Gibson found that our sensory modules are not separate, but rather are perceptual systems that overlap.

Comment #46916

Posted by SEF on September 7, 2005 5:12 PM (e)

As far as I can make out from a google search (using the unfortunately all too obvious terms) the only people to have put stuff on the internet are the freakazoids - aromatherapists and other alt.med. and religious loonies. Aarrgghh! There wasn’t all that rubbish clogging up the system when the internet was restricted to a select few at universities such as Cambridge….

I tried again with some more combinations. This time I could at least narrow it down to a most likely candidate, Luca Turin, who may or may not be sane (just because the crackpots cite him doesn’t prove a thing as we know from the ID/creationists). I’m so bad at names though that I really don’t have a clue whether it is the same person or not. The limited information and different format mean that I also can’t really tell if the content is the same as I saw before either. I had to exclude some of the more unusual keywords which I thought I had recalled in order to get hits at all, which is not a good sign. Judge for yourself:
PDF of review
PDF of a paper
mixed reviews of a book

Comment #46917

Posted by SEF on September 7, 2005 5:16 PM (e)

While I’ve never fallen prey to this new formatting’s tendency to totally wipe the posts of the unwary (replacing it with an error message), this is the second time it has messed up between preview and posting. Both times involved taking some dots and new-lines (which I was using to separate things and indicate passage of time) and turning them into an ellipsis which it then inserted before the full-stop of the preceding paragraph only in the final version. Having it do different things on preview and post when we then can’t re-edit the posts to undo the damage it has done to them is just not fair.

Comment #46918

Posted by SEF on September 7, 2005 5:20 PM (e)

Hmm… I have a cunning plan (which doesn’t involve turnips). I’ll use asterisks instead (as long as I remember) and see if it is less likely to mess with those:

* * *

There should now be a line of them and gaps before this fresh start. There certainly was on preview but, as has been seen with the dots, that proves nothing about final post format.

Comment #46920

Posted by SEF on September 7, 2005 5:22 PM (e)

Ha! Gotcha that time, you evil KwickXML formatting. [laughs maniacally]