Monday, October 31, 2005

"I wish I had but a moment of consciousness," lamented the zombie.

` Okay, it's Halloween today. Whaddaya want?

` Seriously, though - no one really knows how consciousness works. While you are focusing on some sort of mental activity, certain areas of the brain will certainly light up - yet at the same time there are a lot of neural background activities which no one really understands. In fact, many neurons actually seem to fire without being triggered by anything else, though no one knows why that occurs.


` And yet, from this seemingly random tirade of signals comes... your mind.

` And does your conscious mind perceive things as continuously as you might think? Are your mental flittings recorded as discrete moments - as in movie frames - or are they all melded together?
` Furthermore, are different aspects of one event processed in synchrony, or it that just the way it looks to us?
` That's what I'm writing about today. Lucky you.

` According to Talis Bachman of the University of Tartu in Estonia, the conscious perception of any one sensation may some take time to develop - like a Polaroid.
` There is enough delay-time in the brain for this to work: For example, it takes about half a second for one to be consciously aware of anything coming from the senses, which can be evident to the layman when one remembers how one automatically reacts to stimuli before they realize it.
` Indeed, your reactions to things tend to be remembered closer in time to when you actually become consciously aware of them!

` For example, one experiment by Simon J. Thorpe involved an EEG hooked up to the heads of subjects looking at a screen. On this screen were flashed images for mere fractions of a second, and the subjects involved were to decide whether or not each image contained an animal of some kind of another.
` These individuals required less than half a second to give the correct answer! (There were similar results when they were asked to press a button to indicate whether an image showed a car or another means of transportation.)
` In the first fractions of a second after each animal-or-no-animal pictures were presented, the EEG patterns were nearly identical. Now, it takes about 30-50 milliseconds for nerve impulses to travel from the retina to the visual centers of the cerebral cortex all the way in the back of the head. By 150 milliseconds, the EEG readings showed that the brain's evoked potential was different, depending on if the image seen had an animal or not.
` This means that it took about a tenth of second for the cerebral cortex to tell the difference between pictures with animals and pictures without. This is so amazingly fast that it can only be explained by a very large amount of parallel processing at work.

` And yet... as it takes about half a second for someone to consciously perceive something, the person is not actually aware of the distinction until after it is recognized by certain parts of the brain.

` This ultra-fast processing gives the mind enough room in time to, in hindsight, perceive two different stimuli as one.
` For example, it's been known for decades that two flashes of light that are very close-together in time can be perceived as one flash. Also, different notes in a trilling bird call sound like a mere buzzing to human ears, while the birds would seem to be able to pick out each note.
` Not only this, but the visual registering of one image can distort or suppress images that were perceived before or afterwards if they are flashed quickly on a monitor.

` This effect is called 'masking'.

` Robert Efron of the University of California at Davis discovered that this blending together of two sensations in rapid succession is the most common. When he flashed a red light for ten milliseconds, and then a green light for ten milliseconds, his subjects reported the sensation of seeing both red and green light at the same time - in other words, they said the light was yellow!

` Stanislas Dehaene at INSERM in Orsay, France, decided to study the way the brain processes words in a similar way. Dahene's subjects were lying in an fMRI scanner with a series of slides flashing in rapid succession. On them were simple words, which appeared for barely 30 milliseconds - just long enough for them to be decoded. However, if random images appeared before and after the word, it was not nearly as likely to be recognized.
` When the word was able to be read, it could be seen on the fMRI that the brain suddenly acted up in various locations (such as the vision and speech centers). However, when the random images were flashed before and after the word, brain activity was confined to parts of the visual cortex that operate in the early input phases of vision, and the processing to recognize the word was more or less intercepted.

` But even when two images are shown a tenth of a second after one another, the second image can still keep the first from being consciously processed. In this case, however, if a subject were to guess the first image, they can guess correctly, as it has had time to be processed subconsciously.
` This isn't so surprising, considering that people who lack only the parts of the brain to consciously process visual signals are still able to correctly guess at what they must be seeing on a subconscious level. (That phenomena is known as 'blind sight.')

` Brain damage aside, it takes time to get a subconscious 'idea' of what something is - about 100 milliseconds - and it takes a little longer - 50 more milliseconds - to be consciously aware of this recognition without having to guess.
` In other words, there is a tiny amount of time within which you can become aware of something. It is called a 'minimal perceptual moment'. However, it seems that brain activity just after a stimulus - causing 'backwards masking' - disturbs the signals trying to make their way to the parts of the brain which consciously register things.
` Not only that, but signals from a previous stimulus may interrupt the development of new signals in your conscious mind.

