What is cognitive neuroscience?

Transcription

1 Fall 2005 Lecture 7 Ben Balas Courtesy of Ben Balas. Used with permission. What is cognitive neuroscience? Experimental Psychology is more or less the study of subjects behavior subject to various manipulations of stimuli or environment. Cognitive Science is an outgrowth of this field that is particularly interested in characterizing the mind in computational terms. Cognitive Neuroscience is a further extension of that philosophy, explicitly aimed at discovering the neural substrates that support various cognitive processes. 1

2 What is cognitive neuroscience? Cog. Neuro. = What does the brain do? Q: But how do we study brain and behavior? A: Find ways to look at brain activity Roadmap 1. Lesion Studies The birth of cog. neuro. 2. MEG and EEG - When does your brain process stuff? 3. PET and fmri Where does your brain process stuff? 4. TMS Do you need that part of your brain to do that? 5. The view from here 2

3 Brain Lesions Necessity In a sense, brain lesions and their effects on behavior and perception gave birth to cog. neuro. HM Dissociation of memory systems (episodic v. procedural) DF Dissociation of visual systems for recognition v. action Phineas Gage Frontal lobe as the seat of personality and many many others. But what about normal subjects? We can t just wait around for people to have brain damage to learn about neural function. With rats and other animals, we can just use single unit recording to work out what s going on. This is still the Gold Standard for work in neuroscience unfortunately, it is unavailable to us for work with humans 3

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5 EEG and MEG Passive Recording Both EEG and MEG are ways to measure WHEN various things happen in the brain. They are called passive recording techniques because they do not alter the brain in any way. EEG and MEG Passive Recording The general idea in both of these methodologies is to present subjects wi th stimuli, and record the changes in the local electric or magnetic fields at the surface of the scalp. What would make us choose between measuring an E-field v. a B-field? 5

6 What are you measuring? The EEG is sensitive to all the three components of the electric activity of the brain. The MEG is sensitive only to the two tangential components. This means that the MEG is especially good for getting information from sulci, whereas the EEG is good for getting information from gyri as well. Image by MIT OCW. The gear Image removed due to copyright reasons. Typical MEG hairdryer set-up. Typical EEG caps Image removed due to copyright reasons. 6

7 So what kind of stuff can you learn? On a particular trial, the data from any given electrode will look very noisy. This is because the changes in electric field potential brought on by your brain activity are like whisper in a VERY loud room. Both your heartbeat, eyeblinks, and nearby muscle movements can more or less obliterate the signal. So what kind of stuff can you learn? What we can do is average the response over many hundreds of trials. The noise should come out in the wash, and the signal should be all that remains. This way we can see any consistent changes in potential at a given site. Note that all of these peaks are time-locked to the stimulus presentation. 7

8 When not Where A crucial point to remember is that these peaks of activity w ill tell you only WHEN something was happening at that electrode (latency), and HOW MUCH activity you saw (magnitude). What we don t know much about from either technique is where exactly in the brain the signal was coming from. When not Where But wait! You say. Aren t there tons of electrodes at different places on the scalp? Doesn t that tell us where this stuff is happening? The answer is yes and no. The reason for the No is that you re not ONLY listening to one set of neurons at any one electrode. 8

9 When not Where A good metaphor for this is to imagine that you re listening to the Super Bow l from the roof of the stadium. You ll know when big plays happen, but just try to work out field position I dare you. Dipole fitting Are we totally out of luck? Not entirely you can do some fancy stuff by reasoning about the potential across the entire head to do what is called dipole fitting. A nice try, but this is something of a black art still. 9

10 EEG/MEG recap Passive methods for recording brain activity to different stimuli Extremely good temporal resolution Poor spatial resolution Completely non-invasive Useful for children or other special populations EEG can take a lot of prep time (electrode goo) MEG is less involved in terms of prep, but needs more gear. Also, B-fields and E-fields are measuring activity in different places Where are you thinking? PET and fmri So, we can learn when different things happen in the brain, but how do we find out about where? The answer comes in the form of two similar techniques for cortical localization called: 1) PET (Positron Emission Tomography) 2) fmri (function Magnetic Resonance Imaging) Both of which fall under the category of active recording techniques due to the ways in which they interfere with normal brain metabolism. 10

