Electrophysiology
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Transcript of 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)
• Review pgs 31 – 50 if this isn’t familiar to you
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?
Intracranial and “single” Unit
• Single or multiple electrodes are inserted into the brain
• “chronic” implant may be left in place for long periods
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
Intracranial and “single” Unit
• Simultaneous recording from several electrodes allows recording of multiple cells
Intracranial and “single” Unit
• Output of unit recordings is often depicted as a “spike train” and measured in spikes/second
Stimulus on
Spikes
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
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
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?
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?
1. Area A “drives” area B
2. Area B “drives” area A
3. Area A and B are controlled by a third area independently
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?
1. Area A “drives” area B
2. Area B “drives” area A
3. Area A and B are controlled by a third area independently and their activity is unrelated
How might you differentiate these possibilities
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
Subdural Grid
• Intracranial electrodes typically cannot be used in human studies
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
Electroencephalography
• It is also possible to record from outside the skull altogether!
Electroencephalography
• pyramidal cells span layers of cortex and have parallel cell bodies
• their combined extracellular field is small but measurable at the scalp!
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)
Electroencephalography
• Electrical potential is usually measured at many sites on the head surface
Electroencephalography
• Electrical potential is usually measured at many sites on the head surface
• More is sometimes better
Electroencephalography
• EEG changes with various states and in response to stimuli