` In other words, signals compete for conscious awareness. And when sensory signals are just going wild in your perception, blended together in different ways, it is a wonder that the brain is able to integrate everything into a coherant picture without much in the way of time lags.

` Semir Zeki of University College London is a Neurobiologist who's spent immense amounts of time studying things like this. For example, he measured how squares that change color while also changing direction register in people's brains as they watch them on a screen.
` A change in the square's color is actually seen 60 to 80 milliseconds faster than its simultaneous change in direction.
` In other words, it looks as if the square changes color before it changes direction. That's because the brain is composed of many complex areas - one processes color, one processes form, another processes sound, and there are others for speed, direction, and so forth.
` Of course, each different area of the brain takes a different amount of time to process each aspect.

` And yet, the modular brain somehow seems to create a unified consciousness: Usually, the time differences in processing are not noticed by the conscious mind and the entire picture is perceived as happening at the same moment.

` This isn't always the case, of course, if there is something wrong with one's brain. As an example, some poeple have 'cinematographic vision', which can be caused by migraine headaches.
` Oliver Sacks was the neurologist/writer who came up with the term for the times when people see a flickering series of still images which don't overlap at all. Each image seems to last for a long time by itself - instead of superimposed onto other images - and then suddenly jumps to another image further along in time.
` He described one woman in a hospital ward who had begun to draw a bath and had been transfixed by the spigot when the water got about an inch deep. Eventually, the tub filled up without her noticing and the water began to overflow.
` Sacks, who had happened to see the tub overflowing, touched her, whereupon she seemed to snap out of it and finally saw the water overflowing. She said later that what she had seen while the tub was filling up was merely a still image of the pouring faucet with an inch of water in the basin.
` He had also experienced cinematographic vision after ingesting a popular drink of Micronesia called sakau, seeing "a succession of stills, like a film run too slow, its continuity no longer maintained."

` And yet, normal moment-to-moment perception does not appear to be split into discrete moments. Each frame is fused to the next, more or less seamlessly. Events which happen at almost the same time - whether due to external stimuli or merely differences in processing speeds of various brain components - appear to happen at exactly the same time: Information that differs temporally is somehow poured into the same perceptual moment in time, in order to make the world more coherant.
` Additionally, time can appear to pass more slowly or more quickly - depending on such things as how much fun you're having.
` Like a film, when fewer mental 'frames' are taken per second, time seems to speed up. When more frames are taken per second, things seem to go in 'slow motion'.

` For example, when when people are faced with tornadoes or earthquakes tearing up the structures around them, or even simple high-speed car crashes, time does indeed seem to go in slow-motion. In fact, police officers are sometimes able to describe in great detail the sight of their bullets striking the criminals they're shooting at, entering their flesh, and bursting out the other side.
` Sometimes, other senses besides sight fade out at the same time, perhaps due to the immense levels of processing needed just for registering so many perceived visual moments in time. In other words, in order to keep vision coherant, other parts of one's awareness are trumped.

` How exactly the brain does this, no one knows.

` And studying the perception of moments is a very tricky business to begin with: Devices like the fMRI can only record in time-scales of seconds rather than milliseconds, and EEGs can only tell you the firing rate of just a handful of neurons.
` So really, we don't have the means to probe very deep into the workings in consciousness, though I trust that, as it has been, we will continue to be better and better equipped for this kind of study.

` I'll keep you posted.

` Nevertheless, I still have the darndest craving for human brains... BRAAAIIINNNSSS!!

5 comments:

Galtron said...

Braaaaaaaaaaiiiinnnnnnnnnnsssss! They make your world go round.

Even if the way they work seems incoherant...

I think yours is even more incoherant than the average, but I mean that in the best possible way!
Sometimes it's good to be disjointed. But not evil.

S E E Quine said...

` Sometimes disjointed is good... though usually, it sucks.
` My brain has a tendency to actually run things together, which is not fun. I can't pick apart my memories very well. Grrrr.

cassie d said...

brain stuff is SO interresting! Actually the human body is incredibly interresting!!! i mean, how do they even know how long it takes for the retina to send info to the brain? i mean really? sure - a very fast stopwatch, but how cool is that?

sometimes i wish i went into the medical, as opposed to landscaping. hmmm....i'd probably be making more money, true, but then i wouldn't know spanish.

tough call.
tough call.

Galtron said...

Spanish makes your brain work differently!

S E E Quine said...

` Yes, knowing more than one language DOES rock. You're right, Galtron, it really does cause you to think differntly!
` That's why... if you're to be a Ranger, you must know how to speak like a Minbari in order to think like one.
` Or something like that.