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