Sensory Processes &amp; Perception PSY 343 Fall Session 2007 - Lecture 7 Measuring Neural Activity Instructor: Sean Green Studying Perception Neural Activity Review - Neural Activity Neurons carry information through action potentials When an action potential travels across a neuron, charged particles move into and out of the cell through ion channels We can get some idea of which neurons are active in spreading information by measuring the physiological effects of action potentials Studying Perception Neural Activity Techniques for measuring brain activity Measuring electrical/magnetic activity Single-cell recording EEG (Electroencephalography) MEG (Magnetoencephalography) Voltage sensitive dye 2-DG (2-Deoxyglucose) PET (<a href="/keyword/positron-emission-tomography/" ><a href="/keyword/positron-emission/" >positron emission</a> tomography</a> ) fMRI Blood oxygenation level dependent (BOLD) Measuring metabolic activity Studying Perception Neural Activity Single-cell recording Done by positioning a microelectrode in a neuron When an action potential reaches that neuron, the microelectrode detects the change in electric charge The electrode's response can be seen on an oscilloscope One limitation is that it's sometimes hard to get the electrode to the neurons without causing damage Studying Perception Neural Activity EEG (Electroencephalography) How EEG works Neurons in the brain produce electrical activity Electrodes on the head record changes in electrical activity through the scalp Usually electrodes are placed in many locations on the scalp Good temporal resolution (you can tell when activity happens) Bad spatial resolution (hard to tell where it happened) Here's a Brookhaven lab site with pictures and info http://www.bnl.gov/neuropsychology/ERPs_al.asp Studying Perception Neural Activity Event related potentials Electrodes will detect electrical noise and brain activity that has nothing to do with what you are studying To get rid of irrelevant activity, researchers often measure ERPs First, present the stimulus many times (many trials) Line up the EEGs for each trial Irrelevant activity will (hopefully) cancel out Activity related to the stimulus event (Event related potentials) will show up in the average of all the EEGs ERPs will show up as peaks and valleys Event related potentials -60 Voltage (Microvolts) -40 -20 -0 20 Stimulus happens here 40 60 -100 0 100 200 300 400 500 600 800 900 1000 Time (milliseconds) Event related potentials Average of 10 trials -60 Voltage (Microvolts) -40 -20 -0 20 Stimulus happens here 40 60 -100 0 100 200 300 400 500 600 800 900 1000 Time (milliseconds) Event related potentials -60 Voltage (Microvolts) -40 -20 -0 20 Stimulus happens here 40 60 -100 0 100 200 300 400 500 600 800 900 1000 Time (milliseconds) Studying Perception Neural Activity Here's a real example Armstrong, et al., 2002 Comparing ERPs for deaf and hearing people as they watch moving stimuli INZ and PZ are different electrodes Scale Studying Perception Neural Activity MEG (Magnetoencephalography) A more recent technique How MEG works Electrical activity in the brain generates measurable magnetic fields Devices called SQUIDs placed near the scalp measure these fields Good temporal resolution, like EEG, somewhat better spatial resolution The skull doesn't interfere with magnetic fields as much as electric fields Studying Perception Neural Activity Voltage Sensitive Dye How Voltage sensitive dyes work Voltage sensitive dyes are chemicals that behave differently when exposed to electricity (some of them fluoresce) By introducing them into the brain we can tell what parts of the brain are active by looking for the effects of the dyes Studying Perception Neural Activity Voltage Sensitive Dye Example of voltage sensitive dye from the Delaney lab University of Victoria in BC Olfactory neuron terminals www.sfu.ca/~kdelaney/ glomeruli/glomeruli.html Studying Perception Neural Activity Measuring metabolic activity Neurons are living cells To function, neurons need Glucose (sugar) Oxygen (carried to neurons by hemoglobin) Instead of measuring action potentials directly, we can measure the effects of a neurons increased use of glucose and oxygen when it is more active Studying Perception Neural Activity 2-Deoxyglucose How the 2-DG technique works Neurons in the brain that are more active use more glucose Give an animal 2-Deoxyglucose with a radioactive atom (like carbon-14 or tritium) in it 2-Deoxyglucose is taken up by brain cells (like sugar) but is not metabolized More active brain areas will take up more 2-DG (along with regular glucose) By measuring the level of radioactivity, you can see what brain areas have been