To wholegroup from Bill P.
("Wholegroup" is a new mailbox nickname I'm starting in Eudora. It will have all the above addresses in it, though I'll delete anyone's if they ask. Saves using "reply to all" every time, which is easy to forget.)
HY: Few bothered to examine the firing of these neurons under other conditions, because the tuning curve experiments appear to be the most scientifically rigorous. Needless to say, the question of what the animal is 'seeing' is entirely neglected. It's really quite outrageous when all the perceptual neuroscientists talk incessantly about 'coding' and 'decoding' and so on, and really all they are doing is to replace any scientific explanation with the results of a simple tuning curve experiment, to let the cells they carefully selected to answer to the question of vision. See, this cell could be a detector for 'x' when the animal is anesthetized, so that explains how animals can see 'x'. Lord, they still think they are studying receptors...
BP: Thanks for that, Henry. I am encouraged. It never occurred to me that people weren't challenging these results all over the place. Isn't that what we're supposed to do when we say we're doing science? I've been taking the "tuning curve" idea as a well-established fact, but as soon as I read your post another possibility popped up.
The basic perceptual model in PCT is one in which a perceptual input function receives multiple input signals from lower-order perceptual functions and applies some computation to the set to generate the next level of perceptual signal. Generally many different input functions may receive copies of the same lower signal, but of course use different computations so the same signal can contribute differently to several (maybe many) perceptions at the higher level.
If you just stuck multiple electrodes into a visual nucleus where there were many perceptual input functions and then presented some simple scene to the eyes, what (according to the PCT model) would the electrodes show you? You would see many neurons responding to different degrees. If you changed the scene, the degree of the responses would change: some would increase, some would decrease.
This would tell you that whatever it is about the scene that you're changing, some perception of it at one level contributes to a number of different perceptions at the next level up. If the scene were a set of parallel lines, we might think of perceptions of spacing. Shading. Slant. Perspective. Deleted section of a paragraph. Cross section. Mineral content in a geological map. Political subdivision. All these perceptions could change when a set of parallel lines is presented in different orientations. The change in multiple neuronal responses doesn't tell you what variable is being represented; it tells you only that the lines contribute something to detecting it at some level in the perceptual hierarchy. Maybe somewhere, buried in the neurons preceding (signal-wise) the ones being measured, we might find one or two signals, say X and Y coordinate signals, which are providing directional information to all the cells we're looking at, with different weightings. Wouldn't that account for the H&W observations?
HY: So the answers to your questions are all 'don't knows', though if you ask a visual neuroscientist he will try to overwhelm you with details.
BP earlier: what does the magnitude of the response from one of these cells represent, and more to the point, to what system would those signals be an input? And what would the destination system do with those signals? Is there some principle of mapping or conformal transformation or something else esoteric that we need to know about to understand how this thing works?
HY: It's actually pretty easy to falsify the conclusions of all the tuning curve experiments, but I will refrain from that experiment, not only because I'm occupied now with setting up new things, but also because there are a few labs already doing it. And I don't mean to attack visual neuroscience, as most other fields in neuroscience are even worse. At least H&W collected a lot of replicable data, and Wiesel's deprivation experiments were excellent and started the productive field of plasticity in sensory cortical areas. So they did the simple experiments and deserved the Nobel. Their followers are the problem.
The Purves textbook you have should have decent, though fairly mainstream, chapters on this topic, as 3 of the authors--Purves, Katz, and Fitzpatrick--have studied vision. Katz used to work with Wiesel. Purves, an outsider who is interested in real vision (as opposed to tuning curves), actually agrees with me on H&W, but he is in a minority. You've watched his lecture on visual illusions.
BP: Well, I am much comforted, though we haven't heard the rebuttals yet and still could be quite wrong. How fast is that light at the end of the tunnel approaching? Faster than we're moving toward it?
At 12:05 PM 10/29/2011 -0400, Henry Yin wrote: