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Hack 12. Build Your Own Sensory HomunculusEnd Notes 1. Gurney, K. N. (2001). Information processing in dendrites II. Information theoretic complexity. Neural Networks, 14, 1005-1022. 2. You can start finding out details of the delicate electrochemical dance that allows the transmission of these binary electrical signals on the pages about action potentials that are part of a series of lecture notes on human physiology (http://members.aol.com/Bio50/LecNotes/lecnot11.html), the Neuroscience for Kids site (http://faculty.washington.edu/chudler/ap.html), and The Brain from Top to Bottom project (http://www.thebrain.mcgill.ca/flash/a/a_01/a_01_m/a_01_m_fon/a_01_m_fon.html). 3. But this is another storya story called learning. See Also · How neurons are born, develop, and die is another interesting story and one that we're not covering here. These notes from the National Institutes of Health are a good introduction: http://www.ninds.nih.gov/health_and_medical/pubs/NINDS_Neuron.htm. · Neurons actually make up less than a tenth of the cells in the brain. The other 90-98%, by number, are glial cells, which are involved in development and maintenancethe sysadmins of the brain. Recent research also suggests that they play more of a role in information processing than was previously thought. You can read about this in the cover story from the April 2004 edition of Scientific American (volume 290 #4), "The Other Half of the Brain." |
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Hack 10. Detect the Effect of Cognitive Function on Cerebral Blood Flow
 
When you think really hard, your heart rate noticeably increases.
The brain requires approximately 20% of the oxygen in the body, even during times of rest. Like the other organs in our body, our brain needs more glucose, oxygen, and other essential nutrients as it takes on more work. Many of the scanning technologies that aim to measure aspects of brain function take advantage of this. Functional magnetic resonance imaging (fMRI) [Hack #4] benefits from the fact that oxygenated blood produces slightly different electromagnetic signals when exposed to strong magnetic fields than deoxygenated blood and that oxygenated blood is more concentrated in active brain areas. Positron emission tomography (PET) [Hack #3] involves being injected with weakly radioactive glucose and reading the subsequent signals from the most active, glucose-hungry areas of the brain.
A technology called transcranial Doppler sonography takes a different approach and measures blood flow through veins and arteries. It takes advantage of the fact that the pitch of reflected ultrasound will be altered in proportion to the rate of flow and has been used to measure moment-to-moment changes in blood supply to the brain. It has been found to be particularly useful in making comparisons between different mental tasks. However, even without transcranial Doppler sonography, you can measure the effect of increased brain activity on blood flow by measuring the pulse.
1.11.1. In Action
For this exercise you will need to get someone to measure your carotid pulse, taken from either side of the front of the neck, just below the angle of the jaw. It is important that only very light pressure be useda couple of fingertips pressed lightly to the neck, next to the windpipe, should enable your friend to feel your pulse with little trouble.
First you need to take a measure of a resting pulse. Sit down and relax for a few minutes. When you are calm, ask your friend to count your pulse for 60 seconds. During this time, close your eyes and try to empty your mind.
With a baseline established, ask your friend to measure your pulse for a second time, using exactly the same method. This time, however, try and think of as many species of animals as you can. Keeping still and with your eyes closed, think hard, and if you get stuck, try thinking up a new strategy to give you some more ideas.
During the second session, your pulse rate is likely to increase as your brain requires more glucose and oxygen to complete its task. Just how much increase you'll see varies from person to person.
1.11.2. How It Works
Thinking of as many animals as possible is a type of verbal fluency task, testing how easily you can come up with words. To complete the task successfully, you needed to be able to coordinate various cognitive skills, for example, searching your memory for category examples, generating and using strategies to think up more names (perhaps you thought about walking through the jungle or animals from your local area) and checking you were not repeating yourself.
Neuropsychologists often use this task to test the executive system, the notional system that allows us to coordinate mental tasks to solve problems and work toward a goal, skills that you were using to think up examples of animals. After brain injury (particularly to the frontal cortex), this system can break down, and the verbal fluency task can be one of the tests used to assess the function of this system.
Research using PET scanning has shown similar verbal fluency tasks use a significant amount of brain resources and large areas of the cortex, particularly the frontal, temporal, and parietal areas.1
Interestingly, in this study people who did best used less blood glucose than people who did not perform as well. You can examine this relationship yourself by trying the earlier exercise on a number of people. Do the people who do best show a slightly lower pulse than others? In these cases, high performers seem to be using their brain more efficiently, rather than simply using more brain resources.
Although measuring the carotid pulse is a fairly crude measure of brain activity compared to PET scanning, it is still a good indirect measure of brain activity for this type of high-demand mental task, as the carotid arteries supply both the middle and anterior cerebral arteries. They supply blood to most major parts of the cortex, including the frontal, temporal, parietal, and occipital areas, and so would be important in supplying the needed glucose and oxygen as your brain kicks into gear.
One problem with PET scanning is that, although it can localize activity to certain brain areas, it has poor temporal resolution, meaning it is not very good at detecting quick changes in the rate of blood flow. In contrast, transcranial Doppler sonography can detect differences in blood flow over very short periods of time (milliseconds). Frauenfelder and colleagues used this technique to measure blood flow through the middle and anterior cerebral arteries while participants were completing tasks that are known to need similar cognitive skills as the verbal fluency exercise.2 They found that the rate of blood flow changed second by second, depending on exactly which part of the task the participant was tackling. While brain scanning can provide important information about which areas of the brain are involved in completing a mental activity, sometimes measuring something as simple as blood flow can fill in the missing pieces.
1.11.3. End Notes
1. Parks, R. W., Loewenstein, D. A., Dodrill, K. L., Barker, W. W., Yoshii, F., Chang, J. Y., Emran, A., Apicella, A., Sheramata, W. A., & Duara, R. (1988). Cerebral metabolic effects of a verbal fluency test: A PET scan study. Journal of Clinical and Experimental Neuropsychology, 10(5), 565-575.