11 From electrical fields to blood flow One thing that is different about these techniques is that our dependent variable is no longer electric field potentials we instead will be using blood flow as a proxy for neuronal activity. Images removed due to copyright reasons. PET injecting you with science Image removed due to copyright reasons. PET works by injecting you with a tracer substance that will be distributed throughout your bloodstream and that gives off positrons. When they decay, one can use a detector to work out where they were, allowing a researcher to determine where blood went in one condition vs. the others. This is accomplished by means of a subtraction analysis. 11

12 PET injecting you with science HOW DOES PET WORK? 511 KeV Annihilation XRay Radiation Detector Coincidence Circuit Atomic Nucleus Positron Electron 180 o 511 KeV Annihilation XRay Radiation Detector Principles of Decay and Detection Figure by MIT OCW. PET Detector Ring Coincidence Imaging PET injecting you with science PROS: 1) Pretty good spatial resolution. 2) One can tag other substances than blood to look for neurotransmitters. CONS: 1) Pretty expensive. 2) Radioactive tracer means a subject can only participate occasionally. 3) Bad tempora l resolution. PET really isn t used a lot anymore in Cog. Neuro., but a lot of seminal work was done using it, so you should know how it works. 12

13 fmri Brain mapping in the modern era fmri has become the method of choice for relating brain anatomy Image removed due to copyright reasons. Image removed due to copyright reasons. to brain function fmri Brain mapping in the modern era Photo courtesy of the National Institute of Mental Health. In case you ve never seen one 13

14 fmri Setup Figure removed due to copyright reasons. Equipment Magnet Gradient Coil RF Coil 14

15 The BIG magnet Very strong 1 Tesla (T) = 10,000 Gauss Earth s magnetic field = 0.5 Gauss 4 Tesla = 4 x 10, = 80,000X Earth s magnetic field Continuously on Main field = B 0 R Images removed due to copyright reasons. B 0 S Magnet Safety The whopping strength of the magnet makes safety essential. Things fly Even big things! Images removed due to copyright reasons. 15

16 Artifacts from metal This subject was wearing a hair band with a ~2 mm copper clamp. Left: with hair band. Right: without. Source: Jorge Jovicich Courtesy of Jorge Jovicich. Used with permission. MRI v. fmri MRI fmri Images removed due to copyright reasons. fmri Blood Oxygenation Level D ependent (BOLD) signal indirect measure of neural activity neural activity Î blood oxygen Î fmri signal 16

17 Proton Alignment Outside magnetic field randomly oriented Inside magnetic field Applied Magnetic Field M spins tend to align parallel or anti-parallel to B 0 net magnetization (M) along B 0 spins precess with random phase no net magnetization in transverse plane only % of protons/t align with field Image by MIT OCW. Hemoglobin (deoxygenated) ) Hemoglogin (Hgb): - four globin chains - each globin chain contains a heme group - at center of each heme group is an iron atom (Fe) - each heme group can attach an oxygen atom (O 2 ) - oxy-hgb (four O2 ) i s diamagnetic no B effects - deoxy-hgb is paramagnetic if [ deoxy-hgb] local B Figures removed due to copyright reasons. 17

18 Recipe for fmri 1) Put subject in big magnetic field (leave him there) 2) Transmit radio waves i nto subject [about 3 ms] 3) Turn off radio wave transmitter 4) Receive radio waves re-transmitted by subject Manipulate re-transmission with magnetic fields during this readout interval [ ms: MRI is not a snapshot] 5) Store measured radio wave data vs. time Now go back to 2) to get some more data 6) Process raw data to reconstruct images 7) Allow subject to leave scanner (this is optional) Source: Robert Cox s web slides Statistical Analysis ~ fmri ROI Signal Time (% change) Course Time Condition Images removed due to copyright reasons. Statistical Map superimposed on anatomical MRI image 18