more active Note: the last step involves cutting out slices of the brain. This technique is not done on humans. Studying Perception Neural Activity 2-Deoxyglucose How the 2-DG technique works Neurons in the brain that are more active use more glucose Zzz... Glucose Glucose Studying Perception Neural Activity 2-Deoxyglucose How the 2-DG technique works If you give the neurons radioactive 2-deoxyglucose, it builds up in the cells Zzz... 2-deoxyglucose 2-deoxyglucose Studying Perception Neural Activity 2-Deoxyglucose How the 2-DG technique works If you give the neurons radioactive 2-deoxyglucose, it builds up in the cells Zzz... 2-deoxyglucose 2-deoxyglucose Studying Perception Neural Activity 2-Deoxyglucose How the 2-DG technique works You can tell which cells were more active because they give off more radiation from the 2-DG inside them Radiation Radiation Studying Perception Neural Activity 2-Deoxyglucose This is an image obtained using 2-deoxyglucose Note how the pattern of the stimulus is reflected in the pattern of activity in the cortex Tootell, et al., 1982 Studying Perception Neural Activity <a href="/keyword/positron-emission-tomography/" ><a href="/keyword/positron-emission/" >positron emission</a> tomography</a> (PET) How PET works Inject a person with a positron-emitting radioactive chemical that is taken up by the brain Metabolically active parts of the brain will take up more of the radioactive chemical The chemical naturally emits positrons Positrons emitted from the brain can be detected to measure brain activity Image: www.alzheimers.org/unraveling/07.htm <a href="/keyword/functional-magnetic-resonance-imaging/" >functional <a href="/keyword/magnetic-resonance-imaging/" >magnetic <a href="/keyword/resonance-imaging/" >resonance imaging</a> </a> </a> (fMRI) How fMRI works Studying Perception Neural Activity Hydrogen nuclei possess a fundamental property called &quot;spin&quot; When the brain is placed in a magnetic field, the spin of hydrogen atoms lines up with the field If a radio wave is sent in, the wave temporarily disturbs this alignment As the atoms return to their original state, they produce their own radio waves, which can be measured Here's a site on the web that explains fMRI in more detail http://www.neuroguide.com/gregg.html <a href="/keyword/functional-magnetic-resonance-imaging/" >functional <a href="/keyword/magnetic-resonance-imaging/" >magnetic <a href="/keyword/resonance-imaging/" >resonance imaging</a> </a> </a> (fMRI) Blood Oxygen Level Dependent (BOLD) fMRI Hemoglobin carries oxygen to cells, including neurons Neurons that are more active receive more oxygen fMRI can detect this as a change in the oxygenation level of hemoglobin One advantage of fMRI over PET is that it does not expose the brain to radioactivity Studying Perception Neural Activity <a href="/keyword/functional-magnetic-resonance-imaging/" >functional <a href="/keyword/magnetic-resonance-imaging/" >magnetic <a href="/keyword/resonance-imaging/" >resonance imaging</a> </a> </a> (fMRI) fMRI has good spatial resolution, but poor temporal resolution Studying Perception Neural Activity Easier to tell where brain activity occurs Hard to tell when brain activity occurs This is because controlling oxygen intake is a slower process It's like trying to figure out when people are driving by finding out how much they spend on gas <a href="/keyword/functional-magnetic-resonance-imaging/" >functional <a href="/keyword/magnetic-resonance-imaging/" >magnetic <a href="/keyword/resonance-imaging/" >resonance imaging</a> </a> </a> (fMRI) Demo: http://www.cns.nyu.edu/~heegerlab/ Studying Perception Neural Activity Studying Perception Neural Activity The Subtraction Technique Measuring brain activity The subtraction technique The brain doesn't just do one thing at a time Subtraction technique helps to correct for activity that is not related to what you are trying to measure. How it works: Measure brain activity when the stimulus isn't there Measure brain activity while the stimulus is there Try to make experimental conditions the same for both measurements except for the stimulus Example Finger tapping versus not moving fingers Subtract one from the other to find out which brain areas show differences in activation Studying Perception Neural Activity The Subtraction Technique Subtraction Baseline Technique Example Brain Regions A B C D Experimental Difference Studying Perception Neural Activity Overview Measuring Brain Activity How do these work: 2DG MEG EEG (ERP) PET fMRI voltage sensitive dyes What does each technique measure? Is it invasive? What sort of information does it provide? What is BOLD? What is the subtraction technique?