1. Opportunity to Participate
EEG studies of vision/hearing/decision making – takes about 2 hours
Sign up at
Keep checking back there for more time slots
Two extra points added to your final grade!

2. Functional Imaging
blood flow overshoots baseline after a brain region is activated
More 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 stimulus
Signal
Active
Rest
Active
Rest
60 sec
60 sec
60 sec
60 sec

4. Experimental Design in fMRI
A real example of fMRI block design done well:
alternate moving, blank and stationary visual input
Moving
Blank
Stationary
Blank
40 sec
40 sec
40 sec
40 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 motion
How did they know to look in area MT?

8. PET: another way to measure blood Oxygenation
Positron Emission Tomography (PET)
Injects a radioisotope of oxygen
PET 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 PET
Structural MRI is usually needed anyway
No radioactivity in MRI
Better 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 operation
Poor temporal resolution - depends on slow changes in blood flow
expensive

13. Electrophysiology

14. Neurons are Electrical
Remember that Neurons have electrically charged membranes
they 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 signals
but 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 cell
An electrode may pick up the combined activity from several nearby cells
spike-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/second
Stimulus on
Spikes

20. Intracranial and “single” Unit
Output of unit recordings is often depicted as a “spike train” and measured in spikes/second
Spike rate is almost never zero, even without sensory input
in visual cortex this gives rise to “cortical grey”
Stimulus on
Spikes

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 Grid
Intracranial electrodes typically cannot be used in human studies

23. Subdural Grid
Intracranial electrodes typically cannot be used in human studies
It is possible to record from the cortical surface
Subdural 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
Voltage
Time
Place an electrode on the scalp and another one somewhere else on the body
Amplify the signal to record the voltage difference across these electrodes
Keep a running measurement of how that voltage changes over time
This 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 significance
Currently Google Scholar search for “human EEG” returns “about 383,000” hits
That’s about 13 papers per day

28. Electroencephalography
pyramidal cells span layers of cortex and have parallel cell bodies
their 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 surface
More is sometimes better

31. Electroencephalography
EEG changes with various states and in response to stimuli