19 Visualizing brains What about the folds? Inflated Brains (This used to be done physically, but now we can do it mathematically) Visualizing brains Across-subject analysis Talairach coords. 1) Based on one French woman. 2) Widely used standard 3) Many tools and atlases to support this. 19

20 Visualizing brains Across-subject analysis Images removed due to copyright reasons. Brodmann s Areas Cytoarchitectonic designations for brain regions. Allows one to parcel up the cortex into regions with uniform characteristics. Visualizing brains Across-subject analysis Images removed due to copyright reasons. Spherical brain registry 20

21 ROI ROI analysis analysis functionally functionally defined defined regions regions Figure removed due to copyright reasons. Please see: Kanwisher, Nancy, Josh McDermott, and Marvin M. Chun. Figure 3 in "The Fusiform Face Area: A Module in Human Extrastriate Cortex Specialized for Face Perception." J Neurosci 17 (1997): Another strategy employed by many researchers both in fmri and MEG/EEG studies is ROI analysis. ROI = Region of Interest The idea is to isolate a part of the brain using functional criteria, and then only look at what happens there when you show other stimuli. ROI ROI analysis analysis looking looking at at other other stimuli stimuli Images removed due to copyright reasons. Please see: Tong, F., K. Nakayama, M. Moscovitch, O. Weinrib, and N. Kanwisher. "Response properties of the human fusiform face area." Cogn Neuropsychol 17 (2000):

22 ROI analysis looking at other stimuli Figure removed due to copyright reasons. Please see: Tong, F., K. Nakayama, M. Moscovitch, O. Weinrib, and N. Kanwisher. "Response properties of the human fusiform face area." Cogn Neuropsychol 17 (2000): ROI analysis looking at other stimuli Figure removed due to copyright reasons. Please see: Tong, F., K. Nakayama, M. Moscovitch, O. Weinrib, and N. Kanwisher. "Response properties of the human fusiform face area." Cogn Neuropsychol 17 (2000):

23 fmri sufficiency and necessity? So fmri is a great tool for discovering what brain regions show activity for various kinds of tasks. But are we able to really make claims about the necessity of a particular area for some process with fmri? Well lesions were useful for this But we can t just give subjects lesions! Or can we? 23

24 TMS Virtual Lesions Enter TMS, or Transcranial Magnetic Stimulation TMS uses transient magnetic fields to either stimulate or suppress brain activi ty in a focused region Lesioning a subject with TMS 24

25 A simple experiment F P W A simple experiment 25

26 A simple experiment F P W Visual suppression Figure removed due to copyright reasons. 26

27 Is it it safe? 1. If used properly, single-pulse TMS has no known harmful side effects. TMS has been used since 1985 and today some 3,000 stimulators are in use. 2. If used properly, also repetitive TMS (rtms) is thought to be safe. Beginners should consult literature and competent personnel, since rtms can cause seizures. 3. It is extremely important for the future of TMS/ rtms that the experimenters document all harmful effects and the stimulation parameters that produced them. From: International Federation of Clinical Neurophysiology (IFCN) Roadmap 1. Lesion Studies The birth of cog. neuro. 2. MEG and EEG - When does your brain process stuff? 3. PET and fmri Where does your brain process stuff? 4. TMS Do you need that part of your brain to do that? 5. The view from here Q: What s left for these methods? And what s left for cog. neuro? A: Plenty. 27

28 Technological advances EEG/MEG More and more electrodes, and thus better dipole fitting. Better mathematical tools for combating noise in measurements. Better analytical solutions for source localization. New kinds of analysis. Images removed due to copyright reasons. Technological advances fmri New pulse sequences to get better resolution. Bigger and bigger magnets Clever ways to get more out of the BOLD signal integrating fmri and MEG Images removed due to copyright reasons. 28

29 Methodological advances Are we really making progress? Is Cog. Neuro. just legitimized phrenology? On some level, it is and we need to decide if we re okay with that, or if we want to do something more than geography. Finding other ways to use the technology we have is where the real fun will be in the next decade or so. 29