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Electrophysiology

Electrophysiology. Neurons are Electrical. Remember that Neurons have electrically charged membranes they also rapidly discharge and recharge those membranes (graded potentials and action potentials). Neurons are Electrical.

johnavon-gavin
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Electrophysiology

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  1. Electrophysiology

  2. Neurons are Electrical • Remember that Neurons have electrically charged membranes • they also rapidly discharge and recharge those membranes (graded potentials and action potentials)

  3. 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?

  4. Intracranial and “single” Unit • Single or multiple electrodes are inserted into the brain • may be left in place for long periods

  5. Intracranial and “single” Unit • Single electrodes may pick up action potentials from a single cell • An electrode may pick up the signals from several nearby cells • spike-sorting attempts to isolate individual cells

  6. Intracranial and “single” Unit • Simultaneous recording from several electrodes allows recording of multiple cells

  7. Intracranial and “single” Unit • Output of unit recordings is often depicted as a “spike train” and measured in spikes/second Stimulus on Spikes

  8. 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

  9. 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

  10. Intracranial and “single” Unit • Some complications: • Suppose we observe an increase in spike rate in two discrete regions of the brain in response to a sensory stimulus: What are the possible interpretations?

  11. Intracranial and “single” Unit • Some complications: • Suppose we observe an increase in spike rate in two discrete regions of the brain in response to a sensory stimulus: What are the possible interpretations? • Area A “drives” area B • Area B “drives” area A • Area A and B are controlled by a third area independently

  12. Intracranial and “single” Unit • Some complications: • Suppose we observe an increase in spike rate in two discrete regions of the brain in response to a sensory stimulus: What are the possible interpretations? • Area A “drives” area B • Area B “drives” area A • Area A and B are controlled by a third area independently and their activity is unrelated How might you differentiate these possibilities

  13. Intracranial and “single” Unit How might you differentiate these possibilities • Timing of spikes might help: • if A and B are synchronized they are probably functionally related • if A leads B then it is likely to be the first in the signal chain

  14. Subdural Grid • Intracranial electrodes cannot be used in human studies

  15. Subdural Grid • Intracranial electrodes cannot be used in human studies • It is possible to record from the cortical surface Subdural grid on surface of Human cortex

  16. Electroencephalography • It is also possible to record from outside the skull altogether!

  17. Electroencephalography • pyramidal cells span layers of cortex and have parallel cell bodies • their combined extracellular field is small but measurable at the scalp!

  18. 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

  19. Electroencephalography • Electrical potential is usually measured at many sites on the head surface

  20. Electroencephalography • Electrical potential is usually measured at many sites on the head surface • More is sometimes better

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

  22. The Event-Related Potential (ERP) • Embedded in the EEG signal is the small electrical response due to specific events such as stimulus or task onsets, motor actions, etc.

  23. The Event-Related Potential (ERP) • Embedded in the EEG signal is the small electrical response due to specific events such as stimulus or task onsets, motor actions, etc. • Averaging all such events together isolates this event-related potential

  24. The Event-Related Potential (ERP) • We have an ERP waveform for every electrode

  25. The Event-Related Potential (ERP) • We have an ERP waveform for every electrode

  26. The Event-Related Potential (ERP) • We have an ERP waveform for every electrode • Sometimes that isn’t very useful

  27. The Event-Related Potential (ERP) • We have an ERP waveform for every electrode • Sometimes that isn’t very useful • Sometimes we want to know the overall pattern of potentials across the head surface • isopotential map

  28. The Event-Related Potential (ERP) • We have an ERP waveform for every electrode • Sometimes that isn’t very useful • Sometimes we want to know the overall pattern of potentials across the head surface • isopotential map Sometimes that isn’t very useful - we want to know the generator source in 3D

  29. Brain Electrical Source Analysis • Given this pattern on the scalp, can you guess where the current generator was?

  30. Brain Electrical Source Analysis • Given this pattern on the scalp, can you guess where the current generator was? Duracell

  31. Brain Electrical Source Analysis • Source Analysis models neural activity as one or more equivalent current dipoles inside a head-shaped volume with some set of electrical characteristics

  32. Brain Electrical Source Analysis Initiate the model

  33. Brain Electrical Source Analysis Project “Forward Solution” Initiate the model

  34. Brain Electrical Source Analysis Project “Forward Solution” Initiate the model Compare to actual data

  35. Brain Electrical Source Analysis Project “Forward Solution” Adjust the model Compare to actual data

  36. Brain Electrical Source Analysis Project “Forward Solution” This is most likely location of dipole Compare to actual data

  37. Brain Electrical Source Analysis • EEG data can now be coregistered with high-resolution MRI image Anatomical MRI

  38. Brain Electrical Source Analysis • EEG data can now be coregistered with high-resolution MRI image 3D volume is rendered and electrode locations are superimposed Anatomical MRI

  39. Brain Electrical Source Analysis • EEG data can now be coregistered with high-resolution MRI image

  40. Magnetoencephalography • For any electric current, there is an associated magnetic field Electric Current Magnetic Field

  41. Magnetoencephalography • For any electric current, there is an associated magnetic field • magnetic sensors called “SQuID”s can measure very small fields associated with current flowing through extracellular space Electric Current Magnetic Field SquID Amplifier

  42. Magnetoencephalography • MEG systems use many sensors to accomplish source analysis • MEG and EEG are complementary because they are sensitive to orthogonal current flows • MEG is very expensive

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