Presentation on theme: "Opportunity to Participate"— Presentation transcript:
1 Opportunity to Participate EEG studies of vision/hearing/decision making – takes about 2 hoursSign up atKeep checking back there for more time slotsTwo extra points added to your final grade!
2 Functional Imagingblood flow overshoots baseline after a brain region is activatedMore oxygenated blood in that region increases MR signal from that region
3 Experimental Design in fMRI A voxel in tissue that responds to the task shows signal change that matches the timecourse of the stimulusSignalActiveRestActiveRest60 sec60 sec60 sec60 sec
4 Experimental Design in fMRI A real example of fMRI block design done well:alternate moving, blank and stationary visual inputMovingBlankStationaryBlank40 sec40 sec40 sec40 sec
5 Experimental Design in fMRI Voxels in Primary cortex tracked all stimuli
6 Experimental Design in fMRI Voxels in area MT tracked only the onset of motion
7 Experimental Design in fMRI Voxels in area MT tracked only the onset of motionHow did they know to look in area MT?
8 PET: another way to measure blood Oxygenation Positron Emission Tomography (PET)Injects a radioisotope of oxygenPET scanner detects the concentration of this isotope as it decays
9 Advantages of fMRI Advantages of MRI: Most hospitals have MRI scanners that can be used for fMRI (PET is rare)Better spatial resolution in fMRI than PETStructural MRI is usually needed anywayNo radioactivity in MRIBetter temporal resolution in MRI
10 Advantages of PET Advantages of PET: Quiet A number of different molecules can be labeled and imaged in the body
11 Limitations of fMRI All techniques have constraints and limitations A good scientist is careful to interpret data within those constraints
12 Limitations of fMRI Limitations of MRI and PET: BOLD signal change does not necessarily mean a region was specifically engaged in a cognitive operationPoor temporal resolution - depends on slow changes in blood flowexpensive
14 Neurons are Electrical Remember that Neurons have electrically charged membranesthey also rapidly discharge and recharge those membranes (graded potentials and action potentials)Review relevant textbook sections if this isn’t familiar to you
15 Neurons are Electrical Importantly, we think the electrical signals are fundamental to brain function, so it makes sense that we should try to directly measure these signalsbut how?
16 Intracranial and “single” Unit Single or multiple electrodes are inserted into the brain“chronic” implant may be left in place for long periods
17 Intracranial and “single” Unit Single electrodes may pick up action potentials from a single cellAn electrode may pick up the combined activity from several nearby cellsspike-sorting attempts to isolate individual cells
18 Intracranial and “single” Unit Simultaneous recording from many electrodes allows recording of multiple cells
19 Intracranial and “single” Unit Output of unit recordings is often depicted as a “spike train” and measured in spikes/secondStimulus onSpikes
20 Intracranial and “single” Unit Output of unit recordings is often depicted as a “spike train” and measured in spikes/secondSpike rate is almost never zero, even without sensory inputin visual cortex this gives rise to “cortical grey”Stimulus onSpikes
21 Intracranial and “single” Unit By carefully associating changes in spike rate with sensory stimuli or cognitive task, one can map the functional circuitry of one or more brain regions
22 Subdural GridIntracranial electrodes typically cannot be used in human studies
23 Subdural GridIntracranial electrodes typically cannot be used in human studiesIt is possible to record from the cortical surfaceSubdural grid on surface of Human cortex
24 Electroencephalography and the Event-Related Potential Could you measure these electric fields without inserting electrodes through the skull?
25 Electroencephalography and the Event-Related Potential 1929 – first measurement of brain electrical activity from scalp electrodes (Berger, 1929)
26 Electroencephalography and the Event-Related Potential VoltageTimePlace an electrode on the scalp and another one somewhere else on the bodyAmplify the signal to record the voltage difference across these electrodesKeep a running measurement of how that voltage changes over timeThis is the human EEG
27 Electroencephalography and the Event-Related Potential 1929 – first measurement of brain electrical activity from scalp electrodes (Berger, 1929)Believed to be artifactual and/or of no significanceCurrently Google Scholar search for “human EEG” returns “about 383,000” hitsThat’s about 13 papers per day
28 Electroencephalography pyramidal cells span layers of cortex and have parallel cell bodiestheir combined extracellular field is small but measurable at the scalp!
29 Electroencephalography The field generated by a patch of cortex can be modeled as a single equivalent dipolar current source with some orientation (assumed to be perpendicular to cortical surface)Duracell
30 Electroencephalography Electrical potential is usually measured at many sites on the head surfaceMore is sometimes better
31 Electroencephalography EEG changes with various states and in response to stimuli