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Hypnosis Gets Positive Recognition
An anonymous reader writes to tell us the New York Times is reporting that, despite its negative history, hypnosis is now getting some favorable attention from neuroscientists. From the article: "These extensive feedback circuits mean that consciousness, what people see, hear, feel and believe, is based on what neuroscientists call "top down processing." What you see is not always what you get, because what you see depends on a framework built by experience that stands ready to interpret the raw information - as a flower or a hammer or a face."
"The probe, called the Stroop test, presents words in block letters in the colors red, blue, green and yellow. The subject has to press a button identifying the color of the letters. The difficulty is that sometimes the word RED is colored green. Or the word YELLOW is colored blue. " Hypnotised subjects recognised the words more often than unhypnotised subjects.
The Stroop test also differentiates between subjects with a thick corpus callosum and those with a thin corpus callosum - eg: left handers and right handers. Considering the small sample was this factor controlled for?
Also psych experiments use very small samples and have to use the repeated measures statistical technique. This can identify significance but is restricted in other information it can provide.
The whole idea of 'top-down' or cognitive drive for the sensory systems is very addictive, since among other things, it allows you to explain perception as some type of baysian method. However it is simply untrue. The visual system is replete with examples, from the Craik-O'Brien-Cornsweet illusion, to stuff like Julesz' Random Dot stereograms (that CANNOT have top down effects), that defy a top-down framework.
Even with effects that might be top-down modulated (like illusory contours) the physiological evidence is totally towards these things happening in the early nervous system. Although there is definitely some feedback present even in this area, one has to consider that RC constants for most neurons are about ~10ms, and much of our perception takes place in ~100ms. These timeframes are VERY well studied, and generally accepted.. and of that 100ms, about 50ms of the time is the signal travelling from the retina to the cortex (see Bullier & DeAngelis, among others). That doesn't leave much room for dramatic top down feedback for general sensory perception.... Your visual system, bottom up, manages to figure out edges, what colors to fill them in with, various levels of depth, what's moving (in relation to your eye movements.. no easy challenge.. how can you tell when your eye moves whether you're looking at a pen, or a moving streak?) and in relation to what else, all within 100-150ms of the stimulus. That just doesn't leave time for very dramatic 'high level' feedback like this article assumes.
Although I've only mentioned vision, there are similar issues in all sensory modalities except audition, which is a special case, since audition is optimized for temporal accuity, but it has its own issues that make it look like much of your perception happens without much top-down activity.
From our current understanding it appears that top-down activity does two things: 1) Equalize 'gain' in the sensory system.. if the amplification levels across you're visual field were different, you wouldn't be able to tell whether a line was something that had to do with the outside world or noise. And 2) Modulate acuity for attention.. which is very complicated in and of itself, but there is good evidence that most early perception occurs even in areas we aren't attending to.
The main 'evidence' in this article is from a 'brain scanner' which is probably fMRI. As one of my professor's liked to say, "In fMRI we show people a picture of their ass, then a picture of a hole in the ground, and subract them." Most fMRI statistics include averaging across areas... which is nice, until you remember that our brain isn't on a sphere, but something with fissures in it, and so you just averaged two things that were (cortically speaking) in other worlds (since because of the fissure they might be centimeters apart! Remember the Cortex is a laminar archiecture around the surface)... so I'm highly skeptical, to say the least.