2. Schuepbach, D., Merlo, M. C., Goenner, F., Staikov, I., Mattle, H. P., Dierks, T., & Brenner, H. D. (2002). Cerebral hemodynamic response induced by the Tower of Hanoi puzzle and the Wisconsin card sorting test. Neuropsychologia, 40(1), 39-53.
Vaughan Bell
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Hack 11. Why People Don't Work Like Elevator Buttons
More intense signals cause faster reaction times, but there are diminishing returns: as a stimulus grows in intensity, eventually the reaction speed can't get any better. The formula that relates intensity and reaction speed is Pieron's Law.
It's a common illusion that if you are in a hurry for the elevator you can make it come quicker by pressing the button harder. Or more often. Or all the buttons at once. It somehow feels as if it ought to work, although of course we know it doesn't. Either the elevator has heard you, or it hasn't. How loud you call doesn't make any difference to how long it'll take to arrive.
But then elevators aren't like people. People do respond quicker to more stimulation, even on the most fundamental level. We press the brake quicker for brighter stoplights, jump higher at louder bangs. And it's because we all do this that we all fall so easily into thinking that things, including elevators, should behave the same way.
1.12.1. In Action
Give someone this simple task: she must sit in front of a screen and press a button as quickly as she can as soon as she sees a light flash on. If people were like elevators, the time it takes to press the button wouldn't be affected by the brightness of the light or the number of lights.
But people aren't like elevators and we respond quicker to brighter lights; in fact, the relationship between the physical intensity of the light and the average speed of response follows a precise mathematical form. This form is captured by an equation called Pieron's Law. Pieron's Law says that the time to respond to a stimulus is related to the stimulus intensity by the formula:
Reaction Time R0 + kI-b
Reaction Time is the time between the stimulus appearing and you responding. I is the physical intensity of the signal. R0 is the minimum time for any response, the asymptotic value representing all the components of the reaction time that don't vary, such as the time for light to reach your eye. k and b are constants that vary depending on the exact setup and the particular person involved. But whatever the setup and whoever the person, graphically the equation looks like Figure 1-2.
Figure 1-2. How reaction time changes with stimulus intensity
1.12.2. How It Works
In fact, Pieron's Law holds for the brightness of light, the loudness of sound, and even the strength of taste.1 It says something fundamental about how we process signals and make decisionsthe physical nature of a stimulus carries through the whole system to affect the nature of the response. We are not binary systems! The actual number of photons of light or the amplitude of the sound waves that triggers us to respond influences how we respond. In fact, as well as affecting response time, the physical intensity of the stimulus also affects response force as well (e.g., how hard we press the button).
A consequence of the form of Pieron's Law is that increases in speed are easy for low-intensity stimuli and get harder as the stimulus gains more intensity. It follows a log scale, like a lot of things in psychophysics. The converse is also true: for quick reaction times, it's easier to slow people down than to speed them up.
Pieron's Law probably results because of the fundamental way the decisions have to be made with uncertain information. Although it might be clear to you that the light is either there or not, that's only because your brain has done the work of removing the uncertainty for you. And on a neural level, everything is uncertain because neural signals always have noise in them.
So as you wait for light to appear, your neuronal decision-making hardware is inspecting noisy inputs and trying to decide if there is enough evidence to say "Yes, it's there!" Looking at it like this, your response time is the time to collect enough neural evidence that something has really appeared. This is why Pieron's Law applies; more intense stimuli provide more evidence, and the way in which they provide more evidence results in the equation shown earlier.
To see why, think of it like this: Pieron's Law is a way of saying that the response time improves but at a decreasing rate, as the intensity (i.e., the rate at which evidence accumulates) increases. Try this analogy: stimulus intensity is your daily wage and making a response is buying a $900 holiday. If you get paid $10 a day, it'll take 90 days to get the money for the holiday. If you get a raise of $5, you could afford the holiday in 60 days30 days sooner. If you got two $5 raises, you'd be able to afford the holiday in 45 daysonly 15 days sooner than how long it would take with just one $5 raise. The time until you can afford a holiday gets shorter as your wage goes up, but it gets shorter more slowly, and if you do the math it turns out to be an example of Pieron's Law.
1.12.3. End Note
1. Pins, D., & Bonnet, C. (1996). On the relation between stimulus intensity and processing time: Pieron's law and choice reaction time. Perception & Psychophysics, 58(3), 390-400.
1.12.4. See Also
· Stafford, T., & Gurney, K. G. (in press). The role of response mechanisms in determining reaction time performance: Pieron's law revisited. Psychonomic Bulletin & Review (in press).
· Luce, R. D. (1986). Response Times: Their Role in Inferring Elementary Mental Organisation. New York: Clarendon Press. An essential one stop for all you need to know about modeling reaction times.
· Pieron, H. (1952). The Sensations: Their Functions, Processes and Mechanisms. London: Frederick Muller Ltd. The book in which Pieron first proposed his law.
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Hack 12. Build Your Own Sensory Homunculus
All abilities are skills; practice something and your brain will devote more resources to it. The sensory homunculus looks like a person, but swollen and out of all proportion. It has hands as big as its head; huge eyes, lips, ears, and nose; and skinny arms and legs. What kind of person is it? It's you, the person in your head. Have a look at the sensory homunculus first, then make your own. In Action You can play around with Jaakko Hakulinen's homunculus applet (http://www.cs.uta.fi/~jh/homunculus.html; Java) to see where different bits of the body are represented in the sensory and motor cortex. There's a screenshot of it in Figure 1-3. Date: 2015-12-11; view: 833 |
