Electrophysiology

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) • Review pgs 31 – 50 if this isn’t familiar to you

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 • “chronic” implant 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 typically cannot be used in human studies

15. 